diff --git a/src/abstract_tree/assignment.rs b/src/abstract_tree/assignment.rs
index 0ae82ec..163f1d6 100644
--- a/src/abstract_tree/assignment.rs
+++ b/src/abstract_tree/assignment.rs
@@ -1,8 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, AssignmentOperator, Error, Format, Identifier, Map, Result, Statement,
-    SyntaxNode, SyntaxPosition, Type, TypeSpecification, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, AssignmentOperator, Error, Format, Identifier, Map, Statement, SyntaxNode,
+    SyntaxPosition, Type, TypeSpecification, Value,
 };
 
 /// Variable assignment, including add-assign and subtract-assign operations.
@@ -17,7 +18,11 @@ pub struct Assignment {
 }
 
 impl AbstractTree for Assignment {
-    fn from_syntax(syntax_node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(
+        syntax_node: SyntaxNode,
+        source: &str,
+        context: &Map,
+    ) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "assignment", syntax_node)?;
 
         let child_count = syntax_node.child_count();
@@ -57,7 +62,7 @@ impl AbstractTree for Assignment {
         })
     }
 
-    fn check_type(&self, source: &str, context: &Map) -> Result<()> {
+    fn check_type(&self, source: &str, context: &Map) -> Result<(), ValidationError> {
         let actual_type = self.statement.expected_type(context)?;
 
         if let Some(type_specification) = &self.type_specification {
@@ -103,7 +108,7 @@ impl AbstractTree for Assignment {
         Ok(())
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let key = self.identifier.inner();
         let value = self.statement.run(source, context)?;
 
@@ -113,7 +118,9 @@ impl AbstractTree for Assignment {
                     previous_value += value;
                     previous_value
                 } else {
-                    return Err(Error::VariableIdentifierNotFound(key.clone()));
+                    return Err(Error::Runtime(RuntimeError::VariableIdentifierNotFound(
+                        key.clone(),
+                    )));
                 }
             }
             AssignmentOperator::MinusEqual => {
@@ -121,7 +128,9 @@ impl AbstractTree for Assignment {
                     previous_value -= value;
                     previous_value
                 } else {
-                    return Err(Error::VariableIdentifierNotFound(key.clone()));
+                    return Err(Error::Runtime(RuntimeError::VariableIdentifierNotFound(
+                        key.clone(),
+                    )));
                 }
             }
             AssignmentOperator::Equal => value,
@@ -132,7 +141,7 @@ impl AbstractTree for Assignment {
         Ok(Value::none())
     }
 
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
         Ok(Type::None)
     }
 }
diff --git a/src/abstract_tree/assignment_operator.rs b/src/abstract_tree/assignment_operator.rs
index 4fe09da..21cb351 100644
--- a/src/abstract_tree/assignment_operator.rs
+++ b/src/abstract_tree/assignment_operator.rs
@@ -1,6 +1,9 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Map, Result, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Map, SyntaxNode, Type, Value,
+};
 
 /// Operators that be used in an assignment statement.
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -11,7 +14,11 @@ pub enum AssignmentOperator {
 }
 
 impl AbstractTree for AssignmentOperator {
-    fn from_syntax(node: SyntaxNode, source: &str, _context: &crate::Map) -> Result<Self> {
+    fn from_syntax(
+        node: SyntaxNode,
+        source: &str,
+        _context: &crate::Map,
+    ) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "assignment_operator", node)?;
 
         let operator_node = node.child(0).unwrap();
@@ -32,13 +39,17 @@ impl AbstractTree for AssignmentOperator {
         Ok(operator)
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
         Ok(Value::none())
     }
 
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
         Ok(Type::None)
     }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        Ok(())
+    }
 }
 
 impl Format for AssignmentOperator {
diff --git a/src/abstract_tree/block.rs b/src/abstract_tree/block.rs
index 9a87881..b1d95fb 100644
--- a/src/abstract_tree/block.rs
+++ b/src/abstract_tree/block.rs
@@ -3,7 +3,10 @@ use std::sync::RwLock;
 use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Map, Result, Statement, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Map, Statement, SyntaxNode, Type, Value,
+};
 
 /// Abstract representation of a block.
 ///
@@ -20,7 +23,7 @@ pub struct Block {
 }
 
 impl AbstractTree for Block {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "block", node)?;
 
         let first_child = node.child(0).unwrap();
@@ -49,7 +52,7 @@ impl AbstractTree for Block {
         })
     }
 
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
         for statement in &self.statements {
             if let Statement::Return(inner_statement) = statement {
                 return inner_statement.check_type(_source, _context);
@@ -61,7 +64,7 @@ impl AbstractTree for Block {
         Ok(())
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         if self.is_async {
             let statements = &self.statements;
             let final_result = RwLock::new(Ok(Value::none()));
@@ -110,7 +113,7 @@ impl AbstractTree for Block {
         }
     }
 
-    fn expected_type(&self, context: &Map) -> Result<Type> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
         if let Some(statement) = self.statements.iter().find(|statement| {
             if let Statement::Return(_) = statement {
                 true
diff --git a/src/abstract_tree/built_in_value.rs b/src/abstract_tree/built_in_value.rs
index 471a75a..8e56203 100644
--- a/src/abstract_tree/built_in_value.rs
+++ b/src/abstract_tree/built_in_value.rs
@@ -5,7 +5,8 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     built_in_functions::{fs::fs_functions, json::json_functions, str::string_functions, Callable},
-    AbstractTree, BuiltInFunction, Format, Function, List, Map, Result, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, BuiltInFunction, Format, Function, List, Map, SyntaxNode, Type, Value,
 };
 
 static ARGS: OnceLock<Value> = OnceLock::new();
@@ -174,7 +175,7 @@ impl BuiltInValue {
 }
 
 impl AbstractTree for BuiltInValue {
-    fn from_syntax(node: SyntaxNode, _source: &str, _context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, _source: &str, _context: &Map) -> Result<Self, SyntaxError> {
         let built_in_value = match node.kind() {
             "args" => BuiltInValue::Args,
             "assert_equal" => BuiltInValue::AssertEqual,
@@ -190,12 +191,16 @@ impl AbstractTree for BuiltInValue {
         Ok(built_in_value)
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
-        Ok(self.get().clone())
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(self.r#type())
     }
 
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(self.r#type())
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
+        Ok(self.get().clone())
     }
 }
 
diff --git a/src/abstract_tree/command.rs b/src/abstract_tree/command.rs
index d67e7a0..b172bb3 100644
--- a/src/abstract_tree/command.rs
+++ b/src/abstract_tree/command.rs
@@ -2,7 +2,10 @@ use std::process;
 
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Map, Result, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Map, Type, Value,
+};
 
 /// An external program invokation.
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -12,7 +15,11 @@ pub struct Command {
 }
 
 impl AbstractTree for Command {
-    fn from_syntax(node: tree_sitter::Node, source: &str, _context: &crate::Map) -> Result<Self> {
+    fn from_syntax(
+        node: tree_sitter::Node,
+        source: &str,
+        _context: &crate::Map,
+    ) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "command", node)?;
 
         let command_text_node = node.child(1).unwrap();
@@ -40,7 +47,15 @@ impl AbstractTree for Command {
         })
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(Type::String)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        todo!()
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
         let output = process::Command::new(&self.command_text)
             .args(&self.command_arguments)
             .spawn()?
@@ -50,10 +65,6 @@ impl AbstractTree for Command {
 
         Ok(Value::String(string))
     }
-
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::String)
-    }
 }
 
 impl Format for Command {
diff --git a/src/abstract_tree/expression.rs b/src/abstract_tree/expression.rs
index b49a536..c9751d7 100644
--- a/src/abstract_tree/expression.rs
+++ b/src/abstract_tree/expression.rs
@@ -1,8 +1,10 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    value_node::ValueNode, AbstractTree, Command, Error, Format, FunctionCall, Identifier, Index,
-    Logic, Map, Math, New, Result, SyntaxNode, Type, Value, Yield,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    value_node::ValueNode,
+    AbstractTree, Command, Error, Format, FunctionCall, Identifier, Index, Logic, Map, Math, New,
+    SyntaxNode, Type, Value, Yield,
 };
 
 /// Abstract representation of an expression statement.
@@ -24,7 +26,7 @@ pub enum Expression {
 }
 
 impl AbstractTree for Expression {
-    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "expression", node)?;
 
         let child = if node.child(0).unwrap().is_named() {
@@ -62,7 +64,21 @@ impl AbstractTree for Expression {
         Ok(expression)
     }
 
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        match self {
+            Expression::Value(value_node) => value_node.expected_type(_context),
+            Expression::Identifier(identifier) => identifier.expected_type(_context),
+            Expression::Math(math) => math.expected_type(_context),
+            Expression::Logic(logic) => logic.expected_type(_context),
+            Expression::FunctionCall(function_call) => function_call.expected_type(_context),
+            Expression::Index(index) => index.expected_type(_context),
+            Expression::Yield(r#yield) => r#yield.expected_type(_context),
+            Expression::New(new) => new.expected_type(_context),
+            Expression::Command(command) => command.expected_type(_context),
+        }
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
         match self {
             Expression::Value(value_node) => value_node.check_type(_source, _context),
             Expression::Identifier(identifier) => identifier.check_type(_source, _context),
@@ -76,7 +92,7 @@ impl AbstractTree for Expression {
         }
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
         match self {
             Expression::Value(value_node) => value_node.run(_source, _context),
             Expression::Identifier(identifier) => identifier.run(_source, _context),
@@ -89,20 +105,6 @@ impl AbstractTree for Expression {
             Expression::Command(command) => command.run(_source, _context),
         }
     }
-
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        match self {
-            Expression::Value(value_node) => value_node.expected_type(_context),
-            Expression::Identifier(identifier) => identifier.expected_type(_context),
-            Expression::Math(math) => math.expected_type(_context),
-            Expression::Logic(logic) => logic.expected_type(_context),
-            Expression::FunctionCall(function_call) => function_call.expected_type(_context),
-            Expression::Index(index) => index.expected_type(_context),
-            Expression::Yield(r#yield) => r#yield.expected_type(_context),
-            Expression::New(new) => new.expected_type(_context),
-            Expression::Command(command) => command.expected_type(_context),
-        }
-    }
 }
 
 impl Format for Expression {
diff --git a/src/abstract_tree/for.rs b/src/abstract_tree/for.rs
index 48d812c..799759a 100644
--- a/src/abstract_tree/for.rs
+++ b/src/abstract_tree/for.rs
@@ -2,8 +2,8 @@ use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Block, Error, Expression, Format, Identifier, Map, Result, SyntaxNode, Type,
-    Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Error, Expression, Format, Identifier, Map, SyntaxNode, Type, Value,
 };
 
 /// Abstract representation of a for loop statement.
@@ -16,7 +16,7 @@ pub struct For {
 }
 
 impl AbstractTree for For {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "for", node)?;
 
         let for_node = node.child(0).unwrap();
@@ -50,7 +50,15 @@ impl AbstractTree for For {
         })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.block.check_type(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let expression_run = self.collection.run(source, context)?;
         let key = self.item_id.inner();
 
@@ -98,10 +106,6 @@ impl AbstractTree for For {
 
         Ok(Value::none())
     }
-
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::None)
-    }
 }
 
 impl Format for For {
diff --git a/src/abstract_tree/function_call.rs b/src/abstract_tree/function_call.rs
index 1839882..4fced7f 100644
--- a/src/abstract_tree/function_call.rs
+++ b/src/abstract_tree/function_call.rs
@@ -1,8 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Error, Expression, Format, FunctionExpression, Map, Result, SyntaxNode,
-    SyntaxPosition, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Expression, Format, FunctionExpression, Map, SyntaxNode, SyntaxPosition,
+    Type, Value,
 };
 
 /// A function being invoked and the arguments it is being passed.
@@ -29,7 +30,7 @@ impl FunctionCall {
 }
 
 impl AbstractTree for FunctionCall {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "function_call", node)?;
 
         let function_node = node.child(0).unwrap();
@@ -54,7 +55,37 @@ impl AbstractTree for FunctionCall {
         })
     }
 
-    fn check_type(&self, source: &str, context: &Map) -> Result<()> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        match &self.function_expression {
+            FunctionExpression::Identifier(identifier) => {
+                let identifier_type = identifier.expected_type(context)?;
+
+                if let Type::Function {
+                    parameter_types: _,
+                    return_type,
+                } = &identifier_type
+                {
+                    Ok(*return_type.clone())
+                } else {
+                    Ok(identifier_type)
+                }
+            }
+            FunctionExpression::FunctionCall(function_call) => function_call.expected_type(context),
+            FunctionExpression::Value(value_node) => {
+                let value_type = value_node.expected_type(context)?;
+
+                if let Type::Function { return_type, .. } = value_type {
+                    Ok(*return_type)
+                } else {
+                    Ok(value_type)
+                }
+            }
+            FunctionExpression::Index(index) => index.expected_type(context),
+            FunctionExpression::Yield(r#yield) => r#yield.expected_type(context),
+        }
+    }
+
+    fn check_type(&self, source: &str, context: &Map) -> Result<(), ValidationError> {
         let function_expression_type = self.function_expression.expected_type(context)?;
 
         let parameter_types = match function_expression_type {
@@ -89,7 +120,7 @@ impl AbstractTree for FunctionCall {
         Ok(())
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let value = match &self.function_expression {
             FunctionExpression::Identifier(identifier) => {
                 let key = identifier.inner();
@@ -121,36 +152,6 @@ impl AbstractTree for FunctionCall {
 
         value.as_function()?.call(&arguments, source, context)
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        match &self.function_expression {
-            FunctionExpression::Identifier(identifier) => {
-                let identifier_type = identifier.expected_type(context)?;
-
-                if let Type::Function {
-                    parameter_types: _,
-                    return_type,
-                } = &identifier_type
-                {
-                    Ok(*return_type.clone())
-                } else {
-                    Ok(identifier_type)
-                }
-            }
-            FunctionExpression::FunctionCall(function_call) => function_call.expected_type(context),
-            FunctionExpression::Value(value_node) => {
-                let value_type = value_node.expected_type(context)?;
-
-                if let Type::Function { return_type, .. } = value_type {
-                    Ok(*return_type)
-                } else {
-                    Ok(value_type)
-                }
-            }
-            FunctionExpression::Index(index) => index.expected_type(context),
-            FunctionExpression::Yield(r#yield) => r#yield.expected_type(context),
-        }
-    }
 }
 
 impl Format for FunctionCall {
diff --git a/src/abstract_tree/function_expression.rs b/src/abstract_tree/function_expression.rs
index 8e2ced9..5590cf8 100644
--- a/src/abstract_tree/function_expression.rs
+++ b/src/abstract_tree/function_expression.rs
@@ -1,8 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Error, Format, FunctionCall, Identifier, Index, Map, Result, SyntaxNode, Type,
-    Value, ValueNode, Yield,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, FunctionCall, Identifier, Index, Map, SyntaxNode, Type, Value,
+    ValueNode, Yield,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -15,7 +16,7 @@ pub enum FunctionExpression {
 }
 
 impl AbstractTree for FunctionExpression {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "function_expression", node)?;
 
         let first_child = node.child(0).unwrap();
@@ -51,17 +52,7 @@ impl AbstractTree for FunctionExpression {
         Ok(function_expression)
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
-        match self {
-            FunctionExpression::Identifier(identifier) => identifier.run(source, context),
-            FunctionExpression::FunctionCall(function_call) => function_call.run(source, context),
-            FunctionExpression::Value(value_node) => value_node.run(source, context),
-            FunctionExpression::Index(index) => index.run(source, context),
-            FunctionExpression::Yield(r#yield) => r#yield.run(source, context),
-        }
-    }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
         match self {
             FunctionExpression::Identifier(identifier) => identifier.expected_type(context),
             FunctionExpression::FunctionCall(function_call) => function_call.expected_type(context),
@@ -70,6 +61,28 @@ impl AbstractTree for FunctionExpression {
             FunctionExpression::Yield(r#yield) => r#yield.expected_type(context),
         }
     }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        match self {
+            FunctionExpression::Identifier(identifier) => identifier.check_type(_source, _context),
+            FunctionExpression::FunctionCall(function_call) => {
+                function_call.check_type(_source, _context)
+            }
+            FunctionExpression::Value(value_node) => value_node.check_type(_source, _context),
+            FunctionExpression::Index(index) => index.check_type(_source, _context),
+            FunctionExpression::Yield(r#yield) => r#yield.check_type(_source, _context),
+        }
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
+        match self {
+            FunctionExpression::Identifier(identifier) => identifier.run(source, context),
+            FunctionExpression::FunctionCall(function_call) => function_call.run(source, context),
+            FunctionExpression::Value(value_node) => value_node.run(source, context),
+            FunctionExpression::Index(index) => index.run(source, context),
+            FunctionExpression::Yield(r#yield) => r#yield.run(source, context),
+        }
+    }
 }
 
 impl Format for FunctionExpression {
diff --git a/src/abstract_tree/function_node.rs b/src/abstract_tree/function_node.rs
index 32ac575..367a525 100644
--- a/src/abstract_tree/function_node.rs
+++ b/src/abstract_tree/function_node.rs
@@ -3,8 +3,9 @@ use std::fmt::{self, Display, Formatter};
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Block, Error, Format, Function, Identifier, Map, Result, SyntaxNode,
-    SyntaxPosition, Type, TypeSpecification, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Error, Format, Function, Identifier, Map, SyntaxNode, SyntaxPosition,
+    Type, TypeSpecification, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
@@ -56,7 +57,12 @@ impl FunctionNode {
         }
     }
 
-    pub fn call(&self, arguments: &[Value], source: &str, outer_context: &Map) -> Result<Value> {
+    pub fn call(
+        &self,
+        arguments: &[Value],
+        source: &str,
+        outer_context: &Map,
+    ) -> Result<Value, RuntimeError> {
         let function_context = Map::new();
         let parameter_argument_pairs = self.parameters.iter().zip(arguments.iter());
 
@@ -79,7 +85,11 @@ impl FunctionNode {
 }
 
 impl AbstractTree for FunctionNode {
-    fn from_syntax(node: SyntaxNode, source: &str, outer_context: &Map) -> Result<Self> {
+    fn from_syntax(
+        node: SyntaxNode,
+        source: &str,
+        outer_context: &Map,
+    ) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "function", node)?;
 
         let child_count = node.child_count();
@@ -132,7 +142,11 @@ impl AbstractTree for FunctionNode {
         })
     }
 
-    fn check_type(&self, source: &str, context: &Map) -> Result<()> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(self.r#type().clone())
+    }
+
+    fn check_type(&self, source: &str, context: &Map) -> Result<(), ValidationError> {
         let function_context = Map::new();
 
         for (key, (_value, r#type)) in context.variables()?.iter() {
@@ -163,15 +177,11 @@ impl AbstractTree for FunctionNode {
         }
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
         let self_as_value = Value::Function(Function::ContextDefined(self.clone()));
 
         Ok(self_as_value)
     }
-
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(self.r#type().clone())
-    }
 }
 
 impl Format for FunctionNode {
diff --git a/src/abstract_tree/identifier.rs b/src/abstract_tree/identifier.rs
index a4a58ef..3861c82 100644
--- a/src/abstract_tree/identifier.rs
+++ b/src/abstract_tree/identifier.rs
@@ -2,7 +2,10 @@ use std::fmt::{self, Display, Formatter};
 
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Map, Result, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Map, SyntaxNode, Type, Value,
+};
 
 /// A string by which a variable is known to a context.
 ///
@@ -26,7 +29,7 @@ impl Identifier {
 }
 
 impl AbstractTree for Identifier {
-    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "identifier", node)?;
 
         let text = &source[node.byte_range()];
@@ -36,21 +39,21 @@ impl AbstractTree for Identifier {
         Ok(Identifier(text.to_string()))
     }
 
-    fn run(&self, _source: &str, context: &Map) -> Result<Value> {
-        if let Some((value, _)) = context.variables()?.get(&self.0) {
-            Ok(value.clone())
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        if let Some((_value, r#type)) = context.variables()?.get(&self.0) {
+            Ok(r#type.clone())
         } else {
             Err(Error::VariableIdentifierNotFound(self.0.clone()))
         }
     }
 
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
-        Ok(())
-    }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        if let Some((_value, r#type)) = context.variables()?.get(&self.0) {
-            Ok(r#type.clone())
+    fn run(&self, _source: &str, context: &Map) -> Result<Value, RuntimeError> {
+        if let Some((value, _)) = context.variables()?.get(&self.0) {
+            Ok(value.clone())
         } else {
             Err(Error::VariableIdentifierNotFound(self.0.clone()))
         }
diff --git a/src/abstract_tree/if_else.rs b/src/abstract_tree/if_else.rs
index 37d0d69..29bb860 100644
--- a/src/abstract_tree/if_else.rs
+++ b/src/abstract_tree/if_else.rs
@@ -1,6 +1,9 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Block, Expression, Format, Map, Result, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Expression, Format, Map, SyntaxNode, Type, Value,
+};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub struct IfElse {
@@ -12,7 +15,7 @@ pub struct IfElse {
 }
 
 impl AbstractTree for IfElse {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         let if_expression_node = node.child(0).unwrap().child(1).unwrap();
         let if_expression = Expression::from_syntax(if_expression_node, source, context)?;
 
@@ -54,7 +57,35 @@ impl AbstractTree for IfElse {
         })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        self.if_block.expected_type(context)
+    }
+
+    fn check_type(&self, _source: &str, context: &Map) -> Result<(), ValidationError> {
+        self.if_expression.check_type(_source, context)?;
+        self.if_block.check_type(_source, context)?;
+
+        let expected_type = self.if_block.expected_type(context)?;
+        let else_ifs = self
+            .else_if_expressions
+            .iter()
+            .zip(self.else_if_blocks.iter());
+
+        for (expression, block) in else_ifs {
+            expression.check_type(_source, context)?;
+            block.check_type(_source, context)?;
+
+            expected_type.check(&block.expected_type(context)?)?;
+        }
+
+        if let Some(expression) = self.else_block {
+            expected_type.check(&expression.expected_type(context)?)?;
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let if_boolean = self.if_expression.run(source, context)?.as_boolean()?;
 
         if if_boolean {
@@ -79,10 +110,6 @@ impl AbstractTree for IfElse {
             }
         }
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        self.if_block.expected_type(context)
-    }
 }
 
 impl Format for IfElse {
diff --git a/src/abstract_tree/index.rs b/src/abstract_tree/index.rs
index 59332dc..f71186c 100644
--- a/src/abstract_tree/index.rs
+++ b/src/abstract_tree/index.rs
@@ -1,7 +1,8 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Error, Format, IndexExpression, List, Map, Result, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, IndexExpression, List, Map, SyntaxNode, Type, Value,
 };
 
 /// Abstract representation of an index expression.
@@ -15,7 +16,7 @@ pub struct Index {
 }
 
 impl AbstractTree for Index {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "index", node)?;
 
         let collection_node = node.child(0).unwrap();
@@ -42,7 +43,27 @@ impl AbstractTree for Index {
         })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        match self.collection.expected_type(context)? {
+            Type::List(item_type) => Ok(*item_type.clone()),
+            Type::Map(_) => Ok(Type::Any),
+            Type::None => Ok(Type::None),
+            r#type => Ok(r#type),
+        }
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.collection.check_type(_source, _context)?;
+        self.index.check_type(_source, _context)?;
+
+        if let Some(index_end) = self.index_end {
+            index_end.check_type(_source, _context)?;
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let collection = self.collection.run(source, context)?;
 
         match collection {
@@ -97,15 +118,6 @@ impl AbstractTree for Index {
             _ => Err(Error::ExpectedCollection { actual: collection }),
         }
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        match self.collection.expected_type(context)? {
-            Type::List(item_type) => Ok(*item_type.clone()),
-            Type::Map(_) => Ok(Type::Any),
-            Type::None => Ok(Type::None),
-            r#type => Ok(r#type),
-        }
-    }
 }
 
 impl Format for Index {
diff --git a/src/abstract_tree/index_assignment.rs b/src/abstract_tree/index_assignment.rs
index 1caa054..defcb18 100644
--- a/src/abstract_tree/index_assignment.rs
+++ b/src/abstract_tree/index_assignment.rs
@@ -1,8 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, AssignmentOperator, Error, Format, Index, IndexExpression, Map, Result,
-    Statement, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, AssignmentOperator, Error, Format, Index, IndexExpression, Map, Statement,
+    SyntaxNode, Type, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -13,7 +14,7 @@ pub struct IndexAssignment {
 }
 
 impl AbstractTree for IndexAssignment {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "index_assignment", node)?;
 
         let index_node = node.child(0).unwrap();
@@ -32,7 +33,16 @@ impl AbstractTree for IndexAssignment {
         })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.index.check_type(_source, _context)?;
+        self.statement.check_type(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let index_collection = self.index.collection.run(source, context)?;
         let index_context = index_collection.as_map().unwrap_or(context);
         let index_key = if let IndexExpression::Identifier(identifier) = &self.index.index {
@@ -73,10 +83,6 @@ impl AbstractTree for IndexAssignment {
 
         Ok(Value::none())
     }
-
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::None)
-    }
 }
 
 impl Format for IndexAssignment {
diff --git a/src/abstract_tree/index_expression.rs b/src/abstract_tree/index_expression.rs
index 2778da2..bc988bb 100644
--- a/src/abstract_tree/index_expression.rs
+++ b/src/abstract_tree/index_expression.rs
@@ -1,8 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    value_node::ValueNode, AbstractTree, Error, Format, FunctionCall, Identifier, Index, Map,
-    Result, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    value_node::ValueNode,
+    AbstractTree, Error, Format, FunctionCall, Identifier, Index, Map, SyntaxNode, Type, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -14,7 +15,7 @@ pub enum IndexExpression {
 }
 
 impl AbstractTree for IndexExpression {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "index_expression", node)?;
 
         let first_child = node.child(0).unwrap();
@@ -48,16 +49,7 @@ impl AbstractTree for IndexExpression {
         Ok(abstract_node)
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
-        match self {
-            IndexExpression::Value(value_node) => value_node.run(source, context),
-            IndexExpression::Identifier(identifier) => identifier.run(source, context),
-            IndexExpression::Index(index) => index.run(source, context),
-            IndexExpression::FunctionCall(function_call) => function_call.run(source, context),
-        }
-    }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
         match self {
             IndexExpression::Value(value_node) => value_node.expected_type(context),
             IndexExpression::Identifier(identifier) => identifier.expected_type(context),
@@ -65,6 +57,26 @@ impl AbstractTree for IndexExpression {
             IndexExpression::FunctionCall(function_call) => function_call.expected_type(context),
         }
     }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        match self {
+            IndexExpression::Value(value_node) => value_node.check_type(_source, _context),
+            IndexExpression::Identifier(identifier) => identifier.check_type(_source, _context),
+            IndexExpression::Index(index) => index.check_type(_source, _context),
+            IndexExpression::FunctionCall(function_call) => {
+                function_call.check_type(_source, _context)
+            }
+        }
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
+        match self {
+            IndexExpression::Value(value_node) => value_node.run(source, context),
+            IndexExpression::Identifier(identifier) => identifier.run(source, context),
+            IndexExpression::Index(index) => index.run(source, context),
+            IndexExpression::FunctionCall(function_call) => function_call.run(source, context),
+        }
+    }
 }
 
 impl Format for IndexExpression {
diff --git a/src/abstract_tree/logic.rs b/src/abstract_tree/logic.rs
index 542c9d8..e7bcbca 100644
--- a/src/abstract_tree/logic.rs
+++ b/src/abstract_tree/logic.rs
@@ -1,7 +1,8 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Error, Expression, Format, LogicOperator, Map, Result, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Expression, Format, LogicOperator, Map, SyntaxNode, Type, Value,
 };
 
 /// Abstract representation of a logic expression.
@@ -13,7 +14,7 @@ pub struct Logic {
 }
 
 impl AbstractTree for Logic {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "logic", node)?;
 
         let first_node = node.child(0).unwrap();
@@ -39,7 +40,16 @@ impl AbstractTree for Logic {
         })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(Type::Boolean)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.left.check_type(_source, _context)?;
+        self.right.check_type(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let left = self.left.run(source, context)?;
         let right = self.right.run(source, context)?;
         let result = match self.operator {
@@ -67,10 +77,6 @@ impl AbstractTree for Logic {
 
         Ok(Value::Boolean(result))
     }
-
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::Boolean)
-    }
 }
 
 impl Format for Logic {
diff --git a/src/abstract_tree/logic_operator.rs b/src/abstract_tree/logic_operator.rs
index 4ec9035..c31fb26 100644
--- a/src/abstract_tree/logic_operator.rs
+++ b/src/abstract_tree/logic_operator.rs
@@ -1,6 +1,9 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Map, Result, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Map, SyntaxNode, Type, Value,
+};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub enum LogicOperator {
@@ -15,7 +18,7 @@ pub enum LogicOperator {
 }
 
 impl AbstractTree for LogicOperator {
-    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> crate::Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "logic_operator", node)?;
 
         let operator_node = node.child(0).unwrap();
@@ -41,12 +44,16 @@ impl AbstractTree for LogicOperator {
         Ok(operator)
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
-        Ok(Value::none())
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(Type::None)
     }
 
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::None)
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
     }
 }
 
diff --git a/src/abstract_tree/match.rs b/src/abstract_tree/match.rs
index e86c584..0ad5767 100644
--- a/src/abstract_tree/match.rs
+++ b/src/abstract_tree/match.rs
@@ -5,7 +5,8 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Error, Expression, Format, Map, Result, Statement, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Expression, Format, Map, Statement, SyntaxNode, Type, Value,
 };
 
 /// Abstract representation of a match statement.
@@ -17,7 +18,7 @@ pub struct Match {
 }
 
 impl AbstractTree for Match {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "match", node)?;
 
         let matcher_node = node.child(1).unwrap();
@@ -58,7 +59,28 @@ impl AbstractTree for Match {
         })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        let (_, first_statement) = self.options.first().unwrap();
+
+        first_statement.expected_type(context)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.matcher.check_type(_source, _context)?;
+
+        for (expression, statement) in self.options {
+            expression.check_type(_source, _context)?;
+            statement.check_type(_source, _context)?;
+        }
+
+        if let Some(statement) = self.fallback {
+            statement.check_type(_source, _context)?;
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let matcher_value = self.matcher.run(source, context)?;
 
         for (expression, statement) in &self.options {
@@ -75,12 +97,6 @@ impl AbstractTree for Match {
             Ok(Value::none())
         }
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        let (_, first_statement) = self.options.first().unwrap();
-
-        first_statement.expected_type(context)
-    }
 }
 
 impl Format for Match {
diff --git a/src/abstract_tree/math.rs b/src/abstract_tree/math.rs
index 2a37bf0..c7a80aa 100644
--- a/src/abstract_tree/math.rs
+++ b/src/abstract_tree/math.rs
@@ -1,7 +1,8 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Error, Expression, Format, Map, MathOperator, Result, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Expression, Format, Map, MathOperator, SyntaxNode, Type, Value,
 };
 
 /// Abstract representation of a math operation.
@@ -16,7 +17,7 @@ pub struct Math {
 }
 
 impl AbstractTree for Math {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "math", node)?;
 
         let left_node = node.child(0).unwrap();
@@ -35,7 +36,16 @@ impl AbstractTree for Math {
         })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        self.left.expected_type(context)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.left.check_type(_source, _context)?;
+        self.right.check_type(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let left = self.left.run(source, context)?;
         let right = self.right.run(source, context)?;
         let value = match self.operator {
@@ -48,10 +58,6 @@ impl AbstractTree for Math {
 
         Ok(value)
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        self.left.expected_type(context)
-    }
 }
 
 impl Format for Math {
diff --git a/src/abstract_tree/math_operator.rs b/src/abstract_tree/math_operator.rs
index 8b74fbd..497f7d0 100644
--- a/src/abstract_tree/math_operator.rs
+++ b/src/abstract_tree/math_operator.rs
@@ -1,6 +1,9 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Map, Result, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Map, SyntaxNode, Type, Value,
+};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub enum MathOperator {
@@ -12,7 +15,7 @@ pub enum MathOperator {
 }
 
 impl AbstractTree for MathOperator {
-    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, _context: &Map) -> Result<Self, SyntaxError> {
         let operator_node = node.child(0).unwrap();
         let operator = match operator_node.kind() {
             "+" => MathOperator::Add,
@@ -33,12 +36,16 @@ impl AbstractTree for MathOperator {
         Ok(operator)
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
-        Ok(Value::none())
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(Type::None)
     }
 
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::None)
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
     }
 }
 
diff --git a/src/abstract_tree/mod.rs b/src/abstract_tree/mod.rs
index 1808cbc..95f46b0 100644
--- a/src/abstract_tree/mod.rs
+++ b/src/abstract_tree/mod.rs
@@ -44,7 +44,10 @@ pub use {
 
 use serde::{Deserialize, Serialize};
 
-use crate::{Error, Map, Result, SyntaxNode, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    Error, Map, SyntaxNode, Value,
+};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
 pub struct SyntaxPosition {
@@ -78,7 +81,7 @@ pub struct Root {
 // instead of indexes. This will be more performant when there are a lot of
 // top-level statements in the tree.
 impl AbstractTree for Root {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "root", node)?;
 
         let statement_count = node.child_count();
@@ -94,7 +97,7 @@ impl AbstractTree for Root {
         Ok(Root { statements })
     }
 
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
         for statement in &self.statements {
             if let Statement::Return(inner_statement) = statement {
                 return inner_statement.check_type(_source, _context);
@@ -106,7 +109,7 @@ impl AbstractTree for Root {
         Ok(())
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let mut value = Value::none();
 
         for statement in &self.statements {
@@ -120,7 +123,7 @@ impl AbstractTree for Root {
         Ok(value)
     }
 
-    fn expected_type(&self, context: &Map) -> Result<Type> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
         self.statements.last().unwrap().expected_type(context)
     }
 }
@@ -149,17 +152,15 @@ pub trait AbstractTree: Sized + Format {
     ///
     /// If necessary, the source code can be accessed directly by getting the
     /// node's byte range.
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self>;
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError>;
 
     /// Verify the type integrity of the node.
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
-        Ok(())
-    }
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError>;
 
     /// Execute dust code by traversing the tree.
-    fn run(&self, source: &str, context: &Map) -> Result<Value>;
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError>;
 
-    fn expected_type(&self, context: &Map) -> Result<Type>;
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError>;
 }
 
 pub trait Format {
diff --git a/src/abstract_tree/new.rs b/src/abstract_tree/new.rs
index 93a4d7e..a517d66 100644
--- a/src/abstract_tree/new.rs
+++ b/src/abstract_tree/new.rs
@@ -2,7 +2,8 @@ use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, Format, Identifier, Map, Result, Type, TypeSpecification, Value, ValueNode,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Format, Identifier, Map, Type, TypeSpecification, Value, ValueNode,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
@@ -12,7 +13,7 @@ pub struct New {
 }
 
 impl AbstractTree for New {
-    fn from_syntax(node: Node, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: Node, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         let identifier_node = node.child(1).unwrap();
         let identifier = Identifier::from_syntax(identifier_node, source, context)?;
 
@@ -24,11 +25,15 @@ impl AbstractTree for New {
         })
     }
 
-    fn run(&self, _source: &str, _context: &crate::Map) -> crate::Result<Value> {
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
         todo!()
     }
 
-    fn expected_type(&self, _context: &crate::Map) -> crate::Result<Type> {
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        todo!()
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
         todo!()
     }
 }
diff --git a/src/abstract_tree/statement.rs b/src/abstract_tree/statement.rs
index 4054ad2..db1cc23 100644
--- a/src/abstract_tree/statement.rs
+++ b/src/abstract_tree/statement.rs
@@ -1,8 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
     AbstractTree, Assignment, Block, Error, Expression, For, Format, IfElse, IndexAssignment, Map,
-    Match, Result, SyntaxNode, Type, Value, While,
+    Match, SyntaxNode, Type, Value, While,
 };
 
 /// Abstract representation of a statement.
@@ -20,7 +21,7 @@ pub enum Statement {
 }
 
 impl AbstractTree for Statement {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "statement", node)?;
 
         let child = node.child(0).unwrap();
@@ -65,7 +66,21 @@ impl AbstractTree for Statement {
         }
     }
 
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        match self {
+            Statement::Assignment(assignment) => assignment.expected_type(context),
+            Statement::Expression(expression) => expression.expected_type(context),
+            Statement::IfElse(if_else) => if_else.expected_type(context),
+            Statement::Match(r#match) => r#match.expected_type(context),
+            Statement::While(r#while) => r#while.expected_type(context),
+            Statement::Block(block) => block.expected_type(context),
+            Statement::For(r#for) => r#for.expected_type(context),
+            Statement::IndexAssignment(index_assignment) => index_assignment.expected_type(context),
+            Statement::Return(statement) => statement.expected_type(context),
+        }
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
         match self {
             Statement::Assignment(assignment) => assignment.check_type(_source, _context),
             Statement::Expression(expression) => expression.check_type(_source, _context),
@@ -81,7 +96,7 @@ impl AbstractTree for Statement {
         }
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         match self {
             Statement::Assignment(assignment) => assignment.run(source, context),
             Statement::Expression(expression) => expression.run(source, context),
@@ -94,20 +109,6 @@ impl AbstractTree for Statement {
             Statement::Return(statement) => statement.run(source, context),
         }
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        match self {
-            Statement::Assignment(assignment) => assignment.expected_type(context),
-            Statement::Expression(expression) => expression.expected_type(context),
-            Statement::IfElse(if_else) => if_else.expected_type(context),
-            Statement::Match(r#match) => r#match.expected_type(context),
-            Statement::While(r#while) => r#while.expected_type(context),
-            Statement::Block(block) => block.expected_type(context),
-            Statement::For(r#for) => r#for.expected_type(context),
-            Statement::IndexAssignment(index_assignment) => index_assignment.expected_type(context),
-            Statement::Return(statement) => statement.expected_type(context),
-        }
-    }
 }
 
 impl Format for Statement {
diff --git a/src/abstract_tree/type.rs b/src/abstract_tree/type.rs
index eaf9618..7aa8b2a 100644
--- a/src/abstract_tree/type.rs
+++ b/src/abstract_tree/type.rs
@@ -2,7 +2,10 @@ use std::fmt::{self, Display, Formatter};
 
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Identifier, Map, Result, Structure, SyntaxNode, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Identifier, Map, Structure, SyntaxNode, Value,
+};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub enum Type {
@@ -41,7 +44,7 @@ impl Type {
         Type::Option(Box::new(optional_type))
     }
 
-    pub fn check(&self, other: &Type) -> Result<()> {
+    pub fn check(&self, other: &Type) -> Result<(), ValidationError> {
         log::info!("Checking type {self} against {other}.");
 
         match (self, other) {
@@ -153,7 +156,7 @@ impl Type {
 }
 
 impl AbstractTree for Type {
-    fn from_syntax(node: SyntaxNode, _source: &str, _context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, _source: &str, _context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(_source, "type", node)?;
 
         let type_node = node.child(0).unwrap();
@@ -219,12 +222,16 @@ impl AbstractTree for Type {
         Ok(r#type)
     }
 
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
-        Ok(Value::none())
+    fn expected_type(&self, _context: &Map) -> Result<Type, ValidationError> {
+        Ok(Type::None)
     }
 
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::None)
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
     }
 }
 
diff --git a/src/abstract_tree/type_specification.rs b/src/abstract_tree/type_specification.rs
index c1ed7ff..f5aa116 100644
--- a/src/abstract_tree/type_specification.rs
+++ b/src/abstract_tree/type_specification.rs
@@ -1,6 +1,9 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Error, Format, Map, Result, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Error, Format, Map, SyntaxNode, Type, Value,
+};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub struct TypeSpecification {
@@ -22,7 +25,7 @@ impl TypeSpecification {
 }
 
 impl AbstractTree for TypeSpecification {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "type_specification", node)?;
 
         let type_node = node.child(1).unwrap();
@@ -31,12 +34,16 @@ impl AbstractTree for TypeSpecification {
         Ok(TypeSpecification { r#type })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
-        self.r#type.run(source, context)
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        self.r#type.expected_type(context)
     }
 
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        self.r#type.expected_type(context)
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
+        self.r#type.run(source, context)
     }
 }
 
diff --git a/src/abstract_tree/value_node.rs b/src/abstract_tree/value_node.rs
index 6edf918..1c1e912 100644
--- a/src/abstract_tree/value_node.rs
+++ b/src/abstract_tree/value_node.rs
@@ -1,14 +1,11 @@
-use std::{
-    cmp::Ordering,
-    collections::BTreeMap,
-    ops::{Range, RangeInclusive},
-};
+use std::{cmp::Ordering, collections::BTreeMap, ops::RangeInclusive};
 
 use serde::{Deserialize, Serialize};
 
 use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
     AbstractTree, BuiltInValue, Error, Expression, Format, Function, FunctionNode, Identifier,
-    List, Map, Result, Statement, Structure, SyntaxNode, Type, TypeSpecification, Value,
+    List, Map, Statement, Structure, SyntaxNode, Type, TypeSpecification, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq)]
@@ -27,7 +24,7 @@ pub enum ValueNode {
 }
 
 impl AbstractTree for ValueNode {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "value", node)?;
 
         let child = node.child(0).unwrap();
@@ -189,7 +186,59 @@ impl AbstractTree for ValueNode {
         Ok(value_node)
     }
 
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        let r#type = match self {
+            ValueNode::Boolean(_) => Type::Boolean,
+            ValueNode::Float(_) => Type::Float,
+            ValueNode::Function(function) => function.r#type().clone(),
+            ValueNode::Integer(_) => Type::Integer,
+            ValueNode::String(_) => Type::String,
+            ValueNode::List(expressions) => {
+                let mut previous_type = None;
+
+                for expression in expressions {
+                    let expression_type = expression.expected_type(context)?;
+
+                    if let Some(previous) = previous_type {
+                        if expression_type != previous {
+                            return Ok(Type::List(Box::new(Type::Any)));
+                        }
+                    }
+
+                    previous_type = Some(expression_type);
+                }
+
+                if let Some(previous) = previous_type {
+                    Type::List(Box::new(previous))
+                } else {
+                    Type::List(Box::new(Type::Any))
+                }
+            }
+            ValueNode::Option(option) => {
+                if let Some(expression) = option {
+                    Type::Option(Box::new(expression.expected_type(context)?))
+                } else {
+                    Type::None
+                }
+            }
+            ValueNode::Map(_) => Type::Map(None),
+            ValueNode::BuiltInValue(built_in_value) => built_in_value.expected_type(context)?,
+            ValueNode::Structure(node_map) => {
+                let mut value_map = BTreeMap::new();
+
+                for (key, (_statement_option, r#type)) in node_map {
+                    value_map.insert(key.to_string(), (None, r#type.clone()));
+                }
+
+                Type::Map(Some(Structure::new(value_map)))
+            }
+            ValueNode::Range(_) => Type::Range,
+        };
+
+        Ok(r#type)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
         match self {
             ValueNode::Function(function) => {
                 if let Function::ContextDefined(function_node) = function {
@@ -202,7 +251,7 @@ impl AbstractTree for ValueNode {
         Ok(())
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         let value = match self {
             ValueNode::Boolean(value_source) => Value::Boolean(value_source.parse().unwrap()),
             ValueNode::Float(value_source) => {
@@ -267,58 +316,6 @@ impl AbstractTree for ValueNode {
 
         Ok(value)
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        let r#type = match self {
-            ValueNode::Boolean(_) => Type::Boolean,
-            ValueNode::Float(_) => Type::Float,
-            ValueNode::Function(function) => function.r#type().clone(),
-            ValueNode::Integer(_) => Type::Integer,
-            ValueNode::String(_) => Type::String,
-            ValueNode::List(expressions) => {
-                let mut previous_type = None;
-
-                for expression in expressions {
-                    let expression_type = expression.expected_type(context)?;
-
-                    if let Some(previous) = previous_type {
-                        if expression_type != previous {
-                            return Ok(Type::List(Box::new(Type::Any)));
-                        }
-                    }
-
-                    previous_type = Some(expression_type);
-                }
-
-                if let Some(previous) = previous_type {
-                    Type::List(Box::new(previous))
-                } else {
-                    Type::List(Box::new(Type::Any))
-                }
-            }
-            ValueNode::Option(option) => {
-                if let Some(expression) = option {
-                    Type::Option(Box::new(expression.expected_type(context)?))
-                } else {
-                    Type::None
-                }
-            }
-            ValueNode::Map(_) => Type::Map(None),
-            ValueNode::BuiltInValue(built_in_value) => built_in_value.expected_type(context)?,
-            ValueNode::Structure(node_map) => {
-                let mut value_map = BTreeMap::new();
-
-                for (key, (_statement_option, r#type)) in node_map {
-                    value_map.insert(key.to_string(), (None, r#type.clone()));
-                }
-
-                Type::Map(Some(Structure::new(value_map)))
-            }
-            ValueNode::Range(_) => Type::Range,
-        };
-
-        Ok(r#type)
-    }
 }
 
 impl Format for ValueNode {
diff --git a/src/abstract_tree/while.rs b/src/abstract_tree/while.rs
index 664f154..22243f1 100644
--- a/src/abstract_tree/while.rs
+++ b/src/abstract_tree/while.rs
@@ -1,6 +1,9 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{AbstractTree, Block, Error, Expression, Format, Map, Result, SyntaxNode, Type, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Error, Expression, Format, Map, SyntaxNode, Type, Value,
+};
 
 /// Abstract representation of a while loop.
 ///
@@ -12,7 +15,7 @@ pub struct While {
 }
 
 impl AbstractTree for While {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> crate::Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "while", node)?;
 
         let expression_node = node.child(1).unwrap();
@@ -24,17 +27,22 @@ impl AbstractTree for While {
         Ok(While { expression, block })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        self.block.expected_type(context)
+    }
+
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.expression.check_type(_source, _context)?;
+        self.block.check_type(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
         while self.expression.run(source, context)?.as_boolean()? {
             self.block.run(source, context)?;
         }
 
         Ok(Value::none())
     }
-
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        self.block.expected_type(context)
-    }
 }
 
 impl Format for While {
diff --git a/src/abstract_tree/yield.rs b/src/abstract_tree/yield.rs
index 2400828..9657196 100644
--- a/src/abstract_tree/yield.rs
+++ b/src/abstract_tree/yield.rs
@@ -1,8 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    function_expression::FunctionExpression, AbstractTree, Error, Expression, Format, FunctionCall,
-    Map, Result, SyntaxNode, Type, Value,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    function_expression::FunctionExpression,
+    AbstractTree, Error, Expression, Format, FunctionCall, Map, SyntaxNode, Type, Value,
 };
 
 /// Abstract representation of a yield expression.
@@ -14,7 +15,7 @@ pub struct Yield {
 }
 
 impl AbstractTree for Yield {
-    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Map) -> Result<Self, SyntaxError> {
         Error::expect_syntax_node(source, "yield", node)?;
 
         let input_node = node.child(0).unwrap();
@@ -42,12 +43,16 @@ impl AbstractTree for Yield {
         Ok(Yield { call })
     }
 
-    fn run(&self, source: &str, context: &Map) -> Result<Value> {
-        self.call.run(source, context)
+    fn expected_type(&self, context: &Map) -> Result<Type, ValidationError> {
+        self.call.expected_type(context)
     }
 
-    fn expected_type(&self, context: &Map) -> Result<Type> {
-        self.call.expected_type(context)
+    fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
+        self.call.check_type(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
+        self.call.run(source, context)
     }
 }
 
diff --git a/src/built_in_functions/fs.rs b/src/built_in_functions/fs.rs
index 4983d34..ddbb8ac 100644
--- a/src/built_in_functions/fs.rs
+++ b/src/built_in_functions/fs.rs
@@ -3,7 +3,7 @@ use std::fs::read_to_string;
 use enum_iterator::{all, Sequence};
 use serde::{Deserialize, Serialize};
 
-use crate::{Error, Map, Result, Type, Value};
+use crate::{error::RuntimeError, Error, Map, Type, Value};
 
 use super::Callable;
 
@@ -35,7 +35,12 @@ impl Callable for Fs {
         }
     }
 
-    fn call(&self, arguments: &[Value], _source: &str, _outer_context: &Map) -> Result<Value> {
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        _outer_context: &Map,
+    ) -> Result<Value, RuntimeError> {
         match self {
             Fs::ReadFile => {
                 Error::expect_argument_amount(self.name(), 1, arguments.len())?;
diff --git a/src/built_in_functions/json.rs b/src/built_in_functions/json.rs
index fb34548..ebe1c6a 100644
--- a/src/built_in_functions/json.rs
+++ b/src/built_in_functions/json.rs
@@ -1,7 +1,7 @@
 use enum_iterator::Sequence;
 use serde::{Deserialize, Serialize};
 
-use crate::{Error, Map, Result, Type, Value};
+use crate::{error::RuntimeError, Error, Map, Type, Value};
 
 use super::Callable;
 
@@ -41,7 +41,12 @@ impl Callable for Json {
         }
     }
 
-    fn call(&self, arguments: &[Value], _source: &str, _outer_context: &Map) -> Result<Value> {
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        _outer_context: &Map,
+    ) -> Result<Value, RuntimeError> {
         match self {
             Json::Create => {
                 Error::expect_argument_amount(self.name(), 1, arguments.len())?;
diff --git a/src/built_in_functions/mod.rs b/src/built_in_functions/mod.rs
index 45eb7a3..ef17410 100644
--- a/src/built_in_functions/mod.rs
+++ b/src/built_in_functions/mod.rs
@@ -7,7 +7,7 @@ use std::fmt::{self, Display, Formatter};
 use rand::{random, thread_rng, Rng};
 use serde::{Deserialize, Serialize};
 
-use crate::{Error, Format, Map, Result, Type, Value};
+use crate::{error::RuntimeError, Error, Format, Map, Type, Value};
 
 use self::{fs::Fs, json::Json, str::StrFunction};
 
@@ -15,7 +15,12 @@ pub trait Callable {
     fn name(&self) -> &'static str;
     fn description(&self) -> &'static str;
     fn r#type(&self) -> Type;
-    fn call(&self, arguments: &[Value], source: &str, outer_context: &Map) -> Result<Value>;
+    fn call(
+        &self,
+        arguments: &[Value],
+        source: &str,
+        outer_context: &Map,
+    ) -> Result<Value, RuntimeError>;
 }
 
 #[derive(Debug, Copy, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
@@ -78,7 +83,12 @@ impl Callable for BuiltInFunction {
         }
     }
 
-    fn call(&self, arguments: &[Value], _source: &str, _outer_context: &Map) -> Result<Value> {
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        _outer_context: &Map,
+    ) -> Result<Value, RuntimeError> {
         match self {
             BuiltInFunction::AssertEqual => {
                 Error::expect_argument_amount(self.name(), 2, arguments.len())?;
diff --git a/src/built_in_functions/str.rs b/src/built_in_functions/str.rs
index 2b53dd5..29f03cb 100644
--- a/src/built_in_functions/str.rs
+++ b/src/built_in_functions/str.rs
@@ -1,7 +1,7 @@
 use enum_iterator::Sequence;
 use serde::{Deserialize, Serialize};
 
-use crate::{Error, List, Map, Result, Type, Value};
+use crate::{error::RuntimeError, Error, List, Map, Type, Value};
 
 use super::Callable;
 
@@ -198,7 +198,12 @@ impl Callable for StrFunction {
         }
     }
 
-    fn call(&self, arguments: &[Value], _source: &str, _outer_context: &Map) -> Result<Value> {
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        _outer_context: &Map,
+    ) -> Result<Value, RuntimeError> {
         let value = match self {
             StrFunction::AsBytes => {
                 Error::expect_argument_amount(self.name(), 1, arguments.len())?;
diff --git a/src/error.rs b/src/error.rs
deleted file mode 100644
index 378d024..0000000
--- a/src/error.rs
+++ /dev/null
@@ -1,468 +0,0 @@
-//! Error and Result types.
-//!
-//! To deal with errors from dependencies, either create a new error variant
-//! or use the ToolFailure variant if the error can only occur inside a tool.
-
-use serde::{Deserialize, Serialize};
-use tree_sitter::{LanguageError, Node, Point};
-
-use crate::{value::Value, SyntaxPosition, Type};
-
-use std::{
-    fmt::{self, Formatter},
-    io,
-    num::ParseFloatError,
-    string::FromUtf8Error,
-    sync::PoisonError,
-    time,
-};
-
-pub type Result<T> = std::result::Result<T, Error>;
-
-#[derive(Clone, PartialEq, Serialize, Deserialize)]
-pub enum Error {
-    AtSourcePosition {
-        error: Box<Error>,
-        source: String,
-        start_row: usize,
-        start_column: usize,
-        end_row: usize,
-        end_column: usize,
-    },
-
-    UnexpectedSyntaxNode {
-        expected: String,
-        actual: String,
-        #[serde(skip)]
-        location: Point,
-        relevant_source: String,
-    },
-
-    TypeCheck {
-        expected: Type,
-        actual: Type,
-    },
-
-    TypeCheckExpectedFunction {
-        actual: Type,
-    },
-
-    /// The 'assert' macro did not resolve successfully.
-    AssertEqualFailed {
-        expected: Value,
-        actual: Value,
-    },
-
-    /// The 'assert' macro did not resolve successfully.
-    AssertFailed,
-
-    /// A row was inserted to a table with the wrong amount of values.
-    WrongColumnAmount {
-        expected: usize,
-        actual: usize,
-    },
-
-    /// An operator was called with the wrong amount of arguments.
-    ExpectedOperatorArgumentAmount {
-        expected: usize,
-        actual: usize,
-    },
-
-    /// A function was called with the wrong amount of arguments.
-    ExpectedBuiltInFunctionArgumentAmount {
-        function_name: String,
-        expected: usize,
-        actual: usize,
-    },
-
-    /// A function was called with the wrong amount of arguments.
-    ExpectedFunctionArgumentAmount {
-        expected: usize,
-        actual: usize,
-    },
-
-    /// A function was called with the wrong amount of arguments.
-    ExpectedFunctionArgumentMinimum {
-        source: String,
-        minumum_expected: usize,
-        actual: usize,
-    },
-
-    ExpectedFunctionType {
-        actual: Type,
-    },
-
-    ExpectedString {
-        actual: Value,
-    },
-
-    ExpectedInteger {
-        actual: Value,
-    },
-
-    ExpectedFloat {
-        actual: Value,
-    },
-
-    /// An integer, floating point or value was expected.
-    ExpectedNumber {
-        actual: Value,
-    },
-
-    /// An integer, floating point or string value was expected.
-    ExpectedNumberOrString {
-        actual: Value,
-    },
-
-    ExpectedBoolean {
-        actual: Value,
-    },
-
-    ExpectedList {
-        actual: Value,
-    },
-
-    ExpectedMinLengthList {
-        minimum_len: usize,
-        actual_len: usize,
-    },
-
-    ExpectedFixedLenList {
-        expected_len: usize,
-        actual: Value,
-    },
-
-    ExpectedNone {
-        actual: Value,
-    },
-
-    ExpectedMap {
-        actual: Value,
-    },
-
-    ExpectedTable {
-        actual: Value,
-    },
-
-    ExpectedFunction {
-        actual: Value,
-    },
-
-    ExpectedOption {
-        actual: Value,
-    },
-
-    /// A string, list, map or table value was expected.
-    ExpectedCollection {
-        actual: Value,
-    },
-
-    /// Failed to find a variable with a value for this key.
-    VariableIdentifierNotFound(String),
-
-    /// The function failed due to an external error.
-    External(String),
-
-    /// A custom error explained by its message.
-    CustomMessage(String),
-
-    /// Invalid user input.
-    Syntax {
-        source: String,
-        #[serde(skip)]
-        location: Point,
-    },
-
-    SerdeJson(String),
-
-    ParserCancelled,
-
-    ParseFloat {
-        reason: String,
-    },
-
-    ExpectedIterable {
-        actual: Value,
-    },
-}
-
-impl Error {
-    pub fn at_source_position(self, source: &str, position: SyntaxPosition) -> Self {
-        let byte_range = position.start_byte..position.end_byte;
-
-        Error::AtSourcePosition {
-            error: Box::new(self),
-            source: source[byte_range].to_string(),
-            start_row: position.start_row,
-            start_column: position.start_column,
-            end_row: position.end_row,
-            end_column: position.end_column,
-        }
-    }
-
-    pub fn expect_syntax_node(source: &str, expected: &str, actual: Node) -> Result<()> {
-        log::info!("Converting {} to abstract node", actual.kind());
-
-        if expected == actual.kind() {
-            Ok(())
-        } else if actual.is_error() {
-            Err(Error::Syntax {
-                source: source[actual.byte_range()].to_string(),
-                location: actual.start_position(),
-            })
-        } else {
-            Err(Error::UnexpectedSyntaxNode {
-                expected: expected.to_string(),
-                actual: actual.kind().to_string(),
-                location: actual.start_position(),
-                relevant_source: source[actual.byte_range()].to_string(),
-            })
-        }
-    }
-
-    pub fn expect_argument_amount(
-        function_name: &str,
-        expected: usize,
-        actual: usize,
-    ) -> Result<()> {
-        if expected == actual {
-            Ok(())
-        } else {
-            Err(Error::ExpectedBuiltInFunctionArgumentAmount {
-                function_name: function_name.to_string(),
-                expected,
-                actual,
-            })
-        }
-    }
-
-    pub fn is_error(&self, other: &Error) -> bool {
-        match self {
-            Error::AtSourcePosition { error, .. } => error.as_ref() == other,
-            _ => self == other,
-        }
-    }
-}
-
-impl From<LanguageError> for Error {
-    fn from(error: LanguageError) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl<T> From<PoisonError<T>> for Error {
-    fn from(error: PoisonError<T>) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl From<FromUtf8Error> for Error {
-    fn from(error: FromUtf8Error) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl From<ParseFloatError> for Error {
-    fn from(error: ParseFloatError) -> Self {
-        Error::ParseFloat {
-            reason: error.to_string(),
-        }
-    }
-}
-
-impl From<csv::Error> for Error {
-    fn from(error: csv::Error) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl From<io::Error> for Error {
-    fn from(error: std::io::Error) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl From<reqwest::Error> for Error {
-    fn from(error: reqwest::Error) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl From<serde_json::Error> for Error {
-    fn from(error: serde_json::Error) -> Self {
-        Error::SerdeJson(error.to_string())
-    }
-}
-
-impl From<time::SystemTimeError> for Error {
-    fn from(error: time::SystemTimeError) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl From<toml::de::Error> for Error {
-    fn from(error: toml::de::Error) -> Self {
-        Error::External(error.to_string())
-    }
-}
-
-impl std::error::Error for Error {}
-
-impl fmt::Debug for Error {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{self}")
-    }
-}
-
-impl fmt::Display for Error {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        use Error::*;
-
-        match self {
-            AssertEqualFailed { expected, actual } => {
-                write!(
-                    f,
-                    "Equality assertion failed. {expected} does not equal {actual}."
-                )
-            }
-            AssertFailed => write!(
-                f,
-                "Assertion failed. A false value was passed to \"assert\"."
-            ),
-            ExpectedOperatorArgumentAmount { expected, actual } => write!(
-                f,
-                "An operator expected {} arguments, but got {}.",
-                expected, actual
-            ),
-            ExpectedBuiltInFunctionArgumentAmount {
-                function_name: tool_name,
-                expected,
-                actual,
-            } => write!(
-                f,
-                "{tool_name} expected {expected} arguments, but got {actual}.",
-            ),
-            ExpectedFunctionArgumentAmount { expected, actual } => {
-                write!(f, "Expected {expected} arguments, but got {actual}.",)
-            }
-            ExpectedFunctionArgumentMinimum {
-                source,
-                minumum_expected,
-                actual,
-            } => {
-                write!(
-                    f,
-                    "{source} expected at least {minumum_expected} arguments, but got {actual}."
-                )
-            }
-            ExpectedString { actual } => {
-                write!(f, "Expected a string but got {actual}.")
-            }
-            ExpectedInteger { actual } => write!(f, "Expected an integer, but got {actual}."),
-            ExpectedFloat { actual } => write!(f, "Expected a float, but got {actual}."),
-            ExpectedNumber { actual } => {
-                write!(f, "Expected a float or integer but got {actual}.",)
-            }
-            ExpectedNumberOrString { actual } => {
-                write!(f, "Expected a number or string, but got {actual}.")
-            }
-            ExpectedBoolean { actual } => {
-                write!(f, "Expected a boolean, but got {actual}.")
-            }
-            ExpectedList { actual } => write!(f, "Expected a list, but got {actual}."),
-            ExpectedMinLengthList {
-                minimum_len,
-                actual_len,
-            } => write!(
-                f,
-                "Expected a list of at least {minimum_len} values, but got one with {actual_len}.",
-            ),
-            ExpectedFixedLenList {
-                expected_len,
-                actual,
-            } => write!(
-                f,
-                "Expected a list of len {}, but got {:?}.",
-                expected_len, actual
-            ),
-            ExpectedNone { actual } => write!(f, "Expected an empty value, but got {actual}."),
-            ExpectedMap { actual } => write!(f, "Expected a map, but got {actual}."),
-            ExpectedTable { actual } => write!(f, "Expected a table, but got {actual}."),
-            ExpectedFunction { actual } => {
-                write!(f, "Expected function, but got {actual}.")
-            }
-            ExpectedOption { actual } => write!(f, "Expected option, but got {actual}."),
-            ExpectedCollection { actual } => {
-                write!(
-                    f,
-                    "Expected a string, list, map or table, but got {actual}.",
-                )
-            }
-            VariableIdentifierNotFound(key) => write!(f, "Variable {key} does not exist.",),
-            UnexpectedSyntaxNode {
-                expected,
-                actual,
-                location,
-                relevant_source,
-            } => {
-                let location = get_position(location);
-
-                write!(
-                    f,
-                    "Expected {expected}, but got {actual} at {location}. Code: {relevant_source} ",
-                )
-            }
-            WrongColumnAmount { expected, actual } => write!(
-                f,
-                "Wrong column amount. Expected {expected} but got {actual}."
-            ),
-            External(message) => write!(f, "External error: {message}"),
-            CustomMessage(message) => write!(f, "{message}"),
-            Syntax { source, location } => {
-                let location = get_position(location);
-
-                write!(f, "Syntax error at {location}: {source}")
-            }
-            TypeCheck { expected, actual } => write!(
-                f,
-                "Type check error. Expected type {expected} but got type {actual}."
-            ),
-            TypeCheckExpectedFunction { actual } => {
-                write!(f, "Type check error. Expected a function but got {actual}.")
-            }
-            AtSourcePosition {
-                error,
-                source,
-                start_row,
-                start_column,
-                end_row,
-                end_column,
-            } => {
-                write!(
-                    f,
-                    "{error} Occured at ({start_row}, {start_column}) to ({end_row}, {end_column}). Source: {source}"
-                )
-            }
-            SerdeJson(message) => write!(f, "JSON processing error: {message}"),
-            ParserCancelled => write!(
-                f,
-                "Parsing was cancelled either manually or because it took too long."
-            ),
-            ParseFloat { reason } => {
-                write!(
-                    f,
-                    "Failed to parse a float value. Reason given: {}.",
-                    reason
-                )
-            }
-            ExpectedFunctionType { actual } => write!(f, "Expected a function but got {actual}."),
-            ExpectedIterable { actual } => {
-                write!(f, "Expected an iterable value but got {actual}.")
-            }
-        }
-    }
-}
-
-fn get_position(position: &Point) -> String {
-    format!("column {}, row {}", position.row + 1, position.column)
-}
diff --git a/src/error/mod.rs b/src/error/mod.rs
new file mode 100644
index 0000000..29925ec
--- /dev/null
+++ b/src/error/mod.rs
@@ -0,0 +1,247 @@
+//! Error and Result types.
+//!
+//! To deal with errors from dependencies, either create a new error variant
+//! or use the ToolFailure variant if the error can only occur inside a tool.
+mod runtime_error;
+mod syntax_error;
+mod validation_error;
+
+pub use runtime_error::RuntimeError;
+pub use syntax_error::SyntaxError;
+pub use validation_error::ValidationError;
+
+use serde::{Deserialize, Serialize};
+use tree_sitter::{LanguageError, Node, Point};
+
+use crate::{value::Value, SyntaxPosition};
+
+use std::{
+    fmt::{self, Formatter},
+    io,
+    num::ParseFloatError,
+    string::FromUtf8Error,
+    sync::PoisonError,
+    time,
+};
+
+#[derive(Clone, PartialEq, Serialize, Deserialize)]
+pub enum Error {
+    Parsing(SyntaxError),
+
+    Verification(ValidationError),
+
+    Runtime(RuntimeError),
+
+    AtSourcePosition {
+        error: Box<Error>,
+        source: String,
+        start_row: usize,
+        start_column: usize,
+        end_row: usize,
+        end_column: usize,
+    },
+
+    /// The function failed due to an external error.
+    External(String),
+
+    /// A custom error explained by its message.
+    CustomMessage(String),
+
+    /// Invalid user input.
+    Syntax {
+        source: String,
+        #[serde(skip)]
+        location: Point,
+    },
+
+    SerdeJson(String),
+
+    ParserCancelled,
+
+    ParseFloat {
+        reason: String,
+    },
+
+    ExpectedIterable {
+        actual: Value,
+    },
+}
+
+impl Error {
+    pub fn at_source_position(self, source: &str, position: SyntaxPosition) -> Self {
+        let byte_range = position.start_byte..position.end_byte;
+
+        Error::AtSourcePosition {
+            error: Box::new(self),
+            source: source[byte_range].to_string(),
+            start_row: position.start_row,
+            start_column: position.start_column,
+            end_row: position.end_row,
+            end_column: position.end_column,
+        }
+    }
+
+    pub fn expect_syntax_node(
+        source: &str,
+        expected: &str,
+        actual: Node,
+    ) -> Result<(), SyntaxError> {
+        log::info!("Converting {} to abstract node", actual.kind());
+
+        if expected == actual.kind() {
+            Ok(())
+        } else if actual.is_error() {
+            Error::Syntax {
+                source: source[actual.byte_range()].to_string(),
+                location: actual.start_position(),
+            }
+        } else {
+            SyntaxError::UnexpectedSyntaxNode {
+                expected: expected.to_string(),
+                actual: actual.kind().to_string(),
+                location: actual.start_position(),
+                relevant_source: source[actual.byte_range()].to_string(),
+            }
+        }
+    }
+
+    pub fn expect_argument_amount(
+        function_name: &str,
+        expected: usize,
+        actual: usize,
+    ) -> Result<(), ValidationError> {
+        if expected == actual {
+            Ok(())
+        } else {
+            Err(Error::ExpectedBuiltInFunctionArgumentAmount {
+                function_name: function_name.to_string(),
+                expected,
+                actual,
+            })
+        }
+    }
+
+    pub fn is_error(&self, other: &Error) -> bool {
+        match self {
+            Error::AtSourcePosition { error, .. } => error.as_ref() == other,
+            _ => self == other,
+        }
+    }
+}
+
+impl From<LanguageError> for Error {
+    fn from(error: LanguageError) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl<T> From<PoisonError<T>> for Error {
+    fn from(error: PoisonError<T>) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl From<FromUtf8Error> for Error {
+    fn from(error: FromUtf8Error) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl From<ParseFloatError> for Error {
+    fn from(error: ParseFloatError) -> Self {
+        Error::ParseFloat {
+            reason: error.to_string(),
+        }
+    }
+}
+
+impl From<csv::Error> for Error {
+    fn from(error: csv::Error) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl From<io::Error> for Error {
+    fn from(error: std::io::Error) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl From<reqwest::Error> for Error {
+    fn from(error: reqwest::Error) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl From<serde_json::Error> for Error {
+    fn from(error: serde_json::Error) -> Self {
+        Error::SerdeJson(error.to_string())
+    }
+}
+
+impl From<time::SystemTimeError> for Error {
+    fn from(error: time::SystemTimeError) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl From<toml::de::Error> for Error {
+    fn from(error: toml::de::Error) -> Self {
+        Error::External(error.to_string())
+    }
+}
+
+impl std::error::Error for Error {}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{self}")
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        use Error::*;
+
+        match self {
+            AtSourcePosition {
+                error,
+                source,
+                start_row,
+                start_column,
+                end_row,
+                end_column,
+            } => {
+                write!(
+                    f,
+                    "{error} Occured at ({start_row}, {start_column}) to ({end_row}, {end_column}). Source: {source}"
+                )
+            }
+            SerdeJson(message) => write!(f, "JSON processing error: {message}"),
+            ParserCancelled => write!(
+                f,
+                "Parsing was cancelled either manually or because it took too long."
+            ),
+            ParseFloat { reason } => {
+                write!(
+                    f,
+                    "Failed to parse a float value. Reason given: {}.",
+                    reason
+                )
+            }
+            ExpectedIterable { actual } => {
+                write!(f, "Expected an iterable value but got {actual}.")
+            }
+            Parsing(_) => todo!(),
+            Verification(_) => todo!(),
+            Runtime(_) => todo!(),
+            External(_) => todo!(),
+            CustomMessage(_) => todo!(),
+            Syntax { source, location } => todo!(),
+        }
+    }
+}
+
+fn get_position(position: &Point) -> String {
+    format!("column {}, row {}", position.row + 1, position.column)
+}
diff --git a/src/error/runtime_error.rs b/src/error/runtime_error.rs
new file mode 100644
index 0000000..1363700
--- /dev/null
+++ b/src/error/runtime_error.rs
@@ -0,0 +1,181 @@
+use std::fmt::{self, Display, Formatter};
+
+use serde::{Deserialize, Serialize};
+
+use crate::Value;
+
+#[derive(Clone, PartialEq, Serialize, Deserialize)]
+pub enum RuntimeError {
+    ExpectedString {
+        actual: Value,
+    },
+
+    ExpectedInteger {
+        actual: Value,
+    },
+
+    ExpectedFloat {
+        actual: Value,
+    },
+
+    /// An integer, floating point or value was expected.
+    ExpectedNumber {
+        actual: Value,
+    },
+
+    /// An integer, floating point or string value was expected.
+    ExpectedNumberOrString {
+        actual: Value,
+    },
+
+    ExpectedBoolean {
+        actual: Value,
+    },
+
+    ExpectedList {
+        actual: Value,
+    },
+
+    ExpectedMinLengthList {
+        minimum_len: usize,
+        actual_len: usize,
+    },
+
+    ExpectedFixedLenList {
+        expected_len: usize,
+        actual: Value,
+    },
+
+    ExpectedNone {
+        actual: Value,
+    },
+
+    ExpectedMap {
+        actual: Value,
+    },
+
+    ExpectedTable {
+        actual: Value,
+    },
+
+    ExpectedFunction {
+        actual: Value,
+    },
+
+    ExpectedOption {
+        actual: Value,
+    },
+
+    /// A string, list, map or table value was expected.
+    ExpectedCollection {
+        actual: Value,
+    },
+
+    /// A function was called with the wrong amount of arguments.
+    ExpectedBuiltInFunctionArgumentAmount {
+        function_name: String,
+        expected: usize,
+        actual: usize,
+    },
+
+    /// A function was called with the wrong amount of arguments.
+    ExpectedFunctionArgumentAmount {
+        expected: usize,
+        actual: usize,
+    },
+
+    /// A function was called with the wrong amount of arguments.
+    ExpectedFunctionArgumentMinimum {
+        source: String,
+        minumum_expected: usize,
+        actual: usize,
+    },
+
+    /// Failed to find a variable with a value for this key.
+    VariableIdentifierNotFound(String),
+}
+
+impl Display for RuntimeError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        use RuntimeError::*;
+
+        match self {
+            ExpectedString { actual } => {
+                write!(f, "Expected a string but got {actual}.")
+            }
+            ExpectedInteger { actual } => write!(f, "Expected an integer, but got {actual}."),
+            ExpectedFloat { actual } => write!(f, "Expected a float, but got {actual}."),
+            ExpectedNumber { actual } => {
+                write!(f, "Expected a float or integer but got {actual}.",)
+            }
+            ExpectedNumberOrString { actual } => {
+                write!(f, "Expected a number or string, but got {actual}.")
+            }
+            ExpectedBoolean { actual } => {
+                write!(f, "Expected a boolean, but got {actual}.")
+            }
+            ExpectedList { actual } => write!(f, "Expected a list, but got {actual}."),
+            ExpectedMinLengthList {
+                minimum_len,
+                actual_len,
+            } => write!(
+                f,
+                "Expected a list of at least {minimum_len} values, but got one with {actual_len}.",
+            ),
+            ExpectedFixedLenList {
+                expected_len,
+                actual,
+            } => write!(
+                f,
+                "Expected a list of len {}, but got {:?}.",
+                expected_len, actual
+            ),
+            ExpectedNone { actual } => write!(f, "Expected an empty value, but got {actual}."),
+            ExpectedMap { actual } => write!(f, "Expected a map, but got {actual}."),
+            ExpectedTable { actual } => write!(f, "Expected a table, but got {actual}."),
+            ExpectedFunction { actual } => {
+                write!(f, "Expected function, but got {actual}.")
+            }
+            ExpectedOption { actual } => write!(f, "Expected option, but got {actual}."),
+            ExpectedCollection { actual } => {
+                write!(
+                    f,
+                    "Expected a string, list, map or table, but got {actual}.",
+                )
+            }
+            ExpectedBuiltInFunctionArgumentAmount {
+                function_name,
+                expected,
+                actual,
+            } => todo!(),
+            ExpectedFunctionArgumentAmount { expected, actual } => todo!(),
+            ExpectedFunctionArgumentMinimum {
+                source,
+                minumum_expected,
+                actual,
+            } => todo!(),
+            ExpectedBuiltInFunctionArgumentAmount {
+                function_name: tool_name,
+                expected,
+                actual,
+            } => write!(
+                f,
+                "{tool_name} expected {expected} arguments, but got {actual}.",
+            ),
+            ExpectedFunctionArgumentAmount { expected, actual } => {
+                write!(f, "Expected {expected} arguments, but got {actual}.",)
+            }
+            ExpectedFunctionArgumentMinimum {
+                source,
+                minumum_expected,
+                actual,
+            } => {
+                write!(
+                    f,
+                    "{source} expected at least {minumum_expected} arguments, but got {actual}."
+                )
+            }
+            VariableIdentifierNotFound(_) => todo!(),
+        }
+    }
+}
diff --git a/src/error/syntax_error.rs b/src/error/syntax_error.rs
new file mode 100644
index 0000000..35063f5
--- /dev/null
+++ b/src/error/syntax_error.rs
@@ -0,0 +1,15 @@
+use serde::{Deserialize, Serialize};
+use tree_sitter::Point;
+
+#[derive(Clone, PartialEq, Serialize, Deserialize)]
+pub enum SyntaxError {
+    UnexpectedSyntaxNode {
+        expected: String,
+        actual: String,
+
+        #[serde(skip)]
+        location: Point,
+
+        relevant_source: String,
+    },
+}
diff --git a/src/error/validation_error.rs b/src/error/validation_error.rs
new file mode 100644
index 0000000..4b447b6
--- /dev/null
+++ b/src/error/validation_error.rs
@@ -0,0 +1,27 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{Type, Value};
+
+#[derive(Clone, PartialEq, Serialize, Deserialize)]
+pub enum ValidationError {
+    /// The 'assert' macro did not resolve successfully.
+    AssertEqualFailed {
+        expected: Value,
+        actual: Value,
+    },
+
+    /// The 'assert' macro did not resolve successfully.
+    AssertFailed,
+
+    TypeCheck {
+        expected: Type,
+        actual: Type,
+    },
+
+    TypeCheckExpectedFunction {
+        actual: Type,
+    },
+
+    /// Failed to find a variable with a value for this key.
+    VariableIdentifierNotFound(String),
+}
diff --git a/src/interpret.rs b/src/interpret.rs
index ddc16b0..57ce972 100644
--- a/src/interpret.rs
+++ b/src/interpret.rs
@@ -37,13 +37,16 @@
 //! ```
 use tree_sitter::{Node as SyntaxNode, Parser, Tree as SyntaxTree, TreeCursor};
 
-use crate::{language, AbstractTree, Error, Format, Map, Result, Root, Value};
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    language, AbstractTree, Error, Format, Map, Root, Value,
+};
 
 /// Interpret the given source code. Returns the value of last statement or the
 /// first error encountered.
 ///
 /// See the [module-level docs][self] for more info.
-pub fn interpret(source: &str) -> Result<Value> {
+pub fn interpret(source: &str) -> Result<Value, Error> {
     interpret_with_context(source, Map::new())
 }
 
@@ -55,7 +58,7 @@ pub fn interpret(source: &str) -> Result<Value> {
 /// maps.
 ///
 /// See the [module-level docs][self] for more info.
-pub fn interpret_with_context(source: &str, context: Map) -> Result<Value> {
+pub fn interpret_with_context(source: &str, context: Map) -> Result<Value, Error> {
     let mut interpreter = Interpreter::new(context);
     let value = interpreter.run(source)?;
 
@@ -93,7 +96,7 @@ impl Interpreter {
     ///
     /// Tree sitter is designed to be run on every keystroke, so this is generally a lightweight
     /// function to call.
-    pub fn parse(&mut self, source: &str) -> Result<SyntaxTree> {
+    pub fn parse(&mut self, source: &str) -> Result<SyntaxTree, Error> {
         if let Some(tree) = self.parser.parse(source, None) {
             Ok(tree)
         } else {
@@ -109,8 +112,12 @@ impl Interpreter {
     /// - check the syntax tree for errors
     /// - generate an abstract tree from the source and syntax tree
     /// - check the abstract tree for type errors
-    pub fn verify(&mut self, source: &str) -> Result<Root> {
-        fn check_for_error(node: SyntaxNode, source: &str, cursor: &mut TreeCursor) -> Result<()> {
+    pub fn verify(&mut self, source: &str) -> Result<Root, Error> {
+        fn check_for_error(
+            node: SyntaxNode,
+            source: &str,
+            cursor: &mut TreeCursor,
+        ) -> Result<(), ValidationError> {
             if node.is_error() {
                 Err(Error::Syntax {
                     source: source[node.byte_range()].to_string(),
@@ -142,13 +149,13 @@ impl Interpreter {
     ///
     /// This function [parses][Self::parse], [verifies][Self::verify] and [runs][Root::run] using
     /// the same source code.
-    pub fn run(&mut self, source: &str) -> Result<Value> {
+    pub fn run(&mut self, source: &str) -> Result<Value, Error> {
         self.verify(source)?.run(source, &self.context)
     }
 
     /// Return an s-expression displaying a syntax tree of the source, or the ParserCancelled error
     /// if the parser takes too long.
-    pub fn syntax_tree(&mut self, source: &str) -> Result<String> {
+    pub fn syntax_tree(&mut self, source: &str) -> Result<String, Error> {
         Ok(self.parse(source)?.root_node().to_sexp())
     }
 
@@ -157,7 +164,7 @@ impl Interpreter {
     ///
     /// You should call [verify][Interpreter::verify] before calling this function. You can only
     /// create formatted source from a valid abstract tree.
-    pub fn format(&mut self, source: &str) -> Result<String> {
+    pub fn format(&mut self, source: &str) -> Result<String, Error> {
         let mut formatted_output = String::new();
 
         self.verify(source)?.format(&mut formatted_output, 0);
diff --git a/src/lib.rs b/src/lib.rs
index b9b3593..7c50ec8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,7 +6,7 @@
 //! You can use this library externally by calling either of the "interpret"
 //! functions or by constructing your own Interpreter.
 pub use crate::{
-    abstract_tree::*, built_in_functions::BuiltInFunction, error::*, interpret::*, value::*,
+    abstract_tree::*, built_in_functions::BuiltInFunction, error::Error, interpret::*, value::*,
 };
 
 pub use tree_sitter::Node as SyntaxNode;
diff --git a/src/value/function.rs b/src/value/function.rs
index 9f8c4c3..9690db7 100644
--- a/src/value/function.rs
+++ b/src/value/function.rs
@@ -3,7 +3,8 @@ use std::fmt::{self, Display, Formatter};
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    built_in_functions::Callable, BuiltInFunction, Format, FunctionNode, Map, Result, Type, Value,
+    built_in_functions::Callable, error::RuntimeError, BuiltInFunction, Format, FunctionNode, Map,
+    Type, Value,
 };
 
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
@@ -13,7 +14,12 @@ pub enum Function {
 }
 
 impl Function {
-    pub fn call(&self, arguments: &[Value], source: &str, outer_context: &Map) -> Result<Value> {
+    pub fn call(
+        &self,
+        arguments: &[Value],
+        source: &str,
+        outer_context: &Map,
+    ) -> Result<Value, RuntimeError> {
         match self {
             Function::BuiltIn(built_in_function) => {
                 built_in_function.call(arguments, source, outer_context)
diff --git a/src/value/map.rs b/src/value/map.rs
index ed30e2e..ae8ec8f 100644
--- a/src/value/map.rs
+++ b/src/value/map.rs
@@ -13,10 +13,12 @@ use std::{
     collections::BTreeMap,
     fmt::{self, Display, Formatter},
     marker::PhantomData,
-    sync::{Arc, RwLock, RwLockReadGuard},
+    sync::{Arc, PoisonError, RwLock, RwLockReadGuard},
 };
 
-use crate::{value::Value, Result, Structure, Type};
+use crate::{value::Value, Structure, Type};
+
+pub type MapAccessError<'a> = PoisonError<RwLockReadGuard<'a, BTreeMap<String, (Value, Type)>>>;
 
 /// A collection dust variables comprised of key-value pairs.
 ///
@@ -63,7 +65,7 @@ impl Map {
         }
     }
 
-    pub fn clone_from(other: &Self) -> Result<Self> {
+    pub fn clone_from(other: &Self) -> Result<Self, ()> {
         let mut new_map = BTreeMap::new();
 
         for (key, (value, r#type)) in other.variables()?.iter() {
@@ -76,11 +78,13 @@ impl Map {
         })
     }
 
-    pub fn variables(&self) -> Result<RwLockReadGuard<BTreeMap<String, (Value, Type)>>> {
-        Ok(self.variables.read()?)
+    pub fn variables(
+        &self,
+    ) -> Result<RwLockReadGuard<BTreeMap<String, (Value, Type)>>, MapAccessError> {
+        self.variables.read()
     }
 
-    pub fn set(&self, key: String, value: Value) -> Result<Option<(Value, Type)>> {
+    pub fn set(&self, key: String, value: Value) -> Result<Option<(Value, Type)>, MapAccessError> {
         log::info!("Setting variable {key} = {value}");
 
         let value_type = value.r#type();
@@ -92,7 +96,7 @@ impl Map {
         Ok(previous)
     }
 
-    pub fn set_type(&self, key: String, r#type: Type) -> Result<Option<(Value, Type)>> {
+    pub fn set_type(&self, key: String, r#type: Type) -> Result<Option<(Value, Type)>, ()> {
         log::info!("Setting type {key} = {}", r#type);
 
         let previous = self.variables.write()?.insert(key, (Value::none(), r#type));
diff --git a/src/value/mod.rs b/src/value/mod.rs
index 3a0c3b5..6c62da4 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -1,6 +1,6 @@
 //! Types that represent runtime values.
 use crate::{
-    error::{Error, Result},
+    error::{Error, RuntimeError},
     Identifier, Type, TypeSpecification,
 };
 
@@ -15,7 +15,7 @@ use std::{
     convert::TryFrom,
     fmt::{self, Display, Formatter},
     marker::PhantomData,
-    ops::{Add, AddAssign, Div, Mul, Range, RangeInclusive, Rem, Sub, SubAssign},
+    ops::{Add, AddAssign, Div, Mul, RangeInclusive, Rem, Sub, SubAssign},
 };
 
 pub use self::{function::Function, list::List, map::Map, structure::Structure};
@@ -164,7 +164,7 @@ impl Value {
     }
 
     /// Borrows the value stored in `self` as `&String`, or returns `Err` if `self` is not a `Value::String`.
-    pub fn as_string(&self) -> Result<&String> {
+    pub fn as_string(&self) -> Result<&String, RuntimeError> {
         match self {
             Value::String(string) => Ok(string),
             value => Err(Error::ExpectedString {
@@ -174,7 +174,7 @@ impl Value {
     }
 
     /// Copies the value stored in `self` as `i64`, or returns `Err` if `self` is not a `Value::Int`
-    pub fn as_integer(&self) -> Result<i64> {
+    pub fn as_integer(&self) -> Result<i64, RuntimeError> {
         match self {
             Value::Integer(i) => Ok(*i),
             value => Err(Error::ExpectedInteger {
@@ -184,7 +184,7 @@ impl Value {
     }
 
     /// Copies the value stored in  `self` as `f64`, or returns `Err` if `self` is not a `Primitive::Float`.
-    pub fn as_float(&self) -> Result<f64> {
+    pub fn as_float(&self) -> Result<f64, RuntimeError> {
         match self {
             Value::Float(f) => Ok(*f),
             value => Err(Error::ExpectedFloat {
@@ -195,7 +195,7 @@ impl Value {
 
     /// Copies the value stored in  `self` as `f64`, or returns `Err` if `self` is not a `Primitive::Float` or `Value::Int`.
     /// Note that this method silently converts `i64` to `f64`, if `self` is a `Value::Int`.
-    pub fn as_number(&self) -> Result<f64> {
+    pub fn as_number(&self) -> Result<f64, RuntimeError> {
         match self {
             Value::Float(f) => Ok(*f),
             Value::Integer(i) => Ok(*i as f64),
@@ -206,7 +206,7 @@ impl Value {
     }
 
     /// Copies the value stored in  `self` as `bool`, or returns `Err` if `self` is not a `Primitive::Boolean`.
-    pub fn as_boolean(&self) -> Result<bool> {
+    pub fn as_boolean(&self) -> Result<bool, RuntimeError> {
         match self {
             Value::Boolean(boolean) => Ok(*boolean),
             value => Err(Error::ExpectedBoolean {
@@ -216,7 +216,7 @@ impl Value {
     }
 
     /// Borrows the value stored in `self` as `Vec<Value>`, or returns `Err` if `self` is not a `Value::List`.
-    pub fn as_list(&self) -> Result<&List> {
+    pub fn as_list(&self) -> Result<&List, RuntimeError> {
         match self {
             Value::List(list) => Ok(list),
             value => Err(Error::ExpectedList {
@@ -226,7 +226,7 @@ impl Value {
     }
 
     /// Takes ownership of the value stored in `self` as `Vec<Value>`, or returns `Err` if `self` is not a `Value::List`.
-    pub fn into_inner_list(self) -> Result<List> {
+    pub fn into_inner_list(self) -> Result<List, RuntimeError> {
         match self {
             Value::List(list) => Ok(list),
             value => Err(Error::ExpectedList {
@@ -236,7 +236,7 @@ impl Value {
     }
 
     /// Borrows the value stored in `self` as `Vec<Value>`, or returns `Err` if `self` is not a `Value::Map`.
-    pub fn as_map(&self) -> Result<&Map> {
+    pub fn as_map(&self) -> Result<&Map, RuntimeError> {
         match self {
             Value::Map(map) => Ok(map),
             value => Err(Error::ExpectedMap {
@@ -247,7 +247,7 @@ impl Value {
 
     /// Borrows the value stored in `self` as `Function`, or returns `Err` if
     /// `self` is not a `Value::Function`.
-    pub fn as_function(&self) -> Result<&Function> {
+    pub fn as_function(&self) -> Result<&Function, RuntimeError> {
         match self {
             Value::Function(function) => Ok(function),
             value => Err(Error::ExpectedFunction {
@@ -257,7 +257,7 @@ impl Value {
     }
 
     /// Returns `Option`, or returns `Err` if `self` is not a `Value::Option`.
-    pub fn as_option(&self) -> Result<&Option<Box<Value>>> {
+    pub fn as_option(&self) -> Result<&Option<Box<Value>>, RuntimeError> {
         match self {
             Value::Option(option) => Ok(option),
             value => Err(Error::ExpectedOption {
@@ -267,7 +267,7 @@ impl Value {
     }
 
     /// Returns `()`, or returns `Err` if `self` is not a `Value::none()`.
-    pub fn as_none(&self) -> Result<()> {
+    pub fn as_none(&self) -> Result<(), RuntimeError> {
         match self {
             Value::Option(option) => {
                 if option.is_none() {
@@ -292,7 +292,7 @@ impl Default for &Value {
 }
 
 impl Add for Value {
-    type Output = Result<Value>;
+    type Output = Result<Value, RuntimeError>;
 
     fn add(self, other: Self) -> Self::Output {
         if let (Ok(left), Ok(right)) = (self.as_integer(), other.as_integer()) {
@@ -326,7 +326,7 @@ impl Add for Value {
 }
 
 impl Sub for Value {
-    type Output = Result<Self>;
+    type Output = Result<Self, RuntimeError>;
 
     fn sub(self, other: Self) -> Self::Output {
         if let (Ok(left), Ok(right)) = (self.as_integer(), other.as_integer()) {
@@ -346,7 +346,7 @@ impl Sub for Value {
 }
 
 impl Mul for Value {
-    type Output = Result<Self>;
+    type Output = Result<Self, RuntimeError>;
 
     fn mul(self, other: Self) -> Self::Output {
         if let (Ok(left), Ok(right)) = (self.as_integer(), other.as_integer()) {
@@ -362,7 +362,7 @@ impl Mul for Value {
 }
 
 impl Div for Value {
-    type Output = Result<Self>;
+    type Output = Result<Self, RuntimeError>;
 
     fn div(self, other: Self) -> Self::Output {
         if let (Ok(left), Ok(right)) = (self.as_number(), other.as_number()) {
@@ -383,7 +383,7 @@ impl Div for Value {
 }
 
 impl Rem for Value {
-    type Output = Result<Self>;
+    type Output = Result<Self, RuntimeError>;
 
     fn rem(self, other: Self) -> Self::Output {
         let left = self.as_integer()?;
@@ -583,7 +583,7 @@ impl From<Vec<Value>> for Value {
     }
 }
 
-impl From<Value> for Result<Value> {
+impl From<Value> for Result<Value, RuntimeError> {
     fn from(value: Value) -> Self {
         Ok(value)
     }