diff --git a/src/abstract_tree/assignment.rs b/src/abstract_tree/assignment.rs
index 163f1d6..d32e879 100644
--- a/src/abstract_tree/assignment.rs
+++ b/src/abstract_tree/assignment.rs
@@ -2,8 +2,8 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, AssignmentOperator, Error, Format, Identifier, Map, Statement, SyntaxNode,
-    SyntaxPosition, Type, TypeSpecification, Value,
+    AbstractTree, AssignmentOperator, Error, Format, Identifier, Map, SourcePosition, Statement,
+    SyntaxNode, Type, TypeSpecification, Value,
 };
 
 /// Variable assignment, including add-assign and subtract-assign operations.
@@ -14,7 +14,7 @@ pub struct Assignment {
     operator: AssignmentOperator,
     statement: Statement,
 
-    syntax_position: SyntaxPosition,
+    syntax_position: SourcePosition,
 }
 
 impl AbstractTree for Assignment {
@@ -43,15 +43,15 @@ impl AbstractTree for Assignment {
         let statement_node = syntax_node.child(child_count - 1).unwrap();
         let statement = Statement::from_syntax(statement_node, source, context)?;
 
-        if let AssignmentOperator::Equal = operator {
-            let r#type = if let Some(definition) = &type_specification {
-                definition.inner().clone()
-            } else {
-                statement.expected_type(context)?
-            };
+        // if let AssignmentOperator::Equal = operator {
+        //     let r#type = if let Some(definition) = &type_specification {
+        //         definition.inner().clone()
+        //     } else {
+        //         statement.expected_type(context)?
+        //     };
 
-            context.set_type(identifier.inner().clone(), r#type)?;
-        }
+        //     context.set_type(identifier.inner().clone(), r#type)?;
+        // }
 
         Ok(Assignment {
             identifier,
@@ -63,28 +63,42 @@ impl AbstractTree for Assignment {
     }
 
     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 {
             match self.operator {
                 AssignmentOperator::Equal => {
-                    type_specification
-                        .inner()
-                        .check(&actual_type)
-                        .map_err(|error| error.at_source_position(source, self.syntax_position))?;
+                    let expected = type_specification.inner();
+                    let actual = self.statement.expected_type(context)?;
+
+                    if !expected.accepts(&actual) {
+                        return Err(ValidationError::TypeCheck {
+                            expected: expected.clone(),
+                            actual,
+                            position: self.syntax_position,
+                        });
+                    }
                 }
                 AssignmentOperator::PlusEqual => {
-                    if let Type::List(item_type) = type_specification.inner() {
-                        item_type.check(&actual_type).map_err(|error| {
-                            error.at_source_position(source, self.syntax_position)
-                        })?;
+                    if let Type::List(expected) = type_specification.inner() {
+                        let actual = self.identifier.expected_type(context)?;
+
+                        if !expected.accepts(&actual) {
+                            return Err(ValidationError::TypeCheck {
+                                expected: expected.as_ref().clone(),
+                                actual,
+                                position: self.syntax_position,
+                            });
+                        }
                     } else {
-                        type_specification
-                            .inner()
-                            .check(&self.identifier.expected_type(context)?)
-                            .map_err(|error| {
-                                error.at_source_position(source, self.syntax_position)
-                            })?;
+                        let expected = type_specification.inner();
+                        let actual = self.identifier.expected_type(context)?;
+
+                        if !expected.accepts(&actual) {
+                            return Err(ValidationError::TypeCheck {
+                                expected: expected.clone(),
+                                actual,
+                                position: self.syntax_position,
+                            });
+                        }
                     }
                 }
                 AssignmentOperator::MinusEqual => todo!(),
@@ -93,10 +107,16 @@ impl AbstractTree for Assignment {
             match self.operator {
                 AssignmentOperator::Equal => {}
                 AssignmentOperator::PlusEqual => {
-                    if let Type::List(item_type) = self.identifier.expected_type(context)? {
-                        item_type.check(&actual_type).map_err(|error| {
-                            error.at_source_position(source, self.syntax_position)
-                        })?;
+                    if let Type::List(expected) = self.identifier.expected_type(context)? {
+                        let actual = self.statement.expected_type(context)?;
+
+                        if !expected.accepts(&actual) {
+                            return Err(ValidationError::TypeCheck {
+                                expected: expected.as_ref().clone(),
+                                actual,
+                                position: self.syntax_position,
+                            });
+                        }
                     }
                 }
                 AssignmentOperator::MinusEqual => todo!(),
@@ -118,9 +138,7 @@ impl AbstractTree for Assignment {
                     previous_value += value;
                     previous_value
                 } else {
-                    return Err(Error::Runtime(RuntimeError::VariableIdentifierNotFound(
-                        key.clone(),
-                    )));
+                    return Err(RuntimeError::VariableIdentifierNotFound(key.clone()));
                 }
             }
             AssignmentOperator::MinusEqual => {
@@ -128,9 +146,7 @@ impl AbstractTree for Assignment {
                     previous_value -= value;
                     previous_value
                 } else {
-                    return Err(Error::Runtime(RuntimeError::VariableIdentifierNotFound(
-                        key.clone(),
-                    )));
+                    return Err(RuntimeError::VariableIdentifierNotFound(key.clone()));
                 }
             }
             AssignmentOperator::Equal => value,
diff --git a/src/abstract_tree/assignment_operator.rs b/src/abstract_tree/assignment_operator.rs
index 21cb351..0fa4e30 100644
--- a/src/abstract_tree/assignment_operator.rs
+++ b/src/abstract_tree/assignment_operator.rs
@@ -27,7 +27,7 @@ impl AbstractTree for AssignmentOperator {
             "+=" => AssignmentOperator::PlusEqual,
             "-=" => AssignmentOperator::MinusEqual,
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected: "=, += or -=".to_string(),
                     actual: operator_node.kind().to_string(),
                     location: operator_node.start_position(),
diff --git a/src/abstract_tree/block.rs b/src/abstract_tree/block.rs
index b1d95fb..1d19cc9 100644
--- a/src/abstract_tree/block.rs
+++ b/src/abstract_tree/block.rs
@@ -4,7 +4,7 @@ use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    error::{RuntimeError, SyntaxError, ValidationError},
+    error::{rw_lock_error::RwLockError, RuntimeError, SyntaxError, ValidationError},
     AbstractTree, Error, Format, Map, Statement, SyntaxNode, Type, Value,
 };
 
@@ -91,13 +91,13 @@ impl AbstractTree for Block {
                                 *final_result = result;
                                 None
                             }
-                            Err(error) => Some(Err(error.into())),
+                            Err(_error) => Some(Err(RuntimeError::RwLock(RwLockError))),
                         }
                     } else {
                         None
                     }
                 })
-                .unwrap_or(final_result.into_inner()?)
+                .unwrap_or(final_result.into_inner().map_err(|_| RwLockError)?)
         } else {
             let mut prev_result = None;
 
diff --git a/src/abstract_tree/expression.rs b/src/abstract_tree/expression.rs
index c9751d7..3571986 100644
--- a/src/abstract_tree/expression.rs
+++ b/src/abstract_tree/expression.rs
@@ -50,7 +50,7 @@ impl AbstractTree for Expression {
             "new" => Expression::New(New::from_syntax(child, source, _context)?),
             "command" => Expression::Command(Command::from_syntax(child, source, _context)?),
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected:
                         "value, identifier, index, math, logic, function call, new, context or ->"
                             .to_string(),
diff --git a/src/abstract_tree/for.rs b/src/abstract_tree/for.rs
index 799759a..daad3d3 100644
--- a/src/abstract_tree/for.rs
+++ b/src/abstract_tree/for.rs
@@ -24,7 +24,7 @@ impl AbstractTree for For {
             "for" => false,
             "async for" => true,
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected: "for or async for".to_string(),
                     actual: for_node.kind().to_string(),
                     location: for_node.start_position(),
diff --git a/src/abstract_tree/function_call.rs b/src/abstract_tree/function_call.rs
index 4fced7f..eb72510 100644
--- a/src/abstract_tree/function_call.rs
+++ b/src/abstract_tree/function_call.rs
@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, Error, Expression, Format, FunctionExpression, Map, SyntaxNode, SyntaxPosition,
+    AbstractTree, Error, Expression, Format, FunctionExpression, Map, SourcePosition, SyntaxNode,
     Type, Value,
 };
 
@@ -11,7 +11,7 @@ use crate::{
 pub struct FunctionCall {
     function_expression: FunctionExpression,
     arguments: Vec<Expression>,
-    syntax_position: SyntaxPosition,
+    syntax_position: SourcePosition,
 }
 
 impl FunctionCall {
@@ -19,7 +19,7 @@ impl FunctionCall {
     pub fn new(
         function_expression: FunctionExpression,
         arguments: Vec<Expression>,
-        syntax_position: SyntaxPosition,
+        syntax_position: SourcePosition,
     ) -> Self {
         Self {
             function_expression,
@@ -85,7 +85,7 @@ impl AbstractTree for FunctionCall {
         }
     }
 
-    fn check_type(&self, source: &str, context: &Map) -> Result<(), ValidationError> {
+    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 {
@@ -94,26 +94,32 @@ impl AbstractTree for FunctionCall {
             } => parameter_types,
             Type::Any => return Ok(()),
             _ => {
-                return Err(Error::TypeCheckExpectedFunction {
+                return Err(ValidationError::TypeCheckExpectedFunction {
                     actual: function_expression_type,
-                }
-                .at_source_position(source, self.syntax_position))
+                    position: self.syntax_position,
+                });
             }
         };
 
         if self.arguments.len() != parameter_types.len() {
-            return Err(Error::ExpectedFunctionArgumentAmount {
+            return Err(ValidationError::ExpectedFunctionArgumentAmount {
                 expected: parameter_types.len(),
                 actual: self.arguments.len(),
-            }
-            .at_source_position(source, self.syntax_position));
+                position: self.syntax_position,
+            });
         }
 
         for (index, expression) in self.arguments.iter().enumerate() {
-            if let Some(r#type) = parameter_types.get(index) {
-                r#type
-                    .check(&expression.expected_type(context)?)
-                    .map_err(|error| error.at_source_position(source, self.syntax_position))?;
+            if let Some(expected) = parameter_types.get(index) {
+                let actual = expression.expected_type(context)?;
+
+                if !expected.accepts(&actual) {
+                    return Err(ValidationError::TypeCheck {
+                        expected: expected.clone(),
+                        actual,
+                        position: self.syntax_position,
+                    });
+                }
             }
         }
 
@@ -129,7 +135,7 @@ impl AbstractTree for FunctionCall {
                 if let Some((value, _)) = variables.get(key) {
                     value.clone()
                 } else {
-                    return Err(Error::VariableIdentifierNotFound(
+                    return Err(RuntimeError::VariableIdentifierNotFound(
                         identifier.inner().clone(),
                     ));
                 }
diff --git a/src/abstract_tree/function_expression.rs b/src/abstract_tree/function_expression.rs
index 5590cf8..c957751 100644
--- a/src/abstract_tree/function_expression.rs
+++ b/src/abstract_tree/function_expression.rs
@@ -40,7 +40,7 @@ impl AbstractTree for FunctionExpression {
                 FunctionExpression::Yield(Box::new(Yield::from_syntax(child, source, context)?))
             }
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected: "identifier, function call, value, index or yield".to_string(),
                     actual: child.kind().to_string(),
                     location: child.start_position(),
diff --git a/src/abstract_tree/function_node.rs b/src/abstract_tree/function_node.rs
index 367a525..66d8d92 100644
--- a/src/abstract_tree/function_node.rs
+++ b/src/abstract_tree/function_node.rs
@@ -4,7 +4,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, Block, Error, Format, Function, Identifier, Map, SyntaxNode, SyntaxPosition,
+    AbstractTree, Block, Error, Format, Function, Identifier, Map, SourcePosition, SyntaxNode,
     Type, TypeSpecification, Value,
 };
 
@@ -13,7 +13,7 @@ pub struct FunctionNode {
     parameters: Vec<Identifier>,
     body: Block,
     r#type: Type,
-    syntax_position: SyntaxPosition,
+    syntax_position: SourcePosition,
 }
 
 impl FunctionNode {
@@ -21,7 +21,7 @@ impl FunctionNode {
         parameters: Vec<Identifier>,
         body: Block,
         r#type: Type,
-        syntax_position: SyntaxPosition,
+        syntax_position: SourcePosition,
     ) -> Self {
         FunctionNode {
             parameters,
@@ -43,7 +43,7 @@ impl FunctionNode {
         &self.r#type
     }
 
-    pub fn syntax_position(&self) -> &SyntaxPosition {
+    pub fn syntax_position(&self) -> &SourcePosition {
         &self.syntax_position
     }
 
@@ -164,15 +164,23 @@ impl AbstractTree for FunctionNode {
                 function_context.set_type(parameter.inner().clone(), parameter_type.clone())?;
             }
 
-            return_type
-                .check(&self.body.expected_type(&function_context)?)
-                .map_err(|error| error.at_source_position(source, self.syntax_position))?;
+            let actual = self.body.expected_type(&function_context)?;
+
+            if !return_type.accepts(&actual) {
+                return Err(ValidationError::TypeCheck {
+                    expected: return_type.as_ref().clone(),
+                    actual,
+                    position: self.syntax_position,
+                });
+            }
+
             self.body.check_type(source, &function_context)?;
 
             Ok(())
         } else {
-            Err(Error::TypeCheckExpectedFunction {
+            Err(ValidationError::TypeCheckExpectedFunction {
                 actual: self.r#type.clone(),
+                position: self.syntax_position,
             })
         }
     }
diff --git a/src/abstract_tree/identifier.rs b/src/abstract_tree/identifier.rs
index 3861c82..229606e 100644
--- a/src/abstract_tree/identifier.rs
+++ b/src/abstract_tree/identifier.rs
@@ -47,7 +47,7 @@ impl AbstractTree for Identifier {
         if let Some((_value, r#type)) = context.variables()?.get(&self.0) {
             Ok(r#type.clone())
         } else {
-            Err(Error::VariableIdentifierNotFound(self.0.clone()))
+            Err(ValidationError::VariableIdentifierNotFound(self.clone()))
         }
     }
 
@@ -55,7 +55,7 @@ impl AbstractTree for Identifier {
         if let Some((value, _)) = context.variables()?.get(&self.0) {
             Ok(value.clone())
         } else {
-            Err(Error::VariableIdentifierNotFound(self.0.clone()))
+            Err(RuntimeError::VariableIdentifierNotFound(self.0.clone()))
         }
     }
 }
diff --git a/src/abstract_tree/if_else.rs b/src/abstract_tree/if_else.rs
index 29bb860..4c991f3 100644
--- a/src/abstract_tree/if_else.rs
+++ b/src/abstract_tree/if_else.rs
@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, Block, Expression, Format, Map, SyntaxNode, Type, Value,
+    AbstractTree, Block, Expression, Format, Map, SourcePosition, SyntaxNode, Type, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -12,6 +12,7 @@ pub struct IfElse {
     else_if_expressions: Vec<Expression>,
     else_if_blocks: Vec<Block>,
     else_block: Option<Block>,
+    source_position: SourcePosition,
 }
 
 impl AbstractTree for IfElse {
@@ -54,6 +55,7 @@ impl AbstractTree for IfElse {
             else_if_expressions,
             else_if_blocks,
             else_block,
+            source_position: SourcePosition::from(node.range()),
         })
     }
 
@@ -65,7 +67,7 @@ impl AbstractTree for IfElse {
         self.if_expression.check_type(_source, context)?;
         self.if_block.check_type(_source, context)?;
 
-        let expected_type = self.if_block.expected_type(context)?;
+        let expected = self.if_block.expected_type(context)?;
         let else_ifs = self
             .else_if_expressions
             .iter()
@@ -75,11 +77,27 @@ impl AbstractTree for IfElse {
             expression.check_type(_source, context)?;
             block.check_type(_source, context)?;
 
-            expected_type.check(&block.expected_type(context)?)?;
+            let actual = block.expected_type(context)?;
+
+            if !expected.accepts(&actual) {
+                return Err(ValidationError::TypeCheck {
+                    expected,
+                    actual,
+                    position: self.source_position,
+                });
+            }
         }
 
-        if let Some(expression) = self.else_block {
-            expected_type.check(&expression.expected_type(context)?)?;
+        if let Some(block) = &self.else_block {
+            let actual = block.expected_type(context)?;
+
+            if !expected.accepts(&actual) {
+                return Err(ValidationError::TypeCheck {
+                    expected,
+                    actual,
+                    position: self.source_position,
+                });
+            }
         }
 
         Ok(())
diff --git a/src/abstract_tree/index.rs b/src/abstract_tree/index.rs
index f71186c..cab03d2 100644
--- a/src/abstract_tree/index.rs
+++ b/src/abstract_tree/index.rs
@@ -2,7 +2,8 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, Error, Format, IndexExpression, List, Map, SyntaxNode, Type, Value,
+    AbstractTree, Error, Format, IndexExpression, List, Map, SourcePosition, SyntaxNode, Type,
+    Value,
 };
 
 /// Abstract representation of an index expression.
@@ -13,6 +14,7 @@ pub struct Index {
     pub collection: IndexExpression,
     pub index: IndexExpression,
     pub index_end: Option<IndexExpression>,
+    source_position: SourcePosition,
 }
 
 impl AbstractTree for Index {
@@ -40,6 +42,7 @@ impl AbstractTree for Index {
             collection,
             index,
             index_end,
+            source_position: SourcePosition::from(node.range()),
         })
     }
 
@@ -56,7 +59,7 @@ impl AbstractTree for Index {
         self.collection.check_type(_source, _context)?;
         self.index.check_type(_source, _context)?;
 
-        if let Some(index_end) = self.index_end {
+        if let Some(index_end) = &self.index_end {
             index_end.check_type(_source, _context)?;
         }
 
@@ -64,9 +67,9 @@ impl AbstractTree for Index {
     }
 
     fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError> {
-        let collection = self.collection.run(source, context)?;
+        let value = self.collection.run(source, context)?;
 
-        match collection {
+        match value {
             Value::List(list) => {
                 let index = self.index.run(source, context)?.as_integer()? as usize;
 
@@ -104,7 +107,7 @@ impl AbstractTree for Index {
                 };
 
                 if value.is_none() {
-                    Err(Error::VariableIdentifierNotFound(key))
+                    Err(RuntimeError::VariableIdentifierNotFound(key))
                 } else {
                     Ok(value)
                 }
@@ -115,7 +118,7 @@ impl AbstractTree for Index {
 
                 Ok(Value::string(item.to_string()))
             }
-            _ => Err(Error::ExpectedCollection { actual: collection }),
+            _ => Err(RuntimeError::ExpectedCollection { actual: value }),
         }
     }
 }
diff --git a/src/abstract_tree/index_assignment.rs b/src/abstract_tree/index_assignment.rs
index defcb18..b538806 100644
--- a/src/abstract_tree/index_assignment.rs
+++ b/src/abstract_tree/index_assignment.rs
@@ -48,7 +48,7 @@ impl AbstractTree for IndexAssignment {
         let index_key = if let IndexExpression::Identifier(identifier) = &self.index.index {
             identifier.inner()
         } else {
-            return Err(Error::VariableIdentifierNotFound(
+            return Err(RuntimeError::VariableIdentifierNotFound(
                 self.index.index.run(source, context)?.to_string(),
             ));
         };
diff --git a/src/abstract_tree/index_expression.rs b/src/abstract_tree/index_expression.rs
index bc988bb..11630e0 100644
--- a/src/abstract_tree/index_expression.rs
+++ b/src/abstract_tree/index_expression.rs
@@ -37,7 +37,7 @@ impl AbstractTree for IndexExpression {
                 child, source, context,
             )?)),
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected: "value, identifier, index or function call".to_string(),
                     actual: child.kind().to_string(),
                     location: child.start_position(),
diff --git a/src/abstract_tree/logic_operator.rs b/src/abstract_tree/logic_operator.rs
index c31fb26..58971e3 100644
--- a/src/abstract_tree/logic_operator.rs
+++ b/src/abstract_tree/logic_operator.rs
@@ -32,7 +32,7 @@ impl AbstractTree for LogicOperator {
             ">=" => LogicOperator::GreaterOrEqual,
             "<=" => LogicOperator::LessOrEqual,
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected: "==, !=, &&, ||, >, <, >= or <=".to_string(),
                     actual: operator_node.kind().to_string(),
                     location: operator_node.start_position(),
diff --git a/src/abstract_tree/match.rs b/src/abstract_tree/match.rs
index 0ad5767..8c1d3e4 100644
--- a/src/abstract_tree/match.rs
+++ b/src/abstract_tree/match.rs
@@ -68,12 +68,12 @@ impl AbstractTree for Match {
     fn check_type(&self, _source: &str, _context: &Map) -> Result<(), ValidationError> {
         self.matcher.check_type(_source, _context)?;
 
-        for (expression, statement) in self.options {
+        for (expression, statement) in &self.options {
             expression.check_type(_source, _context)?;
             statement.check_type(_source, _context)?;
         }
 
-        if let Some(statement) = self.fallback {
+        if let Some(statement) = &self.fallback {
             statement.check_type(_source, _context)?;
         }
 
diff --git a/src/abstract_tree/math_operator.rs b/src/abstract_tree/math_operator.rs
index 497f7d0..3a65036 100644
--- a/src/abstract_tree/math_operator.rs
+++ b/src/abstract_tree/math_operator.rs
@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, Error, Format, Map, SyntaxNode, Type, Value,
+    AbstractTree, Format, Map, SyntaxNode, Type, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -24,7 +24,7 @@ impl AbstractTree for MathOperator {
             "/" => MathOperator::Divide,
             "%" => MathOperator::Modulo,
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected: "+, -, *, / or %".to_string(),
                     actual: operator_node.kind().to_string(),
                     location: operator_node.start_position(),
diff --git a/src/abstract_tree/mod.rs b/src/abstract_tree/mod.rs
index 95f46b0..42cef9d 100644
--- a/src/abstract_tree/mod.rs
+++ b/src/abstract_tree/mod.rs
@@ -50,7 +50,7 @@ use crate::{
 };
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
-pub struct SyntaxPosition {
+pub struct SourcePosition {
     pub start_byte: usize,
     pub end_byte: usize,
     pub start_row: usize,
@@ -59,9 +59,9 @@ pub struct SyntaxPosition {
     pub end_column: usize,
 }
 
-impl From<tree_sitter::Range> for SyntaxPosition {
+impl From<tree_sitter::Range> for SourcePosition {
     fn from(range: tree_sitter::Range) -> Self {
-        SyntaxPosition {
+        SourcePosition {
             start_byte: range.start_byte,
             end_byte: range.end_byte,
             start_row: range.start_point.row + 1,
@@ -143,7 +143,8 @@ impl Format for Root {
 /// This trait is implemented by the Evaluator's internal types to form an
 /// executable tree that resolves to a single value.
 pub trait AbstractTree: Sized + Format {
-    /// Interpret the syntax tree at the given node and return the abstraction.
+    /// Interpret the syntax tree at the given node and return the abstraction. Returns a syntax
+    /// error if the source is invalid.
     ///
     /// This function is used to convert nodes in the Tree Sitter concrete
     /// syntax tree into executable nodes in an abstract tree. This function is
@@ -154,13 +155,16 @@ pub trait AbstractTree: Sized + Format {
     /// node's byte range.
     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<(), ValidationError>;
-
-    /// Execute dust code by traversing the tree.
-    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError>;
-
+    /// Return the type of the value that this abstract node will create when run. Returns a
+    /// validation error if the tree is invalid.
     fn expected_type(&self, context: &Map) -> Result<Type, ValidationError>;
+
+    /// Verify the type integrity of the node. Returns a validation error if the tree is invalid.
+    fn check_type(&self, source: &str, context: &Map) -> Result<(), ValidationError>;
+
+    /// Execute this node's logic and return a value. Returns a runtime error if the node cannot
+    /// resolve to a value.
+    fn run(&self, source: &str, context: &Map) -> Result<Value, RuntimeError>;
 }
 
 pub trait Format {
diff --git a/src/abstract_tree/statement.rs b/src/abstract_tree/statement.rs
index db1cc23..8c326f9 100644
--- a/src/abstract_tree/statement.rs
+++ b/src/abstract_tree/statement.rs
@@ -56,7 +56,7 @@ impl AbstractTree for Statement {
 
                 Ok(Statement::Return(Box::new(Statement::from_syntax(statement_node, source, context)?)))
             },
-            _ => Err(Error::UnexpectedSyntaxNode {
+            _ => Err(SyntaxError::UnexpectedSyntaxNode {
                 expected:
                     "assignment, index assignment, expression, block, return, if...else, while, for or match".to_string(),
                 actual: child.kind().to_string(),
diff --git a/src/abstract_tree/type.rs b/src/abstract_tree/type.rs
index 7aa8b2a..3e5ca93 100644
--- a/src/abstract_tree/type.rs
+++ b/src/abstract_tree/type.rs
@@ -44,7 +44,11 @@ impl Type {
         Type::Option(Box::new(optional_type))
     }
 
-    pub fn check(&self, other: &Type) -> Result<(), ValidationError> {
+    /// Returns a boolean indicating whether is type is accepting of the other.
+    ///
+    /// The types do not need to match exactly. For example, the Any variant matches all of the
+    /// others and the Number variant accepts Number, Integer and Float.
+    pub fn accepts(&self, other: &Type) -> bool {
         log::info!("Checking type {self} against {other}.");
 
         match (self, other) {
@@ -67,41 +71,20 @@ impl Type {
             | (Type::Integer, Type::Number)
             | (Type::Float, Type::Number)
             | (Type::None, Type::None)
-            | (Type::String, Type::String) => Ok(()),
-            (Type::Custom(left), Type::Custom(right)) => {
-                if left == right {
-                    Ok(())
-                } else {
-                    Err(Error::TypeCheck {
-                        expected: self.clone(),
-                        actual: other.clone(),
-                    })
-                }
-            }
+            | (Type::String, Type::String) => true,
+            (Type::Custom(left), Type::Custom(right)) => left == right,
+            (Type::Option(_), Type::None) => true,
             (Type::Option(left), Type::Option(right)) => {
-                if left == right {
-                    Ok(())
-                } else if let Type::Any = left.as_ref() {
-                    Ok(())
-                } else if let Type::Any = right.as_ref() {
-                    Ok(())
+                if let Type::Any = left.as_ref() {
+                    true
+                } else if left == right {
+                    true
                 } else {
-                    Err(Error::TypeCheck {
-                        expected: self.clone(),
-                        actual: other.clone(),
-                    })
+                    false
                 }
             }
-            (Type::Option(_), Type::None) | (Type::None, Type::Option(_)) => Ok(()),
             (Type::List(self_item_type), Type::List(other_item_type)) => {
-                if self_item_type.check(other_item_type).is_err() {
-                    Err(Error::TypeCheck {
-                        expected: self.clone(),
-                        actual: other.clone(),
-                    })
-                } else {
-                    Ok(())
-                }
+                self_item_type.accepts(&other_item_type)
             }
             (
                 Type::Function {
@@ -118,27 +101,14 @@ impl Type {
                     .zip(other_parameter_types.iter());
 
                 for (self_parameter_type, other_parameter_type) in parameter_type_pairs {
-                    if self_parameter_type.check(other_parameter_type).is_err() {
-                        return Err(Error::TypeCheck {
-                            expected: self.clone(),
-                            actual: other.clone(),
-                        });
+                    if self_parameter_type == other_parameter_type {
+                        return false;
                     }
                 }
 
-                if self_return_type.check(other_return_type).is_err() {
-                    Err(Error::TypeCheck {
-                        expected: self.clone(),
-                        actual: other.clone(),
-                    })
-                } else {
-                    Ok(())
-                }
+                self_return_type == other_return_type
             }
-            _ => Err(Error::TypeCheck {
-                expected: self.clone(),
-                actual: other.clone(),
-            }),
+            _ => false,
         }
     }
 
@@ -210,7 +180,7 @@ impl AbstractTree for Type {
                 Type::Option(Box::new(inner_type))
             }
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected: "any, bool, float, int, num, str, option, (, [ or {".to_string(),
                     actual: type_node.kind().to_string(),
                     location: type_node.start_position(),
diff --git a/src/abstract_tree/value_node.rs b/src/abstract_tree/value_node.rs
index 1c1e912..e2fe159 100644
--- a/src/abstract_tree/value_node.rs
+++ b/src/abstract_tree/value_node.rs
@@ -79,10 +79,6 @@ impl AbstractTree for ValueNode {
                     if child.kind() == "statement" {
                         let statement = Statement::from_syntax(child, source, context)?;
 
-                        if let Some(type_specification) = &current_type {
-                            type_specification.check(&statement.expected_type(context)?)?;
-                        }
-
                         child_nodes.insert(current_key.clone(), (statement, current_type.clone()));
                     }
                 }
@@ -145,20 +141,20 @@ impl AbstractTree for ValueNode {
                         current_statement =
                             Some(Statement::from_syntax(child_syntax_node, source, context)?);
 
-                        if let Some(identifier) = &current_identifier {
-                            let r#type = if let Some(r#type) = &current_type {
-                                r#type.clone()
-                            } else if let Some(statement) = &current_statement {
-                                statement.expected_type(context)?
-                            } else {
-                                Type::None
-                            };
+                        // if let Some(identifier) = &current_identifier {
+                        //     let r#type = if let Some(r#type) = &current_type {
+                        //         r#type.clone()
+                        //     } else if let Some(statement) = &current_statement {
+                        //         statement.expected_type(context)?
+                        //     } else {
+                        //         Type::None
+                        //     };
 
-                            btree_map.insert(
-                                identifier.inner().clone(),
-                                (current_statement.clone(), r#type.clone()),
-                            );
-                        }
+                        //     btree_map.insert(
+                        //         identifier.inner().clone(),
+                        //         (current_statement.clone(), r#type.clone()),
+                        //     );
+                        // }
                     }
                 }
 
@@ -172,7 +168,7 @@ impl AbstractTree for ValueNode {
                 ValueNode::Range(start..=end)
             }
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected:
                         "string, integer, float, boolean, range, list, map, option, function or structure"
                             .to_string(),
diff --git a/src/built_in_functions/fs.rs b/src/built_in_functions/fs.rs
index ddbb8ac..190d7a2 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::RuntimeError, Error, Map, Type, Value};
+use crate::{error::RuntimeError, Map, Type, Value};
 
 use super::Callable;
 
@@ -43,7 +43,7 @@ impl Callable for Fs {
     ) -> Result<Value, RuntimeError> {
         match self {
             Fs::ReadFile => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let path = arguments.first().unwrap().as_string()?;
                 let file_content = read_to_string(path.as_str())?;
diff --git a/src/built_in_functions/json.rs b/src/built_in_functions/json.rs
index ebe1c6a..c0238d3 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::RuntimeError, Error, Map, Type, Value};
+use crate::{error::RuntimeError, Map, Type, Value};
 
 use super::Callable;
 
@@ -49,7 +49,7 @@ impl Callable for Json {
     ) -> Result<Value, RuntimeError> {
         match self {
             Json::Create => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let value = arguments.first().unwrap();
                 let json_string = serde_json::to_string(value)?;
@@ -57,7 +57,7 @@ impl Callable for Json {
                 Ok(Value::String(json_string))
             }
             Json::CreatePretty => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let value = arguments.first().unwrap();
                 let json_string = serde_json::to_string_pretty(value)?;
@@ -65,7 +65,7 @@ impl Callable for Json {
                 Ok(Value::String(json_string))
             }
             Json::Parse => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let json_string = arguments.first().unwrap().as_string()?;
                 let value = serde_json::from_str(json_string)?;
diff --git a/src/built_in_functions/mod.rs b/src/built_in_functions/mod.rs
index ef17410..033bc0f 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::RuntimeError, Error, Format, Map, Type, Value};
+use crate::{error::RuntimeError, Format, Map, Type, Value};
 
 use self::{fs::Fs, json::Json, str::StrFunction};
 
@@ -91,7 +91,7 @@ impl Callable for BuiltInFunction {
     ) -> Result<Value, RuntimeError> {
         match self {
             BuiltInFunction::AssertEqual => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let left = arguments.first().unwrap();
                 let right = arguments.get(1).unwrap();
@@ -105,7 +105,7 @@ impl Callable for BuiltInFunction {
                 json_function.call(arguments, _source, _outer_context)
             }
             BuiltInFunction::Length => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let value = arguments.first().unwrap();
                 let length = if let Ok(list) = value.as_list() {
@@ -115,7 +115,7 @@ impl Callable for BuiltInFunction {
                 } else if let Ok(str) = value.as_string() {
                     str.chars().count()
                 } else {
-                    return Err(Error::ExpectedCollection {
+                    return Err(RuntimeError::ExpectedCollection {
                         actual: value.clone(),
                     });
                 };
@@ -123,7 +123,7 @@ impl Callable for BuiltInFunction {
                 Ok(Value::Integer(length as i64))
             }
             BuiltInFunction::Output => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let value = arguments.first().unwrap();
 
@@ -132,17 +132,17 @@ impl Callable for BuiltInFunction {
                 Ok(Value::none())
             }
             BuiltInFunction::RandomBoolean => {
-                Error::expect_argument_amount(self.name(), 0, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 0, arguments.len())?;
 
                 Ok(Value::Boolean(random()))
             }
             BuiltInFunction::RandomFloat => {
-                Error::expect_argument_amount(self.name(), 0, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 0, arguments.len())?;
 
                 Ok(Value::Float(random()))
             }
             BuiltInFunction::RandomFrom => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let value = arguments.first().unwrap();
 
@@ -162,7 +162,7 @@ impl Callable for BuiltInFunction {
                 }
             }
             BuiltInFunction::RandomInteger => {
-                Error::expect_argument_amount(self.name(), 0, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 0, arguments.len())?;
 
                 Ok(Value::Integer(random()))
             }
diff --git a/src/built_in_functions/str.rs b/src/built_in_functions/str.rs
index 29f03cb..86a8eb8 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::RuntimeError, Error, List, Map, Type, Value};
+use crate::{error::RuntimeError, List, Map, Type, Value};
 
 use super::Callable;
 
@@ -206,7 +206,7 @@ impl Callable for StrFunction {
     ) -> Result<Value, RuntimeError> {
         let value = match self {
             StrFunction::AsBytes => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let bytes = string
@@ -217,7 +217,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(bytes))
             }
             StrFunction::EndsWith => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -226,7 +226,7 @@ impl Callable for StrFunction {
                 Value::Boolean(string.ends_with(pattern))
             }
             StrFunction::Find => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -238,21 +238,21 @@ impl Callable for StrFunction {
                 Value::Option(find)
             }
             StrFunction::IsAscii => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
 
                 Value::Boolean(string.is_ascii())
             }
             StrFunction::IsEmpty => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
 
                 Value::Boolean(string.is_empty())
             }
             StrFunction::Insert => {
-                Error::expect_argument_amount(self.name(), 3, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 3, arguments.len())?;
 
                 let mut string = arguments.first().unwrap().as_string()?.clone();
                 let index = arguments.get(1).unwrap().as_integer()? as usize;
@@ -263,7 +263,7 @@ impl Callable for StrFunction {
                 Value::String(string)
             }
             StrFunction::Lines => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let lines = string
@@ -274,7 +274,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(lines))
             }
             StrFunction::Matches => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -287,7 +287,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(matches))
             }
             StrFunction::Parse => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
 
@@ -300,7 +300,7 @@ impl Callable for StrFunction {
                 }
             }
             StrFunction::Remove => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let index = arguments.get(1).unwrap().as_integer()? as usize;
@@ -318,7 +318,7 @@ impl Callable for StrFunction {
                 }
             }
             StrFunction::ReplaceRange => {
-                Error::expect_argument_amount(self.name(), 3, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 3, arguments.len())?;
 
                 let mut string = arguments.first().unwrap().as_string()?.clone();
                 let range = arguments.get(1).unwrap().as_list()?.items();
@@ -331,7 +331,7 @@ impl Callable for StrFunction {
                 Value::String(string)
             }
             StrFunction::Retain => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let mut string = arguments.first().unwrap().as_string()?.clone();
                 let predicate = arguments.get(1).unwrap().as_function()?;
@@ -347,7 +347,7 @@ impl Callable for StrFunction {
                 Value::String(string)
             }
             StrFunction::Split => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -360,7 +360,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(sections))
             }
             StrFunction::SplitAt => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let index = arguments.get(1).unwrap().as_integer()?;
@@ -372,7 +372,7 @@ impl Callable for StrFunction {
                 ]))
             }
             StrFunction::SplitInclusive => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -385,7 +385,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(sections))
             }
             StrFunction::SplitN => {
-                Error::expect_argument_amount(self.name(), 3, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 3, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let count = arguments.get(1).unwrap().as_integer()?;
@@ -399,7 +399,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(sections))
             }
             StrFunction::SplitOnce => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -414,7 +414,7 @@ impl Callable for StrFunction {
                 Value::option(sections)
             }
             StrFunction::SplitTerminator => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -427,7 +427,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(sections))
             }
             StrFunction::SplitWhitespace => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let sections = string
@@ -438,7 +438,7 @@ impl Callable for StrFunction {
                 Value::List(List::with_items(sections))
             }
             StrFunction::StartsWith => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -447,7 +447,7 @@ impl Callable for StrFunction {
                 Value::Boolean(string.starts_with(pattern))
             }
             StrFunction::StripPrefix => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let prefix_string = arguments.get(1).unwrap().as_string()?;
@@ -459,7 +459,7 @@ impl Callable for StrFunction {
                 Value::option(stripped)
             }
             StrFunction::ToLowercase => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let lowercase = string.to_lowercase();
@@ -467,7 +467,7 @@ impl Callable for StrFunction {
                 Value::string(lowercase)
             }
             StrFunction::ToUppercase => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let uppercase = string.to_uppercase();
@@ -475,14 +475,14 @@ impl Callable for StrFunction {
                 Value::string(uppercase)
             }
             StrFunction::Trim => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let trimmed = arguments.first().unwrap().as_string()?.trim().to_string();
 
                 Value::string(trimmed)
             }
             StrFunction::TrimEnd => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let trimmed = arguments
                     .first()
@@ -494,7 +494,7 @@ impl Callable for StrFunction {
                 Value::string(trimmed)
             }
             StrFunction::TrimEndMatches => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern_string = arguments.get(1).unwrap().as_string()?;
@@ -504,7 +504,7 @@ impl Callable for StrFunction {
                 Value::string(trimmed)
             }
             StrFunction::TrimMatches => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern = arguments
@@ -518,7 +518,7 @@ impl Callable for StrFunction {
                 Value::string(trimmed)
             }
             StrFunction::TrimStart => {
-                Error::expect_argument_amount(self.name(), 1, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
 
                 let trimmed = arguments
                     .first()
@@ -530,7 +530,7 @@ impl Callable for StrFunction {
                 Value::string(trimmed)
             }
             StrFunction::TrimStartMatches => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let string = arguments.first().unwrap().as_string()?;
                 let pattern = arguments
@@ -544,7 +544,7 @@ impl Callable for StrFunction {
                 Value::string(trimmed)
             }
             StrFunction::Truncate => {
-                Error::expect_argument_amount(self.name(), 2, arguments.len())?;
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
 
                 let input_string = arguments.first().unwrap().as_string()?;
                 let new_length = arguments.get(1).unwrap().as_integer()? as usize;
diff --git a/src/error/mod.rs b/src/error/mod.rs
index 29925ec..369bacc 100644
--- a/src/error/mod.rs
+++ b/src/error/mod.rs
@@ -3,6 +3,7 @@
 //! 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;
+pub(crate) mod rw_lock_error;
 mod syntax_error;
 mod validation_error;
 
@@ -10,77 +11,25 @@ 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 crate::{SourcePosition};
 
-use std::{
-    fmt::{self, Formatter},
-    io,
-    num::ParseFloatError,
-    string::FromUtf8Error,
-    sync::PoisonError,
-    time,
-};
+use std::fmt::{self, Formatter};
 
-#[derive(Clone, PartialEq, Serialize, Deserialize)]
 pub enum Error {
-    Parsing(SyntaxError),
+    Syntax(SyntaxError),
 
-    Verification(ValidationError),
+    Validation(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,
-    },
+    Language(LanguageError),
 }
 
 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,
@@ -91,103 +40,42 @@ impl Error {
         if expected == actual.kind() {
             Ok(())
         } else if actual.is_error() {
-            Error::Syntax {
+            Err(SyntaxError::InvalidSource {
                 source: source[actual.byte_range()].to_string(),
-                location: actual.start_position(),
-            }
+                position: SourcePosition::from(actual.range()),
+            })
         } else {
-            SyntaxError::UnexpectedSyntaxNode {
+            Err(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<SyntaxError> for Error {
+    fn from(error: SyntaxError) -> Self {
+        Error::Syntax(error)
+    }
+}
+
+impl From<ValidationError> for Error {
+    fn from(error: ValidationError) -> Self {
+        Error::Validation(error)
+    }
+}
+
+impl From<RuntimeError> for Error {
+    fn from(error: RuntimeError) -> Self {
+        Error::Runtime(error)
     }
 }
 
 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())
+        Error::Language(error)
     }
 }
 
@@ -200,44 +88,15 @@ impl fmt::Debug for Error {
 }
 
 impl fmt::Display for Error {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    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!(),
+            Syntax(_) => todo!(),
+            Validation(_) => todo!(),
             Runtime(_) => todo!(),
-            External(_) => todo!(),
-            CustomMessage(_) => todo!(),
-            Syntax { source, location } => todo!(),
+            ParserCancelled => todo!(),
+            Language(_) => todo!(),
         }
     }
 }
diff --git a/src/error/runtime_error.rs b/src/error/runtime_error.rs
index 1363700..9e9e3cc 100644
--- a/src/error/runtime_error.rs
+++ b/src/error/runtime_error.rs
@@ -1,11 +1,28 @@
-use std::fmt::{self, Display, Formatter};
+use std::{
+    fmt::{self, Debug, Display, Formatter},
+    io,
+    num::ParseFloatError,
+    string::FromUtf8Error,
+    time::{self, SystemTimeError},
+};
 
-use serde::{Deserialize, Serialize};
+use crate::{Value};
 
-use crate::Value;
+use super::rw_lock_error::RwLockError;
 
-#[derive(Clone, PartialEq, Serialize, Deserialize)]
 pub enum RuntimeError {
+    Csv(csv::Error),
+
+    Io(io::Error),
+
+    Reqwest(reqwest::Error),
+
+    Json(serde_json::Error),
+
+    SystemTime(SystemTimeError),
+
+    Toml(toml::de::Error),
+
     ExpectedString {
         actual: Value,
     },
@@ -71,111 +88,145 @@ pub enum RuntimeError {
         actual: Value,
     },
 
-    /// A function was called with the wrong amount of arguments.
+    /// Failed to read or write a map.
+    ///
+    /// See the [MapError] docs for more info.
+    RwLock(RwLockError),
+
+    ParseFloat(ParseFloatError),
+
+    Utf8(FromUtf8Error),
+
+    /// Failed to find a variable with a value for this key.
+    VariableIdentifierNotFound(String),
+
+    /// A built-in 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,
+impl RuntimeError {
+    pub fn expect_argument_amount(
+        function_name: &str,
+        expected: usize,
+        actual: usize,
+    ) -> Result<(), Self> {
+        if expected == actual {
+            Ok(())
+        } else {
+            Err(RuntimeError::ExpectedBuiltInFunctionArgumentAmount {
+                function_name: function_name.to_string(),
                 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!(),
+            })
         }
     }
 }
+
+impl From<csv::Error> for RuntimeError {
+    fn from(error: csv::Error) -> Self {
+        RuntimeError::Csv(error)
+    }
+}
+
+impl From<io::Error> for RuntimeError {
+    fn from(error: std::io::Error) -> Self {
+        RuntimeError::Io(error)
+    }
+}
+
+impl From<reqwest::Error> for RuntimeError {
+    fn from(error: reqwest::Error) -> Self {
+        RuntimeError::Reqwest(error)
+    }
+}
+
+impl From<serde_json::Error> for RuntimeError {
+    fn from(error: serde_json::Error) -> Self {
+        RuntimeError::Json(error)
+    }
+}
+
+impl From<time::SystemTimeError> for RuntimeError {
+    fn from(error: time::SystemTimeError) -> Self {
+        RuntimeError::SystemTime(error)
+    }
+}
+
+impl From<toml::de::Error> for RuntimeError {
+    fn from(error: toml::de::Error) -> Self {
+        RuntimeError::Toml(error)
+    }
+}
+
+impl From<ParseFloatError> for RuntimeError {
+    fn from(error: ParseFloatError) -> Self {
+        RuntimeError::ParseFloat(error)
+    }
+}
+
+impl From<FromUtf8Error> for RuntimeError {
+    fn from(error: FromUtf8Error) -> Self {
+        RuntimeError::Utf8(error)
+    }
+}
+
+impl Display for RuntimeError {
+    fn fmt(&self, _f: &mut Formatter<'_>) -> fmt::Result {
+        use RuntimeError::*;
+
+        match self {
+            VariableIdentifierNotFound(_) => todo!(),
+            RwLock(_) => todo!(),
+            Csv(_) => todo!(),
+            Io(_) => todo!(),
+            Reqwest(_) => todo!(),
+            Json(_) => todo!(),
+            SystemTime(_) => todo!(),
+            Toml(_) => todo!(),
+            Utf8(_) => todo!(),
+            ParseFloat(_) => todo!(),
+            ExpectedBuiltInFunctionArgumentAmount {
+                function_name: _,
+                expected: _,
+                actual: _,
+            } => todo!(),
+            ExpectedString { actual: _ } => todo!(),
+            ExpectedInteger { actual: _ } => todo!(),
+            ExpectedFloat { actual: _ } => todo!(),
+            ExpectedNumber { actual: _ } => todo!(),
+            ExpectedNumberOrString { actual: _ } => todo!(),
+            ExpectedBoolean { actual: _ } => todo!(),
+            ExpectedList { actual: _ } => todo!(),
+            ExpectedMinLengthList {
+                minimum_len: _,
+                actual_len: _,
+            } => todo!(),
+            ExpectedFixedLenList {
+                expected_len: _,
+                actual: _,
+            } => todo!(),
+            ExpectedNone { actual: _ } => todo!(),
+            ExpectedMap { actual: _ } => todo!(),
+            ExpectedTable { actual: _ } => todo!(),
+            ExpectedFunction { actual: _ } => todo!(),
+            ExpectedOption { actual: _ } => todo!(),
+            ExpectedCollection { actual: _ } => todo!(),
+        }
+    }
+}
+
+impl Debug for RuntimeError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{self}")
+    }
+}
+
+impl From<RwLockError> for RuntimeError {
+    fn from(error: RwLockError) -> Self {
+        RuntimeError::RwLock(error)
+    }
+}
diff --git a/src/error/rw_lock_error.rs b/src/error/rw_lock_error.rs
new file mode 100644
index 0000000..f0938c9
--- /dev/null
+++ b/src/error/rw_lock_error.rs
@@ -0,0 +1,21 @@
+use std::fmt::{self, Debug, Display, Formatter};
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, PartialEq, Serialize, Deserialize)]
+pub struct RwLockError;
+
+impl Display for RwLockError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "Map error: failed to acquire a read/write lock because another thread has panicked."
+        )
+    }
+}
+
+impl Debug for RwLockError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{self}")
+    }
+}
diff --git a/src/error/syntax_error.rs b/src/error/syntax_error.rs
index 35063f5..dfb35d6 100644
--- a/src/error/syntax_error.rs
+++ b/src/error/syntax_error.rs
@@ -1,8 +1,20 @@
 use serde::{Deserialize, Serialize};
 use tree_sitter::Point;
 
+use crate::SourcePosition;
+
+use super::rw_lock_error::RwLockError;
+
 #[derive(Clone, PartialEq, Serialize, Deserialize)]
 pub enum SyntaxError {
+    /// Invalid user input.
+    InvalidSource {
+        source: String,
+        position: SourcePosition,
+    },
+
+    RwLock(RwLockError),
+
     UnexpectedSyntaxNode {
         expected: String,
         actual: String,
@@ -13,3 +25,9 @@ pub enum SyntaxError {
         relevant_source: String,
     },
 }
+
+impl From<RwLockError> for SyntaxError {
+    fn from(error: RwLockError) -> Self {
+        SyntaxError::RwLock(error)
+    }
+}
diff --git a/src/error/validation_error.rs b/src/error/validation_error.rs
index 4b447b6..576fdc2 100644
--- a/src/error/validation_error.rs
+++ b/src/error/validation_error.rs
@@ -1,6 +1,8 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{Type, Value};
+use crate::{Identifier, SourcePosition, Type, Value};
+
+use super::rw_lock_error::RwLockError;
 
 #[derive(Clone, PartialEq, Serialize, Deserialize)]
 pub enum ValidationError {
@@ -8,20 +10,73 @@ pub enum ValidationError {
     AssertEqualFailed {
         expected: Value,
         actual: Value,
+        position: SourcePosition,
     },
 
     /// The 'assert' macro did not resolve successfully.
-    AssertFailed,
+    AssertFailed { position: SourcePosition },
+
+    /// A built-in 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,
+        position: SourcePosition,
+    },
+
+    /// A function was called with the wrong amount of arguments.
+    ExpectedFunctionArgumentMinimum {
+        minumum_expected: usize,
+        actual: usize,
+        position: SourcePosition,
+    },
+
+    /// Failed to read or write a map.
+    ///
+    /// See the [MapError] docs for more info.
+    RwLock(RwLockError),
 
     TypeCheck {
         expected: Type,
         actual: Type,
+        position: SourcePosition,
     },
 
     TypeCheckExpectedFunction {
         actual: Type,
+        position: SourcePosition,
     },
 
     /// Failed to find a variable with a value for this key.
-    VariableIdentifierNotFound(String),
+    VariableIdentifierNotFound(Identifier),
+}
+
+impl ValidationError {
+    pub fn expect_argument_amount(
+        function_name: &str,
+        expected: usize,
+        actual: usize,
+    ) -> Result<(), Self> {
+        if expected == actual {
+            Ok(())
+        } else {
+            Err(ValidationError::ExpectedBuiltInFunctionArgumentAmount {
+                function_name: function_name.to_string(),
+                expected,
+                actual,
+            })
+        }
+    }
+}
+
+impl From<RwLockError> for ValidationError {
+    fn from(_error: RwLockError) -> Self {
+        ValidationError::RwLock(RwLockError)
+    }
 }
diff --git a/src/interpret.rs b/src/interpret.rs
index 57ce972..4faf40e 100644
--- a/src/interpret.rs
+++ b/src/interpret.rs
@@ -38,8 +38,7 @@
 use tree_sitter::{Node as SyntaxNode, Parser, Tree as SyntaxTree, TreeCursor};
 
 use crate::{
-    error::{RuntimeError, SyntaxError, ValidationError},
-    language, AbstractTree, Error, Format, Map, Root, Value,
+    error::SyntaxError, language, AbstractTree, Error, Format, Map, Root, SourcePosition, Value,
 };
 
 /// Interpret the given source code. Returns the value of last statement or the
@@ -84,7 +83,7 @@ impl Interpreter {
             .set_language(language())
             .expect("Language version is incompatible with tree sitter version.");
 
-        parser.set_logger(Some(Box::new(|log_type, message| {
+        parser.set_logger(Some(Box::new(|_log_type, message| {
             log::info!("{}", message)
         })));
 
@@ -117,12 +116,12 @@ impl Interpreter {
             node: SyntaxNode,
             source: &str,
             cursor: &mut TreeCursor,
-        ) -> Result<(), ValidationError> {
+        ) -> Result<(), Error> {
             if node.is_error() {
-                Err(Error::Syntax {
+                Err(Error::Syntax(SyntaxError::InvalidSource {
                     source: source[node.byte_range()].to_string(),
-                    location: node.start_position(),
-                })
+                    position: SourcePosition::from(node.range()),
+                }))
             } else {
                 for child in node.children(&mut cursor.clone()) {
                     check_for_error(child, source, cursor)?;
@@ -150,7 +149,9 @@ 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, Error> {
-        self.verify(source)?.run(source, &self.context)
+        self.verify(source)?
+            .run(source, &self.context)
+            .map_err(|error| Error::Runtime(error))
     }
 
     /// Return an s-expression displaying a syntax tree of the source, or the ParserCancelled error
diff --git a/src/main.rs b/src/main.rs
index 2a2b3b4..688b4f9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -11,7 +11,7 @@ use reedline::{
 
 use std::{borrow::Cow, fs::read_to_string, path::PathBuf, process::Command};
 
-use dust_lang::{built_in_values, Interpreter, Map, Result, Value};
+use dust_lang::{built_in_values, Error, Interpreter, Map, Value};
 
 /// Command-line arguments to be parsed.
 #[derive(Parser, Debug)]
@@ -332,7 +332,7 @@ impl Completer for DustCompleter {
     }
 }
 
-fn run_shell(context: Map) -> Result<()> {
+fn run_shell(context: Map) -> Result<(), Error> {
     let mut interpreter = Interpreter::new(context.clone());
     let mut keybindings = default_emacs_keybindings();
 
diff --git a/src/value/map.rs b/src/value/map.rs
index ae8ec8f..4c280e6 100644
--- a/src/value/map.rs
+++ b/src/value/map.rs
@@ -13,12 +13,10 @@ use std::{
     collections::BTreeMap,
     fmt::{self, Display, Formatter},
     marker::PhantomData,
-    sync::{Arc, PoisonError, RwLock, RwLockReadGuard},
+    sync::{Arc, RwLock, RwLockReadGuard},
 };
 
-use crate::{value::Value, Structure, Type};
-
-pub type MapAccessError<'a> = PoisonError<RwLockReadGuard<'a, BTreeMap<String, (Value, Type)>>>;
+use crate::{error::rw_lock_error::RwLockError, value::Value, Structure, Type};
 
 /// A collection dust variables comprised of key-value pairs.
 ///
@@ -65,7 +63,7 @@ impl Map {
         }
     }
 
-    pub fn clone_from(other: &Self) -> Result<Self, ()> {
+    pub fn clone_from(other: &Self) -> Result<Self, RwLockError> {
         let mut new_map = BTreeMap::new();
 
         for (key, (value, r#type)) in other.variables()?.iter() {
@@ -80,26 +78,35 @@ impl Map {
 
     pub fn variables(
         &self,
-    ) -> Result<RwLockReadGuard<BTreeMap<String, (Value, Type)>>, MapAccessError> {
-        self.variables.read()
+    ) -> Result<RwLockReadGuard<BTreeMap<String, (Value, Type)>>, RwLockError> {
+        self.variables.read().map_err(|_| RwLockError)
     }
 
-    pub fn set(&self, key: String, value: Value) -> Result<Option<(Value, Type)>, MapAccessError> {
+    pub fn set(&self, key: String, value: Value) -> Result<Option<(Value, Type)>, RwLockError> {
         log::info!("Setting variable {key} = {value}");
 
         let value_type = value.r#type();
         let previous = self
             .variables
-            .write()?
+            .write()
+            .map_err(|_| RwLockError)?
             .insert(key, (value, value_type.clone()));
 
         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)>, RwLockError> {
         log::info!("Setting type {key} = {}", r#type);
 
-        let previous = self.variables.write()?.insert(key, (Value::none(), r#type));
+        let previous = self
+            .variables
+            .write()
+            .map_err(|_| RwLockError)?
+            .insert(key, (Value::none(), r#type));
 
         Ok(previous)
     }
diff --git a/src/value/mod.rs b/src/value/mod.rs
index 6c62da4..c9b9be2 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -1,8 +1,5 @@
 //! Types that represent runtime values.
-use crate::{
-    error::{Error, RuntimeError},
-    Identifier, Type, TypeSpecification,
-};
+use crate::{error::RuntimeError, Identifier, Type, TypeSpecification};
 
 use serde::{
     de::{MapAccess, SeqAccess, Visitor},
@@ -167,7 +164,7 @@ impl Value {
     pub fn as_string(&self) -> Result<&String, RuntimeError> {
         match self {
             Value::String(string) => Ok(string),
-            value => Err(Error::ExpectedString {
+            value => Err(RuntimeError::ExpectedString {
                 actual: value.clone(),
             }),
         }
@@ -177,7 +174,7 @@ impl Value {
     pub fn as_integer(&self) -> Result<i64, RuntimeError> {
         match self {
             Value::Integer(i) => Ok(*i),
-            value => Err(Error::ExpectedInteger {
+            value => Err(RuntimeError::ExpectedInteger {
                 actual: value.clone(),
             }),
         }
@@ -187,7 +184,7 @@ impl Value {
     pub fn as_float(&self) -> Result<f64, RuntimeError> {
         match self {
             Value::Float(f) => Ok(*f),
-            value => Err(Error::ExpectedFloat {
+            value => Err(RuntimeError::ExpectedFloat {
                 actual: value.clone(),
             }),
         }
@@ -199,7 +196,7 @@ impl Value {
         match self {
             Value::Float(f) => Ok(*f),
             Value::Integer(i) => Ok(*i as f64),
-            value => Err(Error::ExpectedNumber {
+            value => Err(RuntimeError::ExpectedNumber {
                 actual: value.clone(),
             }),
         }
@@ -209,7 +206,7 @@ impl Value {
     pub fn as_boolean(&self) -> Result<bool, RuntimeError> {
         match self {
             Value::Boolean(boolean) => Ok(*boolean),
-            value => Err(Error::ExpectedBoolean {
+            value => Err(RuntimeError::ExpectedBoolean {
                 actual: value.clone(),
             }),
         }
@@ -219,7 +216,7 @@ impl Value {
     pub fn as_list(&self) -> Result<&List, RuntimeError> {
         match self {
             Value::List(list) => Ok(list),
-            value => Err(Error::ExpectedList {
+            value => Err(RuntimeError::ExpectedList {
                 actual: value.clone(),
             }),
         }
@@ -229,7 +226,7 @@ impl Value {
     pub fn into_inner_list(self) -> Result<List, RuntimeError> {
         match self {
             Value::List(list) => Ok(list),
-            value => Err(Error::ExpectedList {
+            value => Err(RuntimeError::ExpectedList {
                 actual: value.clone(),
             }),
         }
@@ -239,7 +236,7 @@ impl Value {
     pub fn as_map(&self) -> Result<&Map, RuntimeError> {
         match self {
             Value::Map(map) => Ok(map),
-            value => Err(Error::ExpectedMap {
+            value => Err(RuntimeError::ExpectedMap {
                 actual: value.clone(),
             }),
         }
@@ -250,7 +247,7 @@ impl Value {
     pub fn as_function(&self) -> Result<&Function, RuntimeError> {
         match self {
             Value::Function(function) => Ok(function),
-            value => Err(Error::ExpectedFunction {
+            value => Err(RuntimeError::ExpectedFunction {
                 actual: value.clone(),
             }),
         }
@@ -260,7 +257,7 @@ impl Value {
     pub fn as_option(&self) -> Result<&Option<Box<Value>>, RuntimeError> {
         match self {
             Value::Option(option) => Ok(option),
-            value => Err(Error::ExpectedOption {
+            value => Err(RuntimeError::ExpectedOption {
                 actual: value.clone(),
             }),
         }
@@ -273,12 +270,12 @@ impl Value {
                 if option.is_none() {
                     Ok(())
                 } else {
-                    Err(Error::ExpectedNone {
+                    Err(RuntimeError::ExpectedNone {
                         actual: self.clone(),
                     })
                 }
             }
-            value => Err(Error::ExpectedNone {
+            value => Err(RuntimeError::ExpectedNone {
                 actual: value.clone(),
             }),
         }
@@ -319,7 +316,7 @@ impl Add for Value {
             other
         };
 
-        Err(Error::ExpectedNumberOrString {
+        Err(RuntimeError::ExpectedNumberOrString {
             actual: non_number_or_string,
         })
     }
@@ -341,7 +338,7 @@ impl Sub for Value {
 
         let non_number = if !self.is_number() { self } else { other };
 
-        Err(Error::ExpectedNumber { actual: non_number })
+        Err(RuntimeError::ExpectedNumber { actual: non_number })
     }
 }
 
@@ -356,7 +353,7 @@ impl Mul for Value {
         } else {
             let non_number = if !self.is_number() { self } else { other };
 
-            Err(Error::ExpectedNumber { actual: non_number })
+            Err(RuntimeError::ExpectedNumber { actual: non_number })
         }
     }
 }
@@ -377,7 +374,7 @@ impl Div for Value {
         } else {
             let non_number = if !self.is_number() { self } else { other };
 
-            Err(Error::ExpectedNumber { actual: non_number })
+            Err(RuntimeError::ExpectedNumber { actual: non_number })
         }
     }
 }
@@ -596,49 +593,49 @@ impl From<()> for Value {
 }
 
 impl TryFrom<Value> for String {
-    type Error = Error;
+    type Error = RuntimeError;
 
     fn try_from(value: Value) -> std::result::Result<Self, Self::Error> {
         if let Value::String(string) = value {
             Ok(string)
         } else {
-            Err(Error::ExpectedString { actual: value })
+            Err(RuntimeError::ExpectedString { actual: value })
         }
     }
 }
 
 impl TryFrom<Value> for f64 {
-    type Error = Error;
+    type Error = RuntimeError;
 
     fn try_from(value: Value) -> std::result::Result<Self, Self::Error> {
         if let Value::Float(value) = value {
             Ok(value)
         } else {
-            Err(Error::ExpectedFloat { actual: value })
+            Err(RuntimeError::ExpectedFloat { actual: value })
         }
     }
 }
 
 impl TryFrom<Value> for i64 {
-    type Error = Error;
+    type Error = RuntimeError;
 
     fn try_from(value: Value) -> std::result::Result<Self, Self::Error> {
         if let Value::Integer(value) = value {
             Ok(value)
         } else {
-            Err(Error::ExpectedInteger { actual: value })
+            Err(RuntimeError::ExpectedInteger { actual: value })
         }
     }
 }
 
 impl TryFrom<Value> for bool {
-    type Error = Error;
+    type Error = RuntimeError;
 
     fn try_from(value: Value) -> std::result::Result<Self, Self::Error> {
         if let Value::Boolean(value) = value {
             Ok(value)
         } else {
-            Err(Error::ExpectedBoolean { actual: value })
+            Err(RuntimeError::ExpectedBoolean { actual: value })
         }
     }
 }