Implement formatting

src/abstract_tree/assignment.rs

@@ -1,10 +1,8 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    AbstractTree, Error, Identifier, Map, Result, Statement, SyntaxNode, SyntaxPosition, Type,
+    AbstractTree, AssignmentOperator, Error, Format, Identifier, Map, Result, Statement,
-    TypeDefinition, Value,
+    SyntaxNode, SyntaxPosition, Type, TypeDefinition, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -34,24 +32,8 @@ impl AbstractTree for Assignment {
             None
         };
 
-        let operator_node = syntax_node
+        let operator_node = syntax_node.child(child_count - 2).unwrap();
-            .child(child_count - 2)
+        let operator = AssignmentOperator::from_syntax_node(source, operator_node, context)?;
-            .unwrap()
-            .child(0)
-            .unwrap();
-        let operator = match operator_node.kind() {
-            "=" => AssignmentOperator::Equal,
-            "+=" => AssignmentOperator::PlusEqual,
-            "-=" => AssignmentOperator::MinusEqual,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "=, += or -=".to_string(),
-                    actual: operator_node.kind().to_string(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
 
         let statement_node = syntax_node.child(child_count - 1).unwrap();
         let statement = Statement::from_syntax_node(source, statement_node, context)?;
@@ -159,8 +141,8 @@ impl AbstractTree for Assignment {
     }
 }
 
-impl Display for Assignment {
+impl Format for Assignment {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         let Assignment {
             identifier,
             type_definition,
@@ -169,29 +151,16 @@ impl Display for Assignment {
             syntax_position: _,
         } = self;
 
-        write!(f, "{identifier}")?;
+        self.identifier.format(output, indent_level);
 
         if let Some(type_definition) = type_definition {
-            write!(f, " {type_definition}")?;
+            type_definition.format(output, indent_level);
         }
 
-        write!(f, " {operator} {statement}")
+        output.push_str(" ");
-    }
+        self.operator.format(output, indent_level);
-}
+        output.push_str(" ");
-
+
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+        self.statement.format(output, indent_level);
-pub enum AssignmentOperator {
-    Equal,
-    PlusEqual,
-    MinusEqual,
-}
-
-impl Display for AssignmentOperator {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        match self {
-            AssignmentOperator::Equal => write!(f, "="),
-            AssignmentOperator::PlusEqual => write!(f, "-="),
-            AssignmentOperator::MinusEqual => write!(f, "+="),
-        }
     }
 }

src/abstract_tree/assignment_operator.rs

@@ -0,0 +1,55 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{AbstractTree, Error, Format, Map, Result, Type, Value};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum AssignmentOperator {
+    Equal,
+    PlusEqual,
+    MinusEqual,
+}
+
+impl AbstractTree for AssignmentOperator {
+    fn from_syntax_node(
+        source: &str,
+        node: tree_sitter::Node,
+        context: &crate::Map,
+    ) -> Result<Self> {
+        Error::expect_syntax_node(source, "assignment_operator", node)?;
+
+        let operator_node = node.child(0).unwrap();
+        let operator = match operator_node.kind() {
+            "=" => AssignmentOperator::Equal,
+            "+=" => AssignmentOperator::PlusEqual,
+            "-=" => AssignmentOperator::MinusEqual,
+            _ => {
+                return Err(Error::UnexpectedSyntaxNode {
+                    expected: "=, += or -=".to_string(),
+                    actual: operator_node.kind().to_string(),
+                    location: operator_node.start_position(),
+                    relevant_source: source[operator_node.byte_range()].to_string(),
+                })
+            }
+        };
+
+        Ok(operator)
+    }
+
+    fn run(&self, source: &str, context: &Map) -> Result<Value> {
+        Ok(Value::none())
+    }
+
+    fn expected_type(&self, context: &Map) -> Result<Type> {
+        Ok(Type::None)
+    }
+}
+
+impl Format for AssignmentOperator {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        match self {
+            AssignmentOperator::Equal => output.push('='),
+            AssignmentOperator::PlusEqual => output.push_str("+="),
+            AssignmentOperator::MinusEqual => output.push_str("-="),
+        }
+    }
+}

src/abstract_tree/block.rs

@@ -1,13 +1,10 @@
-use std::{
+use std::sync::RwLock;
-    fmt::{self, Display, Formatter},
-    sync::RwLock,
-};
 
 use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Map, Result, Statement, Type, Value};
+use crate::{AbstractTree, Error, Format, Map, Result, Statement, Type, Value};
 
 /// Abstract representation of a block.
 ///
@@ -131,18 +128,22 @@ impl AbstractTree for Block {
     }
 }
 
-impl Display for Block {
+impl Format for Block {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         if self.is_async {
-            writeln!(f, "async {{")?;
+            output.push_str("async {");
         } else {
-            writeln!(f, "{{")?;
+            output.push('{');
         }
 
         for statement in &self.statements {
-            writeln!(f, "    {statement}")?;
+            for _ in 0..=indent_level {
+                output.push_str("    ");
+            }
+
+            statement.format(output, indent_level);
         }
 
-        writeln!(f, "}}")
+        output.push('}');
     }
 }

src/abstract_tree/built_in_value.rs

@@ -1,15 +1,11 @@
-use std::{
+use std::{env::args, sync::OnceLock};
-    env::args,
-    fmt::{self, Display, Formatter},
-    sync::OnceLock,
-};
 
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    built_in_functions::string_functions, AbstractTree, BuiltInFunction, Function, List, Map,
+    built_in_functions::string_functions, AbstractTree, BuiltInFunction, Format, Function, List,
-    Result, Type, Value,
+    Map, Result, Type, Value,
 };
 
 static ARGS: OnceLock<Value> = OnceLock::new();
@@ -31,6 +27,19 @@ pub enum BuiltInValue {
 }
 
 impl BuiltInValue {
+    pub fn name(&self) -> &'static str {
+        match self {
+            BuiltInValue::Args => "args",
+            BuiltInValue::AssertEqual => "assert_equal",
+            BuiltInValue::Fs => "fs",
+            BuiltInValue::Json => "json",
+            BuiltInValue::Length => "length",
+            BuiltInValue::Output => "output",
+            BuiltInValue::Random => "random",
+            BuiltInValue::String => "string",
+        }
+    }
+
     fn r#type(&self) -> Type {
         match self {
             BuiltInValue::Args => Type::list(Type::String),
@@ -72,7 +81,7 @@ impl BuiltInValue {
 
                 json_context
                     .set(
-                        "parse".to_string(),
+                        BuiltInFunction::JsonParse.name().to_string(),
                         Value::Function(Function::BuiltIn(BuiltInFunction::JsonParse)),
                         None,
                     )
@@ -153,8 +162,8 @@ impl AbstractTree for BuiltInValue {
     }
 }
 
-impl Display for BuiltInValue {
+impl Format for BuiltInValue {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        write!(f, "{}", self.get())
+        output.push_str(&self.get().to_string());
     }
 }

src/abstract_tree/expression.rs

@@ -1,10 +1,9 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    value_node::ValueNode, AbstractTree, Error, Identifier, Index, Map, Result, Type, Value, Yield,
+    value_node::ValueNode, AbstractTree, Error, Format, Identifier, Index, Map, Result, Type,
+    Value, Yield,
 };
 
 use super::{function_call::FunctionCall, logic::Logic, math::Math};
@@ -104,16 +103,16 @@ impl AbstractTree for Expression {
     }
 }
 
-impl Display for Expression {
+impl Format for Expression {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         match self {
-            Expression::Value(value_node) => write!(f, "{value_node}"),
+            Expression::Value(value_node) => value_node.format(output, indent_level),
-            Expression::Identifier(identifier) => write!(f, "{identifier}"),
+            Expression::Identifier(identifier) => identifier.format(output, indent_level),
-            Expression::Math(math) => write!(f, "{math}"),
+            Expression::Math(math) => math.format(output, indent_level),
-            Expression::Logic(logic) => write!(f, "{logic}"),
+            Expression::Logic(logic) => logic.format(output, indent_level),
-            Expression::FunctionCall(function_call) => write!(f, "{function_call}"),
+            Expression::FunctionCall(function_call) => function_call.format(output, indent_level),
-            Expression::Index(index) => write!(f, "{index}"),
+            Expression::Index(index) => index.format(output, indent_level),
-            Expression::Yield(r#yield) => write!(f, "{}", r#yield),
+            Expression::Yield(r#yield) => r#yield.format(output, indent_level),
         }
     }
 }

src/abstract_tree/for.rs

@@ -1,10 +1,10 @@
-use std::fmt::{self, Display, Formatter};
+use std::fmt::Write;
 
 use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Block, Error, Expression, Identifier, Map, Result, Type, Value};
+use crate::{AbstractTree, Block, Error, Expression, Format, Identifier, Map, Result, Type, Value};
 
 /// Abstract representation of a for loop statement.
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -81,21 +81,18 @@ impl AbstractTree for For {
     }
 }
 
-impl Display for For {
+impl Format for For {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let For {
+        if self.is_async {
-            is_async,
+            output.write_str("async for ");
-            item_id,
-            collection,
-            block,
-        } = self;
-
-        if *is_async {
-            write!(f, "async for ")?;
         } else {
-            write!(f, "for ")?;
+            output.write_str("for ");
         }
 
-        write!(f, "{item_id} in {collection}, {{{block}}}")
+        self.item_id.format(output, indent_level);
+        output.push_str(" in ");
+        self.collection.format(output, indent_level);
+        output.push_str(" ");
+        self.block.format(output, indent_level);
     }
 }

src/abstract_tree/function_call.rs

@@ -1,10 +1,9 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, Error, Expression, FunctionExpression, Map, Result, SyntaxPosition, Type, Value,
+    AbstractTree, Error, Expression, Format, FunctionExpression, Map, Result, SyntaxPosition, Type,
+    Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -162,20 +161,15 @@ impl AbstractTree for FunctionCall {
     }
 }
 
-impl Display for FunctionCall {
+impl Format for FunctionCall {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let FunctionCall {
+        self.function_expression.format(output, indent_level);
-            function_expression,
+        output.push('(');
-            arguments,
-            ..
-        } = self;
 
-        write!(f, "{function_expression}(")?;
+        for expression in &self.arguments {
-
+            expression.format(output, indent_level);
-        for expression in arguments {
-            write!(f, "{expression}")?;
         }
 
-        write!(f, ")")
+        output.push(')');
     }
 }

src/abstract_tree/function_expression.rs

@@ -1,11 +1,9 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, Error, FunctionCall, Identifier, Index, Map, Result, Type, Value, ValueNode,
+    AbstractTree, Error, Format, FunctionCall, Identifier, Index, Map, Result, Type, Value,
-    Yield,
+    ValueNode, Yield,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -77,14 +75,16 @@ impl AbstractTree for FunctionExpression {
     }
 }
 
-impl Display for FunctionExpression {
+impl Format for FunctionExpression {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         match self {
-            FunctionExpression::Value(value_node) => write!(f, "{value_node}"),
+            FunctionExpression::Value(value_node) => value_node.format(output, indent_level),
-            FunctionExpression::Identifier(identifier) => write!(f, "{identifier}"),
+            FunctionExpression::Identifier(identifier) => identifier.format(output, indent_level),
-            FunctionExpression::FunctionCall(function_call) => write!(f, "{function_call}"),
+            FunctionExpression::FunctionCall(function_call) => {
-            FunctionExpression::Index(index) => write!(f, "{index}"),
+                function_call.format(output, indent_level)
-            FunctionExpression::Yield(r#yield) => write!(f, "{}", r#yield),
+            }
+            FunctionExpression::Index(index) => index.format(output, indent_level),
+            FunctionExpression::Yield(r#yield) => r#yield.format(output, indent_level),
         }
     }
 }

src/abstract_tree/function_node.rs

@@ -4,7 +4,7 @@ use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, Block, Error, Function, Identifier, Map, Result, SyntaxPosition, Type,
+    AbstractTree, Block, Error, Format, Function, Identifier, Map, Result, SyntaxPosition, Type,
     TypeDefinition, Value,
 };
 
@@ -146,31 +146,39 @@ impl AbstractTree for FunctionNode {
     }
 }
 
-impl Display for FunctionNode {
+impl Format for FunctionNode {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let FunctionNode {
-            parameters,
-            body,
-            r#type,
-            ..
-        } = self;
-
         let (parameter_types, return_type) = if let Type::Function {
             parameter_types,
             return_type,
-        } = r#type
+        } = &self.r#type
         {
             (parameter_types, return_type)
         } else {
-            return Ok(());
+            return;
         };
 
-        write!(f, "(")?;
+        output.push('(');
 
-        for (identifier, r#type) in parameters.iter().zip(parameter_types.iter()) {
+        for (identifier, r#type) in self.parameters.iter().zip(parameter_types.iter()) {
-            write!(f, "{identifier} <{}>", r#type)?;
+            identifier.format(output, indent_level);
+            output.push('<');
+            r#type.format(output, indent_level);
+            output.push('>');
         }
 
-        write!(f, ") <{return_type}> {body}")
+        output.push_str(") <");
+        return_type.format(output, indent_level);
+        output.push_str("> ");
+        self.body.format(output, indent_level);
+    }
+}
+
+impl Display for FunctionNode {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        let mut string = String::new();
+
+        self.format(&mut string, 0);
+        f.write_str(&string)
     }
 }

src/abstract_tree/identifier.rs

@@ -3,7 +3,7 @@ use std::fmt::{self, Display, Formatter};
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Map, Result, Type, Value};
+use crate::{AbstractTree, Error, Format, Map, Result, Type, Value};
 
 /// A string by which a variable is known to a context.
 ///
@@ -13,8 +13,8 @@ use crate::{AbstractTree, Error, Map, Result, Type, Value};
 pub struct Identifier(String);
 
 impl Identifier {
-    pub fn new(inner: String) -> Self {
+    pub fn new<T: Into<String>>(inner: T) -> Self {
-        Identifier(inner)
+        Identifier(inner.into())
     }
 
     pub fn take_inner(self) -> String {
@@ -56,6 +56,12 @@ impl AbstractTree for Identifier {
     }
 }
 
+impl Format for Identifier {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        output.push_str(&self.0);
+    }
+}
+
 impl Display for Identifier {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         write!(f, "{}", self.0)

src/abstract_tree/if_else.rs

@@ -1,9 +1,7 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Block, Expression, Map, Result, Type, Value};
+use crate::{AbstractTree, Block, Expression, Format, Map, Result, Type, Value};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub struct IfElse {
@@ -88,26 +86,28 @@ impl AbstractTree for IfElse {
     }
 }
 
-impl Display for IfElse {
+impl Format for IfElse {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let IfElse {
+        output.push_str("if ");
-            if_expression,
+        self.if_expression.format(output, indent_level);
-            if_block,
+        output.push(' ');
-            else_if_expressions,
+        self.if_block.format(output, indent_level);
-            else_if_blocks,
-            else_block,
-        } = self;
 
-        write!(f, "if {if_expression} {{{if_block}}}")?;
+        let else_ifs = self
+            .else_if_expressions
+            .iter()
+            .zip(self.else_if_blocks.iter());
 
-        for (expression, block) in else_if_expressions.iter().zip(else_if_blocks.iter()) {
+        for (expression, block) in else_ifs {
-            write!(f, "else if {expression} {{{block}}}")?;
+            output.push_str("else if ");
+            expression.format(output, indent_level);
+            output.push(' ');
+            block.format(output, indent_level);
         }
 
-        if let Some(block) = else_block {
+        if let Some(block) = &self.else_block {
-            write!(f, "else {{{block}}}")?;
+            output.push_str("else ");
+            block.format(output, indent_level);
         }
-
-        Ok(())
     }
 }

src/abstract_tree/index.rs

@@ -1,9 +1,7 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, IndexExpression, List, Map, Result, Type, Value};
+use crate::{AbstractTree, Error, Format, IndexExpression, List, Map, Result, Type, Value};
 
 /// Abstract representation of an index expression.
 ///
@@ -101,20 +99,15 @@ impl AbstractTree for Index {
     }
 }
 
-impl Display for Index {
+impl Format for Index {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let Index {
+        self.collection.format(output, indent_level);
-            collection,
+        output.push(':');
-            index,
+        self.index.format(output, indent_level);
-            index_end,
-        } = self;
 
-        write!(f, "{collection}:{index}")?;
+        if let Some(expression) = &self.index_end {
-
+            output.push_str("..");
-        if let Some(expression) = index_end {
+            expression.format(output, indent_level);
-            write!(f, "..{expression}")?;
         }
-
-        Ok(())
     }
 }

src/abstract_tree/index_assignment.rs

@@ -1,11 +1,9 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, AssignmentOperator, Error, Index, IndexExpression, Map, Result, Statement, Type,
+    AbstractTree, AssignmentOperator, Error, Format, Index, IndexExpression, Map, Result,
-    Value,
+    Statement, Type, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -22,20 +20,8 @@ impl AbstractTree for IndexAssignment {
         let index_node = node.child(0).unwrap();
         let index = Index::from_syntax_node(source, index_node, context)?;
 
-        let operator_node = node.child(1).unwrap().child(0).unwrap();
+        let operator_node = node.child(1).unwrap();
-        let operator = match operator_node.kind() {
+        let operator = AssignmentOperator::from_syntax_node(source, operator_node, context)?;
-            "=" => AssignmentOperator::Equal,
-            "+=" => AssignmentOperator::PlusEqual,
-            "-=" => AssignmentOperator::MinusEqual,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "=, += or -=".to_string(),
-                    actual: operator_node.kind().to_string(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
 
         let statement_node = node.child(2).unwrap();
         let statement = Statement::from_syntax_node(source, statement_node, context)?;
@@ -94,14 +80,12 @@ impl AbstractTree for IndexAssignment {
     }
 }
 
-impl Display for IndexAssignment {
+impl Format for IndexAssignment {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let IndexAssignment {
+        self.index.format(output, indent_level);
-            index,
+        output.push(' ');
-            operator,
+        self.operator.format(output, indent_level);
-            statement,
+        output.push(' ');
-        } = self;
+        self.statement.format(output, indent_level);
-
-        write!(f, "{index} {operator} {statement}")
     }
 }

src/abstract_tree/index_expression.rs

@@ -1,10 +1,8 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    value_node::ValueNode, AbstractTree, Error, FunctionCall, Identifier, Index, Map, Result, Type,
+    value_node::ValueNode, AbstractTree, Error, Format, FunctionCall, Identifier, Index, Map,
-    Value,
+    Result, Type, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -69,13 +67,21 @@ impl AbstractTree for IndexExpression {
     }
 }
 
-impl Display for IndexExpression {
+impl Format for IndexExpression {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         match self {
-            IndexExpression::Value(value_node) => write!(f, "{value_node}"),
+            IndexExpression::Value(value_node) => {
-            IndexExpression::Identifier(identifier) => write!(f, "{identifier}"),
+                if let ValueNode::BuiltInValue(built_in_value) = value_node {
-            IndexExpression::FunctionCall(function_call) => write!(f, "{function_call}"),
+                    output.push_str(built_in_value.name());
-            IndexExpression::Index(index) => write!(f, "{index}"),
+                } else {
+                    value_node.format(output, indent_level);
+                }
+            }
+            IndexExpression::Identifier(identifier) => identifier.format(output, indent_level),
+            IndexExpression::FunctionCall(function_call) => {
+                function_call.format(output, indent_level)
+            }
+            IndexExpression::Index(index) => index.format(output, indent_level),
         }
     }
 }

src/abstract_tree/logic.rs

@@ -1,9 +1,7 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Expression, Map, Result, Type, Value};
+use crate::{AbstractTree, Error, Expression, Format, LogicOperator, Map, Result, Type, Value};
 
 /// Abstract representation of a logic expression.
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -20,38 +18,17 @@ impl AbstractTree for Logic {
         let first_node = node.child(0).unwrap();
         let (left_node, operator_node, right_node) = {
             if first_node.is_named() {
-                (
+                (first_node, node.child(1).unwrap(), node.child(2).unwrap())
-                    first_node,
-                    node.child(1).unwrap().child(0).unwrap(),
-                    node.child(2).unwrap(),
-                )
             } else {
                 (
                     node.child(1).unwrap(),
-                    node.child(2).unwrap().child(0).unwrap(),
+                    node.child(2).unwrap(),
                     node.child(3).unwrap(),
                 )
             }
         };
         let left = Expression::from_syntax_node(source, left_node, context)?;
-        let operator = match operator_node.kind() {
+        let operator = LogicOperator::from_syntax_node(source, operator_node, context)?;
-            "==" => LogicOperator::Equal,
-            "!=" => LogicOperator::NotEqual,
-            "&&" => LogicOperator::And,
-            "||" => LogicOperator::Or,
-            ">" => LogicOperator::Greater,
-            "<" => LogicOperator::Less,
-            ">=" => LogicOperator::GreaterOrEqual,
-            "<=" => LogicOperator::LessOrEqual,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "==, !=, &&, ||, >, <, >= or <=".to_string(),
-                    actual: operator_node.kind().to_string(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
         let right = Expression::from_syntax_node(source, right_node, context)?;
 
         Ok(Logic {
@@ -95,41 +72,12 @@ impl AbstractTree for Logic {
     }
 }
 
-impl Display for Logic {
+impl Format for Logic {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let Logic {
+        self.left.format(output, indent_level);
-            left,
+        output.push(' ');
-            operator,
+        self.operator.format(output, indent_level);
-            right,
+        output.push(' ');
-        } = self;
+        self.right.format(output, indent_level);
-
-        write!(f, "{left} {operator} {right}")
-    }
-}
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub enum LogicOperator {
-    Equal,
-    NotEqual,
-    And,
-    Or,
-    Greater,
-    Less,
-    GreaterOrEqual,
-    LessOrEqual,
-}
-
-impl Display for LogicOperator {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        match self {
-            LogicOperator::Equal => write!(f, "="),
-            LogicOperator::NotEqual => write!(f, "!="),
-            LogicOperator::And => write!(f, "&&"),
-            LogicOperator::Or => write!(f, "||"),
-            LogicOperator::Greater => write!(f, ">"),
-            LogicOperator::Less => write!(f, "<"),
-            LogicOperator::GreaterOrEqual => write!(f, ">="),
-            LogicOperator::LessOrEqual => write!(f, "<="),
-        }
     }
 }

src/abstract_tree/logic_operator.rs

@@ -0,0 +1,70 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{AbstractTree, Error, Format, Map, Result, Type, Value};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum LogicOperator {
+    Equal,
+    NotEqual,
+    And,
+    Or,
+    Greater,
+    Less,
+    GreaterOrEqual,
+    LessOrEqual,
+}
+
+impl AbstractTree for LogicOperator {
+    fn from_syntax_node(
+        source: &str,
+        node: tree_sitter::Node,
+        _context: &crate::Map,
+    ) -> crate::Result<Self> {
+        Error::expect_syntax_node(source, "logic_operator", node)?;
+
+        let operator_node = node.child(0).unwrap();
+        let operator = match operator_node.kind() {
+            "==" => LogicOperator::Equal,
+            "!=" => LogicOperator::NotEqual,
+            "&&" => LogicOperator::And,
+            "||" => LogicOperator::Or,
+            ">" => LogicOperator::Greater,
+            "<" => LogicOperator::Less,
+            ">=" => LogicOperator::GreaterOrEqual,
+            "<=" => LogicOperator::LessOrEqual,
+            _ => {
+                return Err(Error::UnexpectedSyntaxNode {
+                    expected: "==, !=, &&, ||, >, <, >= or <=".to_string(),
+                    actual: operator_node.kind().to_string(),
+                    location: operator_node.start_position(),
+                    relevant_source: source[operator_node.byte_range()].to_string(),
+                })
+            }
+        };
+
+        Ok(operator)
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+        Ok(Value::none())
+    }
+
+    fn expected_type(&self, _context: &Map) -> Result<Type> {
+        Ok(Type::None)
+    }
+}
+
+impl Format for LogicOperator {
+    fn format(&self, output: &mut String, _indent_level: u8) {
+        match self {
+            LogicOperator::Equal => output.push('='),
+            LogicOperator::NotEqual => output.push_str("!="),
+            LogicOperator::And => output.push_str("&&"),
+            LogicOperator::Or => output.push_str("||"),
+            LogicOperator::Greater => output.push('>'),
+            LogicOperator::Less => output.push('<'),
+            LogicOperator::GreaterOrEqual => output.push_str(">="),
+            LogicOperator::LessOrEqual => output.push_str("<="),
+        }
+    }
+}

src/abstract_tree/match.rs

@@ -2,13 +2,10 @@
 //!
 //! Note that this module is called "match" but is escaped as "r#match" because
 //! "match" is a keyword in Rust.
-
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Expression, Map, Result, Statement, Type, Value};
+use crate::{AbstractTree, Error, Expression, Format, Map, Result, Statement, Type, Value};
 
 /// Abstract representation of a match statement.
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -85,24 +82,23 @@ impl AbstractTree for Match {
     }
 }
 
-impl Display for Match {
+impl Format for Match {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let Match {
+        output.push_str("match ");
-            matcher,
+        self.matcher.format(output, indent_level);
-            options,
+        output.push_str(" {");
-            fallback,
-        } = self;
 
-        write!(f, "match {matcher} {{")?;
+        for (expression, statement) in &self.options {
-
+            expression.format(output, indent_level);
-        for (expression, statement) in options {
+            output.push_str(" => ");
-            write!(f, "{expression} => {statement}")?;
+            statement.format(output, indent_level);
         }
 
-        if let Some(statement) = fallback {
+        if let Some(statement) = &self.fallback {
-            write!(f, "* => {statement}")?;
+            output.push_str("* => ");
+            statement.format(output, indent_level);
         }
 
-        write!(f, "}}")
+        output.push('}');
     }
 }

src/abstract_tree/math.rs

@@ -1,9 +1,7 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Expression, Map, Result, Type, Value};
+use crate::{AbstractTree, Error, Expression, Format, Map, MathOperator, Result, Type, Value};
 
 /// Abstract representation of a math operation.
 ///
@@ -23,22 +21,8 @@ impl AbstractTree for Math {
         let left_node = node.child(0).unwrap();
         let left = Expression::from_syntax_node(source, left_node, context)?;
 
-        let operator_node = node.child(1).unwrap().child(0).unwrap();
+        let operator_node = node.child(1).unwrap();
-        let operator = match operator_node.kind() {
+        let operator = MathOperator::from_syntax_node(source, operator_node, context)?;
-            "+" => MathOperator::Add,
-            "-" => MathOperator::Subtract,
-            "*" => MathOperator::Multiply,
-            "/" => MathOperator::Divide,
-            "%" => MathOperator::Modulo,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "+, -, *, / or %".to_string(),
-                    actual: operator_node.kind().to_string(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
 
         let right_node = node.child(2).unwrap();
         let right = Expression::from_syntax_node(source, right_node, context)?;
@@ -69,35 +53,12 @@ impl AbstractTree for Math {
     }
 }
 
-impl Display for Math {
+impl Format for Math {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let Math {
+        self.left.format(output, indent_level);
-            left,
+        output.push(' ');
-            operator,
+        self.operator.format(output, indent_level);
-            right,
+        output.push(' ');
-        } = self;
+        self.right.format(output, indent_level);
-
-        write!(f, "{left} {operator} {right}")
-    }
-}
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub enum MathOperator {
-    Add,
-    Subtract,
-    Multiply,
-    Divide,
-    Modulo,
-}
-
-impl Display for MathOperator {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        match self {
-            MathOperator::Add => write!(f, "+"),
-            MathOperator::Subtract => write!(f, "-"),
-            MathOperator::Multiply => write!(f, "*"),
-            MathOperator::Divide => write!(f, "/"),
-            MathOperator::Modulo => write!(f, "%"),
-        }
     }
 }

src/abstract_tree/math_operator.rs

@@ -0,0 +1,57 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{AbstractTree, Error, Format, Map, Result, Type, Value};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum MathOperator {
+    Add,
+    Subtract,
+    Multiply,
+    Divide,
+    Modulo,
+}
+
+impl AbstractTree for MathOperator {
+    fn from_syntax_node(source: &str, node: tree_sitter::Node, context: &Map) -> Result<Self> {
+        let operator_node = node.child(0).unwrap();
+        let operator = match operator_node.kind() {
+            "+" => MathOperator::Add,
+            "-" => MathOperator::Subtract,
+            "*" => MathOperator::Multiply,
+            "/" => MathOperator::Divide,
+            "%" => MathOperator::Modulo,
+            _ => {
+                return Err(Error::UnexpectedSyntaxNode {
+                    expected: "+, -, *, / or %".to_string(),
+                    actual: operator_node.kind().to_string(),
+                    location: operator_node.start_position(),
+                    relevant_source: source[operator_node.byte_range()].to_string(),
+                })
+            }
+        };
+
+        Ok(operator)
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+        Ok(Value::none())
+    }
+
+    fn expected_type(&self, _context: &Map) -> Result<Type> {
+        Ok(Type::None)
+    }
+}
+
+impl Format for MathOperator {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        let char = match self {
+            MathOperator::Add => '+',
+            MathOperator::Subtract => '-',
+            MathOperator::Multiply => '*',
+            MathOperator::Divide => '/',
+            MathOperator::Modulo => '%',
+        };
+
+        output.push(char);
+    }
+}

src/abstract_tree/mod.rs

@@ -7,6 +7,7 @@
 //! examples.
 
 pub mod assignment;
+pub mod assignment_operator;
 pub mod block;
 pub mod built_in_value;
 pub mod expression;
@@ -20,23 +21,25 @@ pub mod index;
 pub mod index_assignment;
 pub mod index_expression;
 pub mod logic;
+pub mod logic_operator;
 pub mod r#match;
 pub mod math;
+pub mod math_operator;
 pub mod statement;
+pub mod r#type;
 pub mod type_definition;
 pub mod value_node;
 pub mod r#while;
 pub mod r#yield;
 
 pub use {
-    assignment::*, block::*, built_in_value::*, expression::*, function_call::*,
+    assignment::*, assignment_operator::*, block::*, built_in_value::*, expression::*,
-    function_expression::*, function_node::*, identifier::*, if_else::*, index::*,
+    function_call::*, function_expression::*, function_node::*, identifier::*, if_else::*,
-    index_assignment::IndexAssignment, index_expression::*, logic::*, math::*, r#for::*,
+    index::*, index_assignment::IndexAssignment, index_expression::*, logic::*, logic_operator::*,
-    r#match::*, r#while::*, r#yield::*, statement::*, type_definition::*, value_node::*,
+    math::*, math_operator::*, r#for::*, r#match::*, r#type::*, r#while::*, r#yield::*,
+    statement::*, type_definition::*, value_node::*,
 };
 
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
@@ -118,19 +121,17 @@ impl AbstractTree for Root {
     }
 }
 
-impl Display for Root {
+impl Format for Root {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         for statement in &self.statements {
-            write!(f, "{statement}")?;
+            statement.format(output, indent_level);
         }
-
-        Ok(())
     }
 }
 
 /// 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 {
+pub trait AbstractTree: Sized + Format {
     /// Interpret the syntax tree at the given node and return the abstraction.
     ///
     /// This function is used to convert nodes in the Tree Sitter concrete
@@ -152,3 +153,7 @@ pub trait AbstractTree: Sized {
 
     fn expected_type(&self, context: &Map) -> Result<Type>;
 }
+
+pub trait Format {
+    fn format(&self, output: &mut String, indent_level: u8);
+}

src/abstract_tree/statement.rs

@@ -1,11 +1,9 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, Assignment, Block, Error, Expression, For, IfElse, IndexAssignment, Map, Match,
+    AbstractTree, Assignment, Block, Error, Expression, For, Format, IfElse, IndexAssignment, Map,
-    Result, Type, Value, While,
+    Match, Result, Type, Value, While,
 };
 
 /// Abstract representation of a statement.
@@ -113,18 +111,20 @@ impl AbstractTree for Statement {
     }
 }
 
-impl Display for Statement {
+impl Format for Statement {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         match self {
-            Statement::Assignment(assignment) => write!(f, "{assignment}"),
+            Statement::Assignment(assignment) => assignment.format(output, indent_level),
-            Statement::Expression(expression) => write!(f, "{expression}"),
+            Statement::Expression(expression) => expression.format(output, indent_level),
-            Statement::IfElse(if_else) => write!(f, "{if_else}"),
+            Statement::IfElse(if_else) => if_else.format(output, indent_level),
-            Statement::Match(r#match) => write!(f, "{}", r#match),
+            Statement::Match(r#match) => r#match.format(output, indent_level),
-            Statement::While(r#while) => write!(f, "{}", r#while),
+            Statement::While(r#while) => r#while.format(output, indent_level),
-            Statement::Block(block) => write!(f, "{block}"),
+            Statement::Block(block) => block.format(output, indent_level),
-            Statement::For(r#for) => write!(f, "{}", r#for),
+            Statement::For(r#for) => r#for.format(output, indent_level),
-            Statement::IndexAssignment(index_assignment) => write!(f, "{index_assignment}"),
+            Statement::IndexAssignment(index_assignment) => {
-            Statement::Return(statement) => write!(f, "{statement}"),
+                index_assignment.format(output, indent_level)
+            }
+            Statement::Return(statement) => statement.format(output, indent_level),
         }
     }
 }

src/abstract_tree/type.rs

@@ -0,0 +1,290 @@
+use std::fmt::{self, Display, Formatter};
+
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node;
+
+use crate::{AbstractTree, Error, Format, Map, Result, Structure, Value};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum Type {
+    Any,
+    Boolean,
+    Collection,
+    Float,
+    Function {
+        parameter_types: Vec<Type>,
+        return_type: Box<Type>,
+    },
+    Integer,
+    List(Box<Type>),
+    Map(Option<Structure>),
+    None,
+    Number,
+    String,
+    Option(Box<Type>),
+}
+
+impl Type {
+    pub fn list(item_type: Type) -> Self {
+        Type::List(Box::new(item_type))
+    }
+
+    pub fn function(parameter_types: Vec<Type>, return_type: Type) -> Self {
+        Type::Function {
+            parameter_types,
+            return_type: Box::new(return_type),
+        }
+    }
+
+    pub fn option(optional_type: Type) -> Self {
+        Type::Option(Box::new(optional_type))
+    }
+
+    pub fn check(&self, other: &Type) -> Result<()> {
+        match (self, other) {
+            (Type::Any, _)
+            | (_, Type::Any)
+            | (Type::Boolean, Type::Boolean)
+            | (Type::Collection, Type::Collection)
+            | (Type::Collection, Type::List(_))
+            | (Type::List(_), Type::Collection)
+            | (Type::Collection, Type::Map(_))
+            | (Type::Map(_), Type::Collection)
+            | (Type::Collection, Type::String)
+            | (Type::String, Type::Collection)
+            | (Type::Float, Type::Float)
+            | (Type::Integer, Type::Integer)
+            | (Type::Map(_), Type::Map(_))
+            | (Type::Number, Type::Number)
+            | (Type::Number, Type::Integer)
+            | (Type::Number, Type::Float)
+            | (Type::Integer, Type::Number)
+            | (Type::Float, Type::Number)
+            | (Type::None, Type::None)
+            | (Type::String, Type::String) => Ok(()),
+            (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(())
+                } else {
+                    Err(Error::TypeCheck {
+                        expected: self.clone(),
+                        actual: other.clone(),
+                    })
+                }
+            }
+            (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(())
+                }
+            }
+            (
+                Type::Function {
+                    parameter_types: self_parameter_types,
+                    return_type: self_return_type,
+                },
+                Type::Function {
+                    parameter_types: other_parameter_types,
+                    return_type: other_return_type,
+                },
+            ) => {
+                let parameter_type_pairs = self_parameter_types
+                    .iter()
+                    .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_return_type.check(other_return_type).is_err() {
+                    Err(Error::TypeCheck {
+                        expected: self.clone(),
+                        actual: other.clone(),
+                    })
+                } else {
+                    Ok(())
+                }
+            }
+            _ => Err(Error::TypeCheck {
+                expected: self.clone(),
+                actual: other.clone(),
+            }),
+        }
+    }
+}
+
+impl AbstractTree for Type {
+    fn from_syntax_node(_source: &str, node: Node, _context: &Map) -> Result<Self> {
+        Error::expect_syntax_node(_source, "type", node)?;
+
+        let type_node = node.child(0).unwrap();
+
+        let r#type = match type_node.kind() {
+            "[" => {
+                let item_type_node = node.child(1).unwrap();
+                let item_type = Type::from_syntax_node(_source, item_type_node, _context)?;
+
+                Type::List(Box::new(item_type))
+            }
+            "any" => Type::Any,
+            "bool" => Type::Boolean,
+            "collection" => Type::Collection,
+            "float" => Type::Float,
+            "(" => {
+                let child_count = node.child_count();
+                let mut parameter_types = Vec::new();
+
+                for index in 1..child_count - 2 {
+                    let child = node.child(index).unwrap();
+
+                    if child.is_named() {
+                        let parameter_type = Type::from_syntax_node(_source, child, _context)?;
+
+                        parameter_types.push(parameter_type);
+                    }
+                }
+
+                let final_node = node.child(child_count - 1).unwrap();
+                let return_type = if final_node.is_named() {
+                    Type::from_syntax_node(_source, final_node, _context)?
+                } else {
+                    Type::None
+                };
+
+                Type::Function {
+                    parameter_types,
+                    return_type: Box::new(return_type),
+                }
+            }
+            "int" => Type::Integer,
+            "map" => Type::Map(None),
+            "num" => Type::Number,
+            "none" => Type::None,
+            "str" => Type::String,
+            "option" => {
+                let inner_type_node = node.child(2).unwrap();
+                let inner_type = Type::from_syntax_node(_source, inner_type_node, _context)?;
+
+                Type::Option(Box::new(inner_type))
+            }
+            _ => {
+                return Err(Error::UnexpectedSyntaxNode {
+                    expected: "any, bool, float, int, num, str, option, (, [ or {".to_string(),
+                    actual: type_node.kind().to_string(),
+                    location: type_node.start_position(),
+                    relevant_source: _source[type_node.byte_range()].to_string(),
+                })
+            }
+        };
+
+        Ok(r#type)
+    }
+
+    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+        Ok(Value::none())
+    }
+
+    fn expected_type(&self, _context: &Map) -> Result<Type> {
+        Ok(Type::None)
+    }
+}
+
+impl Format for Type {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        match self {
+            Type::Any => output.push_str("any"),
+            Type::Boolean => output.push_str("bool"),
+            Type::Collection => output.push_str("collection"),
+            Type::Float => output.push_str("float"),
+            Type::Function {
+                parameter_types,
+                return_type,
+            } => {
+                output.push('(');
+
+                for (index, parameter_type) in parameter_types.iter().enumerate() {
+                    parameter_type.format(output, indent_level);
+
+                    if index != parameter_types.len() - 1 {
+                        output.push(' ');
+                    }
+                }
+
+                output.push_str(") -> ");
+                return_type.format(output, indent_level);
+            }
+            Type::Integer => output.push_str("int"),
+            Type::List(item_type) => {
+                output.push('[');
+                item_type.format(output, indent_level);
+                output.push(']');
+            }
+            Type::Map(structure_option) => {
+                if let Some(structure) = structure_option {
+                    output.push_str(&structure.to_string());
+                } else {
+                    output.push_str("map");
+                }
+            }
+            Type::None => output.push_str("none"),
+            Type::Number => output.push_str("number"),
+            Type::String => output.push_str("str"),
+            Type::Option(optional_type) => {
+                output.push_str("option(");
+                optional_type.format(output, indent_level);
+                output.push(')');
+            }
+        }
+    }
+}
+
+impl Display for Type {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            Type::Any => write!(f, "any"),
+            Type::Boolean => write!(f, "bool"),
+            Type::Collection => write!(f, "collection"),
+            Type::Float => write!(f, "float"),
+            Type::Function {
+                parameter_types,
+                return_type,
+            } => {
+                write!(f, "(")?;
+
+                for (index, parameter_type) in parameter_types.iter().enumerate() {
+                    write!(f, "{parameter_type}")?;
+
+                    if index != parameter_types.len() - 1 {
+                        write!(f, " ")?;
+                    }
+                }
+
+                write!(f, ")")?;
+                write!(f, " -> {return_type}")
+            }
+            Type::Integer => write!(f, "int"),
+            Type::List(item_type) => write!(f, "[{item_type}]"),
+            Type::Map(_) => write!(f, "map"),
+            Type::Number => write!(f, "num"),
+            Type::None => write!(f, "none"),
+            Type::String => write!(f, "str"),
+            Type::Option(inner_type) => {
+                write!(f, "option({})", inner_type)
+            }
+        }
+    }
+}

src/abstract_tree/type_definition.rs

@@ -1,9 +1,7 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Map, Result, Structure, Value};
+use crate::{AbstractTree, Error, Format, Map, Result, Type, Value};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub struct TypeDefinition {
@@ -43,243 +41,10 @@ impl AbstractTree for TypeDefinition {
     }
 }
 
-impl Display for TypeDefinition {
+impl Format for TypeDefinition {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        write!(f, "<{}>", self.r#type)
+        output.push('<');
-    }
+        self.r#type.format(output, indent_level);
-}
+        output.push('>');
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub enum Type {
-    Any,
-    Boolean,
-    Collection,
-    Float,
-    Function {
-        parameter_types: Vec<Type>,
-        return_type: Box<Type>,
-    },
-    Integer,
-    List(Box<Type>),
-    Map(Option<Structure>),
-    None,
-    Number,
-    String,
-    Option(Box<Type>),
-}
-
-impl Type {
-    pub fn list(item_type: Type) -> Self {
-        Type::List(Box::new(item_type))
-    }
-
-    pub fn function(parameter_types: Vec<Type>, return_type: Type) -> Self {
-        Type::Function {
-            parameter_types,
-            return_type: Box::new(return_type),
-        }
-    }
-
-    pub fn option(optional_type: Type) -> Self {
-        Type::Option(Box::new(optional_type))
-    }
-
-    pub fn check(&self, other: &Type) -> Result<()> {
-        match (self, other) {
-            (Type::Any, _)
-            | (_, Type::Any)
-            | (Type::Boolean, Type::Boolean)
-            | (Type::Collection, Type::Collection)
-            | (Type::Collection, Type::List(_))
-            | (Type::List(_), Type::Collection)
-            | (Type::Collection, Type::Map(_))
-            | (Type::Map(_), Type::Collection)
-            | (Type::Collection, Type::String)
-            | (Type::String, Type::Collection)
-            | (Type::Float, Type::Float)
-            | (Type::Integer, Type::Integer)
-            | (Type::Map(_), Type::Map(_))
-            | (Type::Number, Type::Number)
-            | (Type::Number, Type::Integer)
-            | (Type::Number, Type::Float)
-            | (Type::Integer, Type::Number)
-            | (Type::Float, Type::Number)
-            | (Type::None, Type::None)
-            | (Type::String, Type::String) => Ok(()),
-            (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(())
-                } else {
-                    Err(Error::TypeCheck {
-                        expected: self.clone(),
-                        actual: other.clone(),
-                    })
-                }
-            }
-            (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(())
-                }
-            }
-            (
-                Type::Function {
-                    parameter_types: self_parameter_types,
-                    return_type: self_return_type,
-                },
-                Type::Function {
-                    parameter_types: other_parameter_types,
-                    return_type: other_return_type,
-                },
-            ) => {
-                let parameter_type_pairs = self_parameter_types
-                    .iter()
-                    .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_return_type.check(other_return_type).is_err() {
-                    Err(Error::TypeCheck {
-                        expected: self.clone(),
-                        actual: other.clone(),
-                    })
-                } else {
-                    Ok(())
-                }
-            }
-            _ => Err(Error::TypeCheck {
-                expected: self.clone(),
-                actual: other.clone(),
-            }),
-        }
-    }
-}
-
-impl AbstractTree for Type {
-    fn from_syntax_node(_source: &str, node: Node, _context: &Map) -> Result<Self> {
-        Error::expect_syntax_node(_source, "type", node)?;
-
-        let type_node = node.child(0).unwrap();
-
-        let r#type = match type_node.kind() {
-            "[" => {
-                let item_type_node = node.child(1).unwrap();
-                let item_type = Type::from_syntax_node(_source, item_type_node, _context)?;
-
-                Type::List(Box::new(item_type))
-            }
-            "any" => Type::Any,
-            "bool" => Type::Boolean,
-            "collection" => Type::Collection,
-            "float" => Type::Float,
-            "(" => {
-                let child_count = node.child_count();
-                let mut parameter_types = Vec::new();
-
-                for index in 1..child_count - 2 {
-                    let child = node.child(index).unwrap();
-
-                    if child.is_named() {
-                        let parameter_type = Type::from_syntax_node(_source, child, _context)?;
-
-                        parameter_types.push(parameter_type);
-                    }
-                }
-
-                let final_node = node.child(child_count - 1).unwrap();
-                let return_type = if final_node.is_named() {
-                    Type::from_syntax_node(_source, final_node, _context)?
-                } else {
-                    Type::None
-                };
-
-                Type::Function {
-                    parameter_types,
-                    return_type: Box::new(return_type),
-                }
-            }
-            "int" => Type::Integer,
-            "map" => Type::Map(None),
-            "num" => Type::Number,
-            "none" => Type::None,
-            "str" => Type::String,
-            "option" => {
-                let inner_type_node = node.child(2).unwrap();
-                let inner_type = Type::from_syntax_node(_source, inner_type_node, _context)?;
-
-                Type::Option(Box::new(inner_type))
-            }
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "any, bool, float, int, num, str, option, (, [ or {".to_string(),
-                    actual: type_node.kind().to_string(),
-                    location: type_node.start_position(),
-                    relevant_source: _source[type_node.byte_range()].to_string(),
-                })
-            }
-        };
-
-        Ok(r#type)
-    }
-
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
-        Ok(Value::none())
-    }
-
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        Ok(Type::None)
-    }
-}
-
-impl Display for Type {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match self {
-            Type::Any => write!(f, "any"),
-            Type::Boolean => write!(f, "bool"),
-            Type::Collection => write!(f, "collection"),
-            Type::Float => write!(f, "float"),
-            Type::Function {
-                parameter_types,
-                return_type,
-            } => {
-                write!(f, "(")?;
-
-                for (index, parameter_type) in parameter_types.iter().enumerate() {
-                    write!(f, "{parameter_type}")?;
-
-                    if index != parameter_types.len() - 1 {
-                        write!(f, " ")?;
-                    }
-                }
-
-                write!(f, ")")?;
-                write!(f, " -> {return_type}")
-            }
-            Type::Integer => write!(f, "int"),
-            Type::List(item_type) => write!(f, "[{item_type}]"),
-            Type::Map(_) => write!(f, "map"),
-            Type::Number => write!(f, "num"),
-            Type::None => write!(f, "none"),
-            Type::String => write!(f, "str"),
-            Type::Option(inner_type) => {
-                write!(f, "option({})", inner_type)
-            }
-        }
     }
 }

src/abstract_tree/value_node.rs

@@ -1,14 +1,11 @@
-use std::{
+use std::{collections::BTreeMap, fmt::Write};
-    collections::BTreeMap,
-    fmt::{self, Display, Formatter},
-};
 
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, BuiltInValue, Error, Expression, Function, Identifier, List, Map, Result,
+    AbstractTree, BuiltInValue, Error, Expression, Format, Function, FunctionNode, Identifier,
-    Statement, Structure, Type, TypeDefinition, Value,
+    List, Map, Result, Statement, Structure, Type, TypeDefinition, Value,
 };
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
@@ -33,7 +30,11 @@ impl AbstractTree for ValueNode {
         let value_node = match child.kind() {
             "boolean" => ValueNode::Boolean(source[child.byte_range()].to_string()),
             "float" => ValueNode::Float(source[child.byte_range()].to_string()),
-            "function" => ValueNode::Function(Function::from_syntax_node(source, child, context)?),
+            "function" => {
+                let function_node = FunctionNode::from_syntax_node(source, child, context)?;
+
+                ValueNode::Function(Function::ContextDefined(function_node))
+            }
             "integer" => ValueNode::Integer(source[child.byte_range()].to_string()),
             "string" => {
                 let without_quotes = child.start_byte() + 1..child.end_byte() - 1;
@@ -300,53 +301,75 @@ impl AbstractTree for ValueNode {
     }
 }
 
-impl Display for ValueNode {
+impl Format for ValueNode {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
         match self {
             ValueNode::Boolean(source)
             | ValueNode::Float(source)
             | ValueNode::Integer(source)
-            | ValueNode::String(source) => write!(f, "{source}"),
+            | ValueNode::String(source) => output.push_str(source),
-            ValueNode::Function(function) => write!(f, "{function}"),
+            ValueNode::Function(function) => function.format(output, indent_level),
             ValueNode::List(expressions) => {
+                output.push('[');
+
                 for expression in expressions {
-                    write!(f, "{expression}")?;
+                    expression.format(output, indent_level);
                 }
 
-                Ok(())
+                output.push(']');
             }
             ValueNode::Option(option) => {
                 if let Some(expression) = option {
-                    write!(f, "some({})", expression)
+                    output.push_str("some(");
+                    expression.format(output, indent_level);
+                    output.push(')');
                 } else {
-                    write!(f, "none")
+                    output.push_str("none");
                 }
             }
             ValueNode::Map(nodes) => {
-                writeln!(f, "{{")?;
+                output.push('{');
 
                 for (key, (statement, type_option)) in nodes {
                     if let Some(r#type) = type_option {
-                        writeln!(f, "  {key} <{}> = {statement}", r#type)?;
+                        output.push_str("    ");
+                        output.push_str(key);
+                        output.push_str(" <");
+                        r#type.format(output, indent_level);
+                        output.push_str("> = ");
+                        statement.format(output, indent_level);
                     } else {
-                        writeln!(f, "  {key} = {statement}")?;
+                        output.push_str("    ");
+                        output.push_str(key);
+                        output.push_str(" = ");
+                        statement.format(output, indent_level);
                     }
                 }
-                write!(f, "}}")
+
+                output.push('}');
             }
-            ValueNode::BuiltInValue(built_in_value) => write!(f, "{built_in_value}"),
+            ValueNode::BuiltInValue(built_in_value) => built_in_value.format(output, indent_level),
             ValueNode::Structure(nodes) => {
-                writeln!(f, "{{")?;
+                output.push('{');
 
                 for (key, (value_option, r#type)) in nodes {
                     if let Some(value) = value_option {
-                        writeln!(f, "  {key} <{}> = {value}", r#type)?;
+                        output.push_str("    ");
+                        output.push_str(key);
+                        output.push_str(" <");
+                        r#type.format(output, indent_level);
+                        output.push_str("> = ");
+                        value.format(output, indent_level);
                     } else {
-                        writeln!(f, "  {key} <{}>", r#type)?;
+                        output.push_str("    ");
+                        output.push_str(key);
+                        output.push_str(" <");
+                        r#type.format(output, indent_level);
+                        output.push('>');
                     }
                 }
 
-                write!(f, "}}")
+                output.push('}');
             }
         }
     }

src/abstract_tree/while.rs

@@ -1,9 +1,7 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
-use crate::{AbstractTree, Block, Error, Expression, Map, Result, Type, Value};
+use crate::{AbstractTree, Block, Error, Expression, Format, Map, Result, Type, Value};
 
 /// Abstract representation of a while loop.
 ///
@@ -40,10 +38,11 @@ impl AbstractTree for While {
     }
 }
 
-impl Display for While {
+impl Format for While {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let While { expression, block } = self;
+        output.push_str("while ");
-
+        self.expression.format(output, indent_level);
-        write!(f, "while {expression} {{{block}}}")
+        output.push(' ');
+        self.expression.format(output, indent_level);
     }
 }

src/abstract_tree/yield.rs

@@ -1,11 +1,9 @@
-use std::fmt::{self, Display, Formatter};
-
 use serde::{Deserialize, Serialize};
 use tree_sitter::Node;
 
 use crate::{
-    function_expression::FunctionExpression, AbstractTree, Error, Expression, FunctionCall, Map,
+    function_expression::FunctionExpression, AbstractTree, Error, Expression, Format, FunctionCall,
-    Result, Type, Value,
+    Map, Result, Type, Value,
 };
 
 /// Abstract representation of a yield expression.
@@ -55,8 +53,8 @@ impl AbstractTree for Yield {
     }
 }
 
-impl Display for Yield {
+impl Format for Yield {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        write!(f, "{}", self.call)
+        self.call.format(output, indent_level);
     }
 }

src/built_in_functions/mod.rs

@@ -1,11 +1,14 @@
 mod string;
 
-use std::fs::read_to_string;
+use std::{
+    fmt::{self, Display, Formatter},
+    fs::read_to_string,
+};
 
 use rand::{random, thread_rng, Rng};
 use serde::{Deserialize, Serialize};
 
-use crate::{Error, Map, Result, Type, Value};
+use crate::{Error, Format, Map, Result, Type, Value};
 
 pub use string::{string_functions, StringFunction};
 
@@ -148,3 +151,15 @@ impl BuiltInFunction {
         }
     }
 }
+
+impl Format for BuiltInFunction {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        output.push_str(self.name());
+    }
+}
+
+impl Display for BuiltInFunction {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{}", self.name())
+    }
+}

src/interpret.rs

@@ -4,7 +4,7 @@
 //! functions or by constructing your own Evaluator.
 use tree_sitter::{Node, Parser, Tree as TSTree, TreeCursor};
 
-use crate::{language, AbstractTree, Error, Map, Result, Root, Value};
+use crate::{language, AbstractTree, Error, Format, Map, Result, Root, Value};
 
 /// Interpret the given source code.
 ///
@@ -132,7 +132,11 @@ impl Interpreter {
 
     pub fn format(&self) -> String {
         if let Some(root_node) = &self.abstract_tree {
-            root_node.to_string()
+            let mut formatted_source = String::new();
+
+            root_node.format(&mut formatted_source, 0);
+
+            formatted_source
         } else {
             "".to_string()
         }

src/value/function.rs

@@ -1,9 +1,8 @@
 use std::fmt::{self, Display, Formatter};
 
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, BuiltInFunction, FunctionNode, Map, Result, Type, Value};
+use crate::{BuiltInFunction, Format, FunctionNode, Map, Result, Type, Value};
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
 pub enum Function {
@@ -11,17 +10,6 @@ pub enum Function {
     ContextDefined(FunctionNode),
 }
 
-impl Display for Function {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match self {
-            Function::BuiltIn(built_in_function) => write!(f, "{}", built_in_function.r#type()),
-            Function::ContextDefined(function_node) => {
-                write!(f, "{}", function_node)
-            }
-        }
-    }
-}
-
 impl Function {
     pub fn call(
         &self,
@@ -50,30 +38,20 @@ impl Function {
     }
 }
 
-impl AbstractTree for Function {
+impl Format for Function {
-    fn from_syntax_node(_source: &str, _node: Node, _context: &Map) -> Result<Self> {
+    fn format(&self, output: &mut String, indent_level: u8) {
-        let inner_function = FunctionNode::from_syntax_node(_source, _node, _context)?;
-
-        Ok(Function::ContextDefined(inner_function))
-    }
-
-    fn check_type(&self, _source: &str, _context: &Map) -> Result<()> {
         match self {
-            Function::BuiltIn(_) => Ok(()),
+            Function::BuiltIn(built_in_function) => built_in_function.format(output, indent_level),
-            Function::ContextDefined(defined_function) => {
+            Function::ContextDefined(function_node) => function_node.format(output, indent_level),
-                defined_function.check_type(_source, _context)
+        }
-            }
+    }
-        }
+}
-    }
+
-
+impl Display for Function {
-    fn run(&self, _source: &str, _context: &Map) -> Result<Value> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        Ok(Value::Function(self.clone()))
+        match self {
-    }
+            Function::BuiltIn(built_in_function) => write!(f, "{built_in_function}"),
-
+            Function::ContextDefined(function_node) => write!(f, "{function_node}"),
-    fn expected_type(&self, _context: &Map) -> Result<Type> {
-        match self {
-            Function::BuiltIn(built_in) => Ok(built_in.r#type()),
-            Function::ContextDefined(context_defined) => Ok(context_defined.r#type().clone()),
         }
     }
 }

src/value/mod.rs

@@ -527,7 +527,7 @@ impl Serialize for Value {
 impl Display for Value {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         match self {
-            Value::String(string) => write!(f, "{}", string.read().unwrap()),
+            Value::String(string) => write!(f, "'{}'", string.read().unwrap()),
             Value::Float(float) => write!(f, "{float}"),
             Value::Integer(int) => write!(f, "{int}"),
             Value::Boolean(boolean) => write!(f, "{boolean}"),