From eb08b3bb37f42c00144b2261e5aab398234d6659 Mon Sep 17 00:00:00 2001
From: Eduard Urbach <eduard@urbach.dev>
Date: Sat, 21 Jun 2025 20:18:27 +0200
Subject: [PATCH] Added expression package

---
 src/expression/Expression.go      |  94 +++++++++++++++
 src/expression/Expression_test.go | 182 ++++++++++++++++++++++++++++++
 src/expression/New.go             |   6 +
 src/expression/NewLeaf.go         |   8 ++
 src/expression/NewList.go         |  17 +++
 src/expression/Parse.go           | 173 ++++++++++++++++++++++++++++
 src/expression/bench_test.go      |  17 +++
 src/expression/isComplete.go      |  27 +++++
 src/expression/operator.go        |  67 +++++++++++
 src/expression/write.go           |  33 ++++++
 10 files changed, 624 insertions(+)
 create mode 100644 src/expression/Expression.go
 create mode 100644 src/expression/Expression_test.go
 create mode 100644 src/expression/New.go
 create mode 100644 src/expression/NewLeaf.go
 create mode 100644 src/expression/NewList.go
 create mode 100644 src/expression/Parse.go
 create mode 100644 src/expression/bench_test.go
 create mode 100644 src/expression/isComplete.go
 create mode 100644 src/expression/operator.go
 create mode 100644 src/expression/write.go

diff --git a/src/expression/Expression.go b/src/expression/Expression.go
new file mode 100644
index 0000000..3450120
--- /dev/null
+++ b/src/expression/Expression.go
@@ -0,0 +1,94 @@
+package expression
+
+import (
+	"strings"
+
+	"git.urbach.dev/cli/q/src/token"
+)
+
+// Expression is a tree that can represent a mathematical expression with precedence levels.
+type Expression struct {
+	Parent     *Expression
+	Children   []*Expression
+	Token      token.Token
+	precedence int8
+}
+
+// AddChild adds a child to the expression.
+func (expr *Expression) AddChild(child *Expression) {
+	if expr.Children == nil {
+		expr.Children = make([]*Expression, 0, 2)
+	}
+
+	expr.Children = append(expr.Children, child)
+	child.Parent = expr
+}
+
+// Reset resets all values to the default.
+func (expr *Expression) Reset() {
+	expr.Parent = nil
+
+	if expr.Children != nil {
+		expr.Children = expr.Children[:0]
+	}
+
+	expr.Token.Reset()
+	expr.precedence = 0
+}
+
+// EachLeaf iterates through all leaves in the tree.
+func (expr *Expression) EachLeaf(call func(*Expression) error) error {
+	if expr.IsLeaf() {
+		return call(expr)
+	}
+
+	for _, child := range expr.Children {
+		err := child.EachLeaf(call)
+
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// RemoveChild removes a child from the expression.
+func (expr *Expression) RemoveChild(child *Expression) {
+	for i, c := range expr.Children {
+		if c == child {
+			expr.Children = append(expr.Children[:i], expr.Children[i+1:]...)
+			child.Parent = nil
+			return
+		}
+	}
+}
+
+// InsertAbove replaces this expression in its parent's children with the given new parent,
+// and attaches this expression as a child of the new parent. Effectively, it promotes the
+// given tree above the current node. It assumes that the caller is the last child.
+func (expr *Expression) InsertAbove(tree *Expression) {
+	if expr.Parent != nil {
+		expr.Parent.Children[len(expr.Parent.Children)-1] = tree
+		tree.Parent = expr.Parent
+	}
+
+	tree.AddChild(expr)
+}
+
+// IsLeaf returns true if the expression has no children.
+func (expr *Expression) IsLeaf() bool {
+	return len(expr.Children) == 0
+}
+
+// LastChild returns the last child.
+func (expr *Expression) LastChild() *Expression {
+	return expr.Children[len(expr.Children)-1]
+}
+
+// String generates a textual representation of the expression.
+func (expr *Expression) String(source []byte) string {
+	builder := strings.Builder{}
+	expr.write(&builder, source)
+	return builder.String()
+}
\ No newline at end of file
diff --git a/src/expression/Expression_test.go b/src/expression/Expression_test.go
new file mode 100644
index 0000000..1303305
--- /dev/null
+++ b/src/expression/Expression_test.go
@@ -0,0 +1,182 @@
+package expression_test
+
+import (
+	"errors"
+	"testing"
+
+	"git.urbach.dev/cli/q/src/expression"
+	"git.urbach.dev/cli/q/src/token"
+	"git.urbach.dev/go/assert"
+)
+
+func TestParse(t *testing.T) {
+	tests := []struct {
+		Name       string
+		Expression string
+		Result     string
+	}{
+		{"Identity", "1", "1"},
+		{"Basic calculation", "1+2", "(+ 1 2)"},
+
+		{"Same operator", "1+2+3", "(+ (+ 1 2) 3)"},
+		{"Same operator 2", "1+2+3+4", "(+ (+ (+ 1 2) 3) 4)"},
+
+		{"Different operator", "1+2-3", "(- (+ 1 2) 3)"},
+		{"Different operator 2", "1+2-3+4", "(+ (- (+ 1 2) 3) 4)"},
+		{"Different operator 3", "1+2-3+4-5", "(- (+ (- (+ 1 2) 3) 4) 5)"},
+
+		{"Grouped identity", "(1)", "1"},
+		{"Grouped identity 2", "((1))", "1"},
+		{"Grouped identity 3", "(((1)))", "1"},
+
+		{"Adding identity", "(1)+(2)", "(+ 1 2)"},
+		{"Adding identity 2", "(1)+(2)+(3)", "(+ (+ 1 2) 3)"},
+		{"Adding identity 3", "(1)+(2)+(3)+(4)", "(+ (+ (+ 1 2) 3) 4)"},
+
+		{"Grouping", "(1+2)", "(+ 1 2)"},
+		{"Grouping 2", "(1+2+3)", "(+ (+ 1 2) 3)"},
+		{"Grouping 3", "((1)+(2)+(3))", "(+ (+ 1 2) 3)"},
+		{"Grouping left", "(1+2)*3", "(* (+ 1 2) 3)"},
+		{"Grouping right", "1*(2+3)", "(* 1 (+ 2 3))"},
+		{"Grouping same operator", "1+(2+3)", "(+ 1 (+ 2 3))"},
+		{"Grouping same operator 2", "1+(2+3)+(4+5)", "(+ (+ 1 (+ 2 3)) (+ 4 5))"},
+
+		{"Two groups", "(1+2)*(3+4)", "(* (+ 1 2) (+ 3 4))"},
+		{"Two groups 2", "(1+2-3)*(3+4-5)", "(* (- (+ 1 2) 3) (- (+ 3 4) 5))"},
+		{"Two groups 3", "(1+2)*(3+4-5)", "(* (+ 1 2) (- (+ 3 4) 5))"},
+
+		{"Operator priority", "1+2*3", "(+ 1 (* 2 3))"},
+		{"Operator priority 2", "1*2+3", "(+ (* 1 2) 3)"},
+		{"Operator priority 3", "1+2*3+4", "(+ (+ 1 (* 2 3)) 4)"},
+		{"Operator priority 4", "1+2*(3+4)+5", "(+ (+ 1 (* 2 (+ 3 4))) 5)"},
+		{"Operator priority 5", "1+2*3*4", "(+ 1 (* (* 2 3) 4))"},
+		{"Operator priority 6", "1+2*3+4*5", "(+ (+ 1 (* 2 3)) (* 4 5))"},
+		{"Operator priority 7", "1+2*3*4*5*6", "(+ 1 (* (* (* (* 2 3) 4) 5) 6))"},
+		{"Operator priority 8", "1*2*3+4*5*6", "(+ (* (* 1 2) 3) (* (* 4 5) 6))"},
+
+		{"Complex", "(1+2-3*4)*(5+6-7*8)", "(* (- (+ 1 2) (* 3 4)) (- (+ 5 6) (* 7 8)))"},
+		{"Complex 2", "(1+2*3-4)*(5+6*7-8)", "(* (- (+ 1 (* 2 3)) 4) (- (+ 5 (* 6 7)) 8))"},
+		{"Complex 3", "(1+2*3-4)*(5+6*7-8)+9-10*11", "(- (+ (* (- (+ 1 (* 2 3)) 4) (- (+ 5 (* 6 7)) 8)) 9) (* 10 11))"},
+
+		{"Unary not", "!", "!"},
+		{"Unary not 2", "!a", "(! a)"},
+		{"Unary not 3", "!(!a)", "(! (! a))"},
+		{"Unary not 4", "!(a||b)", "(! (|| a b))"},
+		{"Unary not 5", "a || !b", "(|| a (! b))"},
+
+		{"Unary minus", "-", "-"},
+		{"Unary minus 2", "-a", "(- a)"},
+		{"Unary minus 3", "-(-a)", "(- (- a))"},
+		{"Unary minus 4", "-a+b", "(+ (- a) b)"},
+		{"Unary minus 5", "-(a+b)", "(- (+ a b))"},
+		{"Unary minus 6", "a + -b", "(+ a (- b))"},
+		{"Unary minus 7", "-a + -b", "(+ (- a) (- b))"},
+
+		{"Assign bitwise operation", "a|=b", "(|= a b)"},
+		{"Assign bitwise operation 2", "a|=b<<c", "(|= a (<< b c))"},
+
+		{"Function calls", "a()", "(λ a)"},
+		{"Function calls 2", "a(1)", "(λ a 1)"},
+		{"Function calls 3", "a(1)+1", "(+ (λ a 1) 1)"},
+		{"Function calls 4", "1+a(1)", "(+ 1 (λ a 1))"},
+		{"Function calls 5", "a(1,2)", "(λ a 1 2)"},
+		{"Function calls 6", "a(1,2,3)", "(λ a 1 2 3)"},
+		{"Function calls 7", "a(1,2+2,3)", "(λ a 1 (+ 2 2) 3)"},
+		{"Function calls 8", "a(1,2+2,3+3)", "(λ a 1 (+ 2 2) (+ 3 3))"},
+		{"Function calls 9", "a(1+1,2,3)", "(λ a (+ 1 1) 2 3)"},
+		{"Function calls 10", "a(1+1,2+2,3+3)", "(λ a (+ 1 1) (+ 2 2) (+ 3 3))"},
+		{"Function calls 11", "a(b())", "(λ a (λ b))"},
+		{"Function calls 12", "a(b(),c())", "(λ a (λ b) (λ c))"},
+		{"Function calls 13", "a(b(),c(),d())", "(λ a (λ b) (λ c) (λ d))"},
+		{"Function calls 14", "a(b(1))", "(λ a (λ b 1))"},
+		{"Function calls 15", "a(b(1),c(2),d(3))", "(λ a (λ b 1) (λ c 2) (λ d 3))"},
+		{"Function calls 16", "a(b(1)+1)", "(λ a (+ (λ b 1) 1))"},
+		{"Function calls 17", "a(b(1)+1,c(2),d(3))", "(λ a (+ (λ b 1) 1) (λ c 2) (λ d 3))"},
+		{"Function calls 18", "a(b(1)*c(2))", "(λ a (* (λ b 1) (λ c 2)))"},
+		{"Function calls 19", "a(b(1)*c(2),d(3)+e(4),f(5)/f(6))", "(λ a (* (λ b 1) (λ c 2)) (+ (λ d 3) (λ e 4)) (/ (λ f 5) (λ f 6)))"},
+		{"Function calls 20", "a(b(1,2)+c(3,4)*d(5,6))", "(λ a (+ (λ b 1 2) (* (λ c 3 4) (λ d 5 6))))"},
+		{"Function calls 21", "a((b(1,2)+c(3,4))*d(5,6))", "(λ a (* (+ (λ b 1 2) (λ c 3 4)) (λ d 5 6)))"},
+		{"Function calls 22", "a((b(1,2)+c(3,4))*d(5,6),e())", "(λ a (* (+ (λ b 1 2) (λ c 3 4)) (λ d 5 6)) (λ e))"},
+		{"Function calls 23", "a((b(1,2)+c(3,4))*d(5,6),e(7+8,9-10*11,12))", "(λ a (* (+ (λ b 1 2) (λ c 3 4)) (λ d 5 6)) (λ e (+ 7 8) (- 9 (* 10 11)) 12))"},
+		{"Function calls 24", "a((b(1,2,bb())+c(3,4,cc(0)))*d(5,6,dd(0)),e(7+8,9-10*11,12,ee(0)))", "(λ a (* (+ (λ b 1 2 (λ bb)) (λ c 3 4 (λ cc 0))) (λ d 5 6 (λ dd 0))) (λ e (+ 7 8) (- 9 (* 10 11)) 12 (λ ee 0)))"},
+		{"Function calls 25", "a(1-2*3)", "(λ a (- 1 (* 2 3)))"},
+		{"Function calls 26", "1+2*a()+4", "(+ (+ 1 (* 2 (λ a))) 4)"},
+		{"Function calls 27", "a(b,c)*2+15*4", "(+ (* (λ a b c) 2) (* 15 4))"},
+
+		{"Package function calls", "a.b(c)", "(λ (. a b) c)"},
+		{"Package function calls 2", "a.b(c,d)", "(λ (. a b) c d)"},
+		{"Package function calls 3", "a.b.c(d,e)", "(λ (. (. a b) c) d e)"},
+
+		{"Array access", "a[0]", "(@ a 0)"},
+		{"Array access 2", "a[b+c]", "(@ a (+ b c))"},
+		{"Array access 3", "a.b[c]", "(@ (. a b) c)"},
+		{"Array access 4", "a.b[c+d]", "(@ (. a b) (+ c d))"},
+		{"Array access 5", "a()[b]", "(@ (λ a) b)"},
+		{"Array access 6", "a.b()[c]", "(@ (λ (. a b)) c)"},
+		{"Array access 7", "a.b(c)[d]", "(@ (λ (. a b) c) d)"},
+		{"Array access 8", "a.b(c)[d][e]", "(@ (@ (λ (. a b) c) d) e)"},
+		{"Array access 9", "a[0](1)[2](3)", "(λ (@ (λ (@ a 0) 1) 2) 3)"},
+
+		{"Dereferencing", "[a]", "(@ a)"},
+		{"Dereferencing 2", "[a+b]", "(@ (+ a b))"},
+		{"Dereferencing 3", "[a+b]=c", "(= (@ (+ a b)) c)"},
+		{"Dereferencing 4", "[a+b]=c+d", "(= (@ (+ a b)) (+ c d))"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.Name, func(t *testing.T) {
+			src := []byte(test.Expression)
+			tokens := token.Tokenize(src)
+			expr := expression.Parse(tokens)
+			defer expr.Reset()
+			assert.NotNil(t, expr)
+			assert.Equal(t, expr.String(src), test.Result)
+		})
+	}
+}
+
+func TestEachLeaf(t *testing.T) {
+	src := []byte("(1+2-3*4)+(5*6-7+8)")
+	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	leaves := []string{}
+
+	err := expr.EachLeaf(func(leaf *expression.Expression) error {
+		leaves = append(leaves, leaf.Token.String(src))
+		return nil
+	})
+
+	assert.Nil(t, err)
+	assert.DeepEqual(t, leaves, []string{"1", "2", "3", "4", "5", "6", "7", "8"})
+
+	err = expr.EachLeaf(func(leaf *expression.Expression) error {
+		return errors.New("error")
+	})
+
+	assert.NotNil(t, err)
+	assert.Equal(t, err.Error(), "error")
+}
+
+func TestRemoveChild(t *testing.T) {
+	src := []byte("(1+2-3*4)+(5*6-7+8)")
+	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	left := expr.Children[0]
+	right := expr.Children[1]
+	expr.RemoveChild(left)
+	assert.Equal(t, expr.Children[0], right)
+}
+
+func TestNilExpression(t *testing.T) {
+	src := []byte("")
+	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	assert.Nil(t, expr)
+}
+
+func TestNilGroup(t *testing.T) {
+	src := []byte("()")
+	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	assert.Nil(t, expr)
+}
\ No newline at end of file
diff --git a/src/expression/New.go b/src/expression/New.go
new file mode 100644
index 0000000..0d49d33
--- /dev/null
+++ b/src/expression/New.go
@@ -0,0 +1,6 @@
+package expression
+
+// New creates a new expression.
+func New() *Expression {
+	return &Expression{}
+}
\ No newline at end of file
diff --git a/src/expression/NewLeaf.go b/src/expression/NewLeaf.go
new file mode 100644
index 0000000..c618e12
--- /dev/null
+++ b/src/expression/NewLeaf.go
@@ -0,0 +1,8 @@
+package expression
+
+import "git.urbach.dev/cli/q/src/token"
+
+// NewLeaf creates a new leaf node.
+func NewLeaf(t token.Token) *Expression {
+	return &Expression{Token: t}
+}
\ No newline at end of file
diff --git a/src/expression/NewList.go b/src/expression/NewList.go
new file mode 100644
index 0000000..c75e441
--- /dev/null
+++ b/src/expression/NewList.go
@@ -0,0 +1,17 @@
+package expression
+
+import (
+	"git.urbach.dev/cli/q/src/token"
+)
+
+// NewList generates a list of expressions from comma separated parameters.
+func NewList(tokens token.List) []*Expression {
+	var list []*Expression
+
+	for param := range tokens.Split {
+		expression := Parse(param)
+		list = append(list, expression)
+	}
+
+	return list
+}
\ No newline at end of file
diff --git a/src/expression/Parse.go b/src/expression/Parse.go
new file mode 100644
index 0000000..110096c
--- /dev/null
+++ b/src/expression/Parse.go
@@ -0,0 +1,173 @@
+package expression
+
+import (
+	"math"
+
+	"git.urbach.dev/cli/q/src/token"
+)
+
+// Parse generates an expression tree from tokens.
+func Parse(tokens []token.Token) *Expression {
+	var (
+		cursor        *Expression
+		root          *Expression
+		groupLevel    = 0
+		groupPosition = 0
+	)
+
+	for i, t := range tokens {
+		if t.Kind == token.GroupStart || t.Kind == token.ArrayStart {
+			groupLevel++
+
+			if groupLevel == 1 {
+				groupPosition = i + 1
+			}
+
+			continue
+		}
+
+		if t.Kind == token.GroupEnd || t.Kind == token.ArrayEnd {
+			groupLevel--
+
+			if groupLevel != 0 {
+				continue
+			}
+
+			// Function call or array access
+			if isComplete(cursor) {
+				parameters := NewList(tokens[groupPosition:i])
+				node := New()
+				node.Token.Position = tokens[groupPosition].Position
+
+				switch t.Kind {
+				case token.GroupEnd:
+					node.Token.Kind = token.Call
+				case token.ArrayEnd:
+					node.Token.Kind = token.Array
+				}
+
+				node.precedence = precedence(node.Token.Kind)
+
+				if cursor.Token.IsOperator() && node.precedence > cursor.precedence {
+					cursor.LastChild().InsertAbove(node)
+				} else {
+					if cursor == root {
+						root = node
+					}
+
+					cursor.InsertAbove(node)
+				}
+
+				for _, param := range parameters {
+					node.AddChild(param)
+				}
+
+				cursor = node
+				continue
+			}
+
+			group := Parse(tokens[groupPosition:i])
+
+			if group == nil {
+				continue
+			}
+
+			group.precedence = math.MaxInt8
+
+			if cursor == nil {
+				if t.Kind == token.ArrayEnd {
+					cursor = New()
+					cursor.Token.Position = tokens[groupPosition].Position
+					cursor.Token.Kind = token.Array
+					cursor.precedence = precedence(token.Array)
+					cursor.AddChild(group)
+					root = cursor
+				} else {
+					cursor = group
+					root = group
+				}
+			} else {
+				cursor.AddChild(group)
+			}
+
+			continue
+		}
+
+		if groupLevel > 0 {
+			continue
+		}
+
+		if t.Kind == token.Identifier || t.Kind == token.Number || t.Kind == token.String || t.Kind == token.Rune {
+			if cursor != nil {
+				node := NewLeaf(t)
+				cursor.AddChild(node)
+			} else {
+				cursor = NewLeaf(t)
+				root = cursor
+			}
+
+			continue
+		}
+
+		if !t.IsOperator() {
+			continue
+		}
+
+		if cursor == nil {
+			cursor = NewLeaf(t)
+			cursor.precedence = precedence(t.Kind)
+			root = cursor
+			continue
+		}
+
+		node := NewLeaf(t)
+		node.precedence = precedence(t.Kind)
+
+		if cursor.Token.IsOperator() {
+			oldPrecedence := cursor.precedence
+			newPrecedence := node.precedence
+
+			if newPrecedence > oldPrecedence {
+				if len(cursor.Children) == numOperands(cursor.Token.Kind) {
+					cursor.LastChild().InsertAbove(node)
+				} else {
+					cursor.AddChild(node)
+				}
+			} else {
+				start := cursor
+
+				for start != nil {
+					precedence := start.precedence
+
+					if precedence < newPrecedence {
+						start.LastChild().InsertAbove(node)
+						break
+					}
+
+					if precedence == newPrecedence {
+						if start == root {
+							root = node
+						}
+
+						start.InsertAbove(node)
+						break
+					}
+
+					start = start.Parent
+				}
+
+				if start == nil {
+					root.InsertAbove(node)
+					root = node
+				}
+			}
+		} else {
+			node.AddChild(cursor)
+			root = node
+		}
+
+		cursor = node
+	}
+
+	return root
+}
\ No newline at end of file
diff --git a/src/expression/bench_test.go b/src/expression/bench_test.go
new file mode 100644
index 0000000..db1cdb4
--- /dev/null
+++ b/src/expression/bench_test.go
@@ -0,0 +1,17 @@
+package expression_test
+
+import (
+	"testing"
+
+	"git.urbach.dev/cli/q/src/expression"
+	"git.urbach.dev/cli/q/src/token"
+)
+
+func BenchmarkExpression(b *testing.B) {
+	src := []byte("(1+2-3*4)+(5*6-7+8)")
+	tokens := token.Tokenize(src)
+
+	for b.Loop() {
+		expression.Parse(tokens)
+	}
+}
\ No newline at end of file
diff --git a/src/expression/isComplete.go b/src/expression/isComplete.go
new file mode 100644
index 0000000..16f9667
--- /dev/null
+++ b/src/expression/isComplete.go
@@ -0,0 +1,27 @@
+package expression
+
+import "git.urbach.dev/cli/q/src/token"
+
+// isComplete returns true if the expression is complete (a binary operation with a single operand is incomplete).
+func isComplete(expr *Expression) bool {
+	if expr == nil {
+		return false
+	}
+
+	switch expr.Token.Kind {
+	case token.Identifier, token.Number, token.String:
+		// These aren't operators but they always count as complete expressions.
+		return true
+	case token.Call:
+		// Even though token.Call is an operator and could be handled by the upcoming branch,
+		// the number of operands is variable.
+		// Therefore we consider every single call expression as complete.
+		return true
+	}
+
+	if expr.Token.IsOperator() && len(expr.Children) == numOperands(expr.Token.Kind) {
+		return true
+	}
+
+	return false
+}
\ No newline at end of file
diff --git a/src/expression/operator.go b/src/expression/operator.go
new file mode 100644
index 0000000..d94eb56
--- /dev/null
+++ b/src/expression/operator.go
@@ -0,0 +1,67 @@
+package expression
+
+import (
+	"math"
+
+	"git.urbach.dev/cli/q/src/token"
+)
+
+// operator represents an operator for mathematical expressions.
+type operator struct {
+	Symbol     string
+	Precedence int8
+	Operands   int8
+}
+
+// operators defines the operators used in the language.
+// The number corresponds to the operator priority and can not be zero.
+var operators = [64]operator{
+	token.Dot:    {".", 13, 2},
+	token.Call:   {"λ", 12, 1},
+	token.Array:  {"@", 12, 2},
+	token.Negate: {"-", 11, 1},
+	token.Not:    {"!", 11, 1},
+	token.Mul:    {"*", 10, 2},
+	token.Div:    {"/", 10, 2},
+	token.Mod:    {"%", 10, 2},
+	token.Add:    {"+", 9, 2},
+	token.Sub:    {"-", 9, 2},
+	token.Shr:    {">>", 8, 2},
+	token.Shl:    {"<<", 8, 2},
+	token.And:    {"&", 7, 2},
+	token.Xor:    {"^", 6, 2},
+	token.Or:     {"|", 5, 2},
+
+	token.Greater:      {">", 4, 2},
+	token.Less:         {"<", 4, 2},
+	token.GreaterEqual: {">=", 4, 2},
+	token.LessEqual:    {"<=", 4, 2},
+	token.Equal:        {"==", 3, 2},
+	token.NotEqual:     {"!=", 3, 2},
+	token.LogicalAnd:   {"&&", 2, 2},
+	token.LogicalOr:    {"||", 1, 2},
+
+	token.Range:     {"..", 0, 2},
+	token.Separator: {",", 0, 2},
+
+	token.Assign:    {"=", math.MinInt8, 2},
+	token.Define:    {":=", math.MinInt8, 2},
+	token.AddAssign: {"+=", math.MinInt8, 2},
+	token.SubAssign: {"-=", math.MinInt8, 2},
+	token.MulAssign: {"*=", math.MinInt8, 2},
+	token.DivAssign: {"/=", math.MinInt8, 2},
+	token.ModAssign: {"%=", math.MinInt8, 2},
+	token.AndAssign: {"&=", math.MinInt8, 2},
+	token.OrAssign:  {"|=", math.MinInt8, 2},
+	token.XorAssign: {"^=", math.MinInt8, 2},
+	token.ShrAssign: {">>=", math.MinInt8, 2},
+	token.ShlAssign: {"<<=", math.MinInt8, 2},
+}
+
+func numOperands(symbol token.Kind) int {
+	return int(operators[symbol].Operands)
+}
+
+func precedence(symbol token.Kind) int8 {
+	return operators[symbol].Precedence
+}
\ No newline at end of file
diff --git a/src/expression/write.go b/src/expression/write.go
new file mode 100644
index 0000000..5db5f14
--- /dev/null
+++ b/src/expression/write.go
@@ -0,0 +1,33 @@
+package expression
+
+import (
+	"strings"
+
+	"git.urbach.dev/cli/q/src/token"
+)
+
+// write generates a textual representation of the expression.
+func (expr *Expression) write(builder *strings.Builder, source []byte) {
+	if expr.IsLeaf() {
+		builder.WriteString(expr.Token.String(source))
+		return
+	}
+
+	builder.WriteByte('(')
+
+	switch expr.Token.Kind {
+	case token.Call:
+		builder.WriteString(operators[token.Call].Symbol)
+	case token.Array:
+		builder.WriteString(operators[token.Array].Symbol)
+	default:
+		builder.WriteString(expr.Token.String(source))
+	}
+
+	for _, child := range expr.Children {
+		builder.WriteByte(' ')
+		child.write(builder, source)
+	}
+
+	builder.WriteByte(')')
+}
\ No newline at end of file