feat: add De Bruijn indexed reduction engine

Add a new interpreter option (-i debruijn) that uses De Bruijn indices
for variable representation, eliminating the need for variable renaming
during substitution.

- Add -i flag to select interpreter (lambda or debruijn)
- Create debruijn package with Expression types (Variable with index,
  Abstraction without parameter, Application)
- Implement shift and substitute operations for De Bruijn indices
- Add conversion functions between lambda and De Bruijn representations
- Update CLI to support switching between interpreters
- Add De Bruijn tests to verify all samples pass

Closes #26

pkg/convert/debruijn_to_lambda.go

@@ -0,0 +1,82 @@
+package convert
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/pkg/debruijn"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+	"git.maximhutz.com/max/lambda/pkg/set"
+)
+
+// DeBruijnToLambda converts a De Bruijn indexed expression back to named lambda calculus.
+func DeBruijnToLambda(expr debruijn.Expression) lambda.Expression {
+	return deBruijnToLambdaWithContext(expr, []string{})
+}
+
+func deBruijnToLambdaWithContext(expr debruijn.Expression, context []string) lambda.Expression {
+	switch e := expr.(type) {
+	case *debruijn.Variable:
+		index := e.Index()
+		if index < len(context) {
+			// Bound variable: look up name in context.
+			name := context[len(context)-1-index]
+			return lambda.NewVariable(name)
+		}
+		// Free variable: use the label if available.
+		if e.Label() != "" {
+			return lambda.NewVariable(e.Label())
+		}
+		// Generate a name for free variables without labels.
+		return lambda.NewVariable(fmt.Sprintf("free%d", index))
+
+	case *debruijn.Abstraction:
+		// Generate a fresh parameter name.
+		used := collectUsedNames(e.Body(), context)
+		paramName := generateFreshName(used)
+		newContext := append(context, paramName)
+		body := deBruijnToLambdaWithContext(e.Body(), newContext)
+		return lambda.NewAbstraction(paramName, body)
+
+	case *debruijn.Application:
+		abs := deBruijnToLambdaWithContext(e.Abstraction(), context)
+		arg := deBruijnToLambdaWithContext(e.Argument(), context)
+		return lambda.NewApplication(abs, arg)
+
+	default:
+		panic("unknown expression type")
+	}
+}
+
+// collectUsedNames gathers all variable labels used in an expression.
+func collectUsedNames(expr debruijn.Expression, context []string) *set.Set[string] {
+	used := set.New[string]()
+	for _, name := range context {
+		used.Add(name)
+	}
+	collectUsedNamesHelper(expr, used)
+	return used
+}
+
+func collectUsedNamesHelper(expr debruijn.Expression, used *set.Set[string]) {
+	switch e := expr.(type) {
+	case *debruijn.Variable:
+		if e.Label() != "" {
+			used.Add(e.Label())
+		}
+	case *debruijn.Abstraction:
+		collectUsedNamesHelper(e.Body(), used)
+	case *debruijn.Application:
+		collectUsedNamesHelper(e.Abstraction(), used)
+		collectUsedNamesHelper(e.Argument(), used)
+	}
+}
+
+// generateFreshName creates a fresh variable name not in the used set.
+func generateFreshName(used *set.Set[string]) string {
+	for i := 0; ; i++ {
+		name := fmt.Sprintf("_%d", i)
+		if !used.Has(name) {
+			return name
+		}
+	}
+}

pkg/convert/lambda_to_debruijn.go

@@ -0,0 +1,44 @@
+package convert
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/debruijn"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+// LambdaToDeBruijn converts a lambda calculus expression to De Bruijn indexed form.
+// The context parameter tracks bound variables from outer abstractions.
+func LambdaToDeBruijn(expr lambda.Expression) debruijn.Expression {
+	return lambdaToDeBruijnWithContext(expr, []string{})
+}
+
+func lambdaToDeBruijnWithContext(expr lambda.Expression, context []string) debruijn.Expression {
+	switch e := expr.(type) {
+	case *lambda.Variable:
+		name := e.Value()
+		// Search for the variable in the context (innermost to outermost).
+		for i := len(context) - 1; i >= 0; i-- {
+			if context[i] == name {
+				index := len(context) - 1 - i
+				return debruijn.NewVariable(index, name)
+			}
+		}
+		// Free variable: use a negative index to mark it.
+		// We encode free variables with index = len(context) + position.
+		// For simplicity, we use a large index that won't conflict.
+		return debruijn.NewVariable(len(context), name)
+
+	case *lambda.Abstraction:
+		// Add the parameter to the context.
+		newContext := append(context, e.Parameter())
+		body := lambdaToDeBruijnWithContext(e.Body(), newContext)
+		return debruijn.NewAbstraction(body)
+
+	case *lambda.Application:
+		abs := lambdaToDeBruijnWithContext(e.Abstraction(), context)
+		arg := lambdaToDeBruijnWithContext(e.Argument(), context)
+		return debruijn.NewApplication(abs, arg)
+
+	default:
+		panic("unknown expression type")
+	}
+}

pkg/debruijn/expression.go

@@ -0,0 +1,119 @@
+// Package debruijn provides De Bruijn indexed lambda calculus expressions.
+// De Bruijn indices eliminate the need for variable names by using numeric
+// indices to refer to bound variables, avoiding capture issues during substitution.
+package debruijn
+
+import "git.maximhutz.com/max/lambda/pkg/expr"
+
+// Expression is the interface for all De Bruijn indexed expression types.
+// It embeds the general expr.Expression interface for cross-mode compatibility.
+type Expression interface {
+	expr.Expression
+	Accept(Visitor)
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Abstraction represents a lambda abstraction without a named parameter.
+// In De Bruijn notation, the parameter is implicit and referenced by index 0
+// within the body.
+type Abstraction struct {
+	body Expression
+}
+
+// Body returns the body of the abstraction.
+func (a *Abstraction) Body() Expression {
+	return a.body
+}
+
+// Accept implements the Visitor pattern.
+func (a *Abstraction) Accept(v Visitor) {
+	v.VisitAbstraction(a)
+}
+
+// String returns the De Bruijn notation string representation.
+func (a *Abstraction) String() string {
+	return Stringify(a)
+}
+
+// NewAbstraction creates a new De Bruijn abstraction with the given body.
+func NewAbstraction(body Expression) *Abstraction {
+	return &Abstraction{body: body}
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Application represents the application of one expression to another.
+type Application struct {
+	abstraction Expression
+	argument    Expression
+}
+
+// Abstraction returns the function expression being applied.
+func (a *Application) Abstraction() Expression {
+	return a.abstraction
+}
+
+// Argument returns the argument expression.
+func (a *Application) Argument() Expression {
+	return a.argument
+}
+
+// Accept implements the Visitor pattern.
+func (a *Application) Accept(v Visitor) {
+	v.VisitApplication(a)
+}
+
+// String returns the De Bruijn notation string representation.
+func (a *Application) String() string {
+	return Stringify(a)
+}
+
+// NewApplication creates a new application expression.
+func NewApplication(abstraction Expression, argument Expression) *Application {
+	return &Application{abstraction: abstraction, argument: argument}
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Variable represents a De Bruijn indexed variable.
+// The index indicates how many binders to skip to find the binding abstraction.
+// The label is an optional hint for display purposes.
+type Variable struct {
+	index int
+	label string
+}
+
+// Index returns the De Bruijn index.
+func (v *Variable) Index() int {
+	return v.index
+}
+
+// Label returns the optional variable label.
+func (v *Variable) Label() string {
+	return v.label
+}
+
+// Accept implements the Visitor pattern.
+func (v *Variable) Accept(visitor Visitor) {
+	visitor.VisitVariable(v)
+}
+
+// String returns the De Bruijn notation string representation.
+func (v *Variable) String() string {
+	return Stringify(v)
+}
+
+// NewVariable creates a new De Bruijn variable with the given index and label.
+func NewVariable(index int, label string) *Variable {
+	return &Variable{index: index, label: label}
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Visitor interface for traversing De Bruijn expressions.
+type Visitor interface {
+	VisitAbstraction(*Abstraction)
+	VisitApplication(*Application)
+	VisitVariable(*Variable)
+}

pkg/debruijn/iterator.go

@@ -0,0 +1,76 @@
+package debruijn
+
+// Iterator provides depth-first traversal of De Bruijn expressions.
+type Iterator struct {
+	trace []*Expression
+}
+
+// NewIterator creates a new iterator starting at the given expression.
+func NewIterator(expr *Expression) *Iterator {
+	return &Iterator{[]*Expression{expr}}
+}
+
+// Done returns true when the iterator has finished traversal.
+func (i *Iterator) Done() bool {
+	return len(i.trace) == 0
+}
+
+// Current returns a pointer to the current expression.
+func (i *Iterator) Current() *Expression {
+	if i.Done() {
+		return nil
+	}
+
+	return i.trace[len(i.trace)-1]
+}
+
+// Parent returns a pointer to the parent expression.
+func (i *Iterator) Parent() *Expression {
+	if len(i.trace) < 2 {
+		return nil
+	}
+
+	return i.trace[len(i.trace)-2]
+}
+
+// Swap replaces the current expression with the given expression.
+func (i *Iterator) Swap(with Expression) {
+	current := i.Current()
+	if current != nil {
+		*current = with
+	}
+}
+
+// Back moves the iterator back to the parent expression.
+func (i *Iterator) Back() bool {
+	if i.Done() {
+		return false
+	}
+
+	i.trace = i.trace[:len(i.trace)-1]
+	return true
+}
+
+// Next advances the iterator to the next expression in leftmost-outermost order.
+func (i *Iterator) Next() {
+	switch typed := (*i.Current()).(type) {
+	case *Abstraction:
+		i.trace = append(i.trace, &typed.body)
+	case *Application:
+		i.trace = append(i.trace, &typed.abstraction)
+	case *Variable:
+		for len(i.trace) > 1 {
+			if app, ok := (*i.Parent()).(*Application); ok {
+				if app.abstraction == *i.Current() {
+					i.Back()
+					i.trace = append(i.trace, &app.argument)
+					return
+				}
+			}
+
+			i.Back()
+		}
+
+		i.trace = []*Expression{}
+	}
+}

pkg/debruijn/reducer.go

@@ -0,0 +1,72 @@
+package debruijn
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"git.maximhutz.com/max/lambda/pkg/expr"
+	"git.maximhutz.com/max/lambda/pkg/reducer"
+)
+
+// NormalOrderReducer implements normal order (leftmost-outermost) reduction
+// for De Bruijn indexed lambda calculus expressions.
+type NormalOrderReducer struct {
+	emitter.BaseEmitter[reducer.Event]
+	expression *Expression
+}
+
+// NewNormalOrderReducer creates a new normal order reducer.
+func NewNormalOrderReducer(expression *Expression) *NormalOrderReducer {
+	return &NormalOrderReducer{
+		BaseEmitter: *emitter.New[reducer.Event](),
+		expression:  expression,
+	}
+}
+
+// Expression returns the current expression state.
+func (r *NormalOrderReducer) Expression() expr.Expression {
+	return *r.expression
+}
+
+// isViable checks if an expression is a redex (reducible expression).
+// A redex is an application of an abstraction to an argument.
+func isViable(e *Expression) (*Abstraction, Expression, bool) {
+	if e == nil {
+		return nil, nil, false
+	} else if app, appOk := (*e).(*Application); !appOk {
+		return nil, nil, false
+	} else if fn, fnOk := app.abstraction.(*Abstraction); !fnOk {
+		return nil, nil, false
+	} else {
+		return fn, app.argument, true
+	}
+}
+
+// betaReduce performs a single beta reduction step.
+// Given (\. body) arg, it substitutes arg for index 0 in body,
+// then shifts the result down to account for the removed abstraction.
+func betaReduce(fn *Abstraction, arg Expression) Expression {
+	// Substitute arg for variable 0 in the body.
+	substituted := Substitute(fn.body, 0, Shift(arg, 1, 0))
+	// Shift down to account for the removed abstraction.
+	return Shift(substituted, -1, 0)
+}
+
+// Reduce performs normal order reduction on a De Bruijn expression.
+func (r *NormalOrderReducer) Reduce() {
+	r.Emit(reducer.StartEvent)
+	it := NewIterator(r.expression)
+
+	for !it.Done() {
+		if fn, arg, ok := isViable(it.Current()); !ok {
+			it.Next()
+		} else {
+			it.Swap(betaReduce(fn, arg))
+			r.Emit(reducer.StepEvent)
+
+			if _, _, ok := isViable(it.Parent()); ok {
+				it.Back()
+			}
+		}
+	}
+
+	r.Emit(reducer.StopEvent)
+}

pkg/debruijn/shift.go

@@ -0,0 +1,32 @@
+package debruijn
+
+// Shift increments all free variable indices in an expression by the given amount.
+// A variable is free if its index is >= the cutoff (depth of nested abstractions).
+// This is necessary when substituting an expression into a different binding context.
+func Shift(expr Expression, amount int, cutoff int) Expression {
+	switch e := expr.(type) {
+	case *Variable:
+		if e.index >= cutoff {
+			return NewVariable(e.index+amount, e.label)
+		}
+		return e
+
+	case *Abstraction:
+		newBody := Shift(e.body, amount, cutoff+1)
+		if newBody == e.body {
+			return e
+		}
+		return NewAbstraction(newBody)
+
+	case *Application:
+		newAbs := Shift(e.abstraction, amount, cutoff)
+		newArg := Shift(e.argument, amount, cutoff)
+		if newAbs == e.abstraction && newArg == e.argument {
+			return e
+		}
+		return NewApplication(newAbs, newArg)
+
+	default:
+		return expr
+	}
+}

pkg/debruijn/stringify.go

@@ -0,0 +1,35 @@
+package debruijn
+
+import (
+	"strconv"
+	"strings"
+)
+
+type stringifyVisitor struct {
+	builder strings.Builder
+}
+
+func (v *stringifyVisitor) VisitVariable(a *Variable) {
+	v.builder.WriteString(strconv.Itoa(a.index))
+}
+
+func (v *stringifyVisitor) VisitAbstraction(f *Abstraction) {
+	v.builder.WriteRune('\\')
+	v.builder.WriteRune('.')
+	f.body.Accept(v)
+}
+
+func (v *stringifyVisitor) VisitApplication(c *Application) {
+	v.builder.WriteRune('(')
+	c.abstraction.Accept(v)
+	v.builder.WriteRune(' ')
+	c.argument.Accept(v)
+	v.builder.WriteRune(')')
+}
+
+// Stringify converts a De Bruijn expression to its string representation.
+func Stringify(e Expression) string {
+	b := &stringifyVisitor{builder: strings.Builder{}}
+	e.Accept(b)
+	return b.builder.String()
+}

pkg/debruijn/substitute.go

@@ -0,0 +1,34 @@
+package debruijn
+
+// Substitute replaces the variable at the given index with the replacement expression.
+// The replacement is shifted appropriately as we descend into nested abstractions.
+func Substitute(expr Expression, index int, replacement Expression) Expression {
+	switch e := expr.(type) {
+	case *Variable:
+		if e.index == index {
+			return replacement
+		}
+		return e
+
+	case *Abstraction:
+		// When entering an abstraction, increment the target index and shift the
+		// replacement to account for the new binding context.
+		shiftedReplacement := Shift(replacement, 1, 0)
+		newBody := Substitute(e.body, index+1, shiftedReplacement)
+		if newBody == e.body {
+			return e
+		}
+		return NewAbstraction(newBody)
+
+	case *Application:
+		newAbs := Substitute(e.abstraction, index, replacement)
+		newArg := Substitute(e.argument, index, replacement)
+		if newAbs == e.abstraction && newArg == e.argument {
+			return e
+		}
+		return NewApplication(newAbs, newArg)
+
+	default:
+		return expr
+	}
+}