feat: fmt.Stringer

Closes #36

.gitea/ISSUE_TEMPLATE/bug.md

@@ -1,6 +1,6 @@
 ---
 name: "Bug Report"
-about: "Report a bug or unexpected behavior in the lambda runtime."
+about: "Report a bug or unexpected behavior in the lambda interpreter."
 title: "fix: "
 ref: "main"
 assignees: []

.gitea/ISSUE_TEMPLATE/feature.md

@@ -1,6 +1,6 @@
 ---
 name: "Feature Request"
-about: "Suggest a new feature or enhancement for the lambda runtime."
+about: "Suggest a new feature or enhancement for the lambda interpreter."
 title: "feat: "
 ref: "main"
 assignees: []

.golangci.yml

@@ -48,7 +48,7 @@ linters:
       # More information: https://golangci-lint.run/usage/false-positives/#comments
       #
       # Please uncomment the following line if your code is not using the godoc format
-      # - comments
+      - comments
 
       # Common false positives
       # feel free to remove this if you don't have any false positives
@@ -126,9 +126,6 @@ linters:
         # Blank import should be only in a main or test package, or have a comment justifying it.
         - name: blank-imports
 
-        # Packages should have comments of the form "Package x ...".
-        - name: package-comments
-
         # context.Context() should be the first parameter of a function when provided as argument.
         - name: context-as-argument
           arguments:

LICENSE

@@ -48,7 +48,7 @@ The "source code" for a work means the preferred form of the work for making mod
 
 A "Standard Interface" means an interface that either is an official standard defined by a recognized standards body, or, in the case of interfaces specified for a particular programming language, one that is widely used among developers working in that language.
 
-The "System Libraries" of an executable work include anything, other than the work as a whole, that (a) is included in the normal form of packaging a Major Component, but which is not part of that Major Component, and (b) serves only to enable use of the work with that Major Component, or to implement a Standard Interface for which an implementation is available to the public in source code form.  A "Major Component", in this context, means a major essential component (kernel, window system, and so on) of the specific operating system (if any) on which the executable work runs, or a compiler used to produce the work, or an object code runtime used to run it.
+The "System Libraries" of an executable work include anything, other than the work as a whole, that (a) is included in the normal form of packaging a Major Component, but which is not part of that Major Component, and (b) serves only to enable use of the work with that Major Component, or to implement a Standard Interface for which an implementation is available to the public in source code form.  A "Major Component", in this context, means a major essential component (kernel, window system, and so on) of the specific operating system (if any) on which the executable work runs, or a compiler used to produce the work, or an object code interpreter used to run it.
 
 The "Corresponding Source" for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities.  However, it does not include the work's System Libraries, or general-purpose tools or generally available free programs which are used unmodified in performing those activities but which are not part of the work.  For example, Corresponding Source includes interface definition files associated with source files for the work, and the source code for shared libraries and dynamically linked subprograms that the work is specifically designed to require, such as by intimate data communication or control flow between those
 subprograms and other parts of the work.

Makefile

@@ -1,21 +1,20 @@
 BINARY_NAME=lambda
 TEST=simple
 
-.PHONY: help build run profile explain graph docs test bench lint clean
+.PHONY: help build run profile explain graph docs test bench clean
 .DEFAULT_GOAL := help
 .SILENT:
 
 help:
 	echo "Available targets:"
 	echo "  build    - Build the lambda executable"
-	echo "  run      - Build and run the lambda runtime (use TEST=<name> to specify sample)"
+	echo "  run      - Build and run the lambda interpreter (use TEST=<name> to specify sample)"
 	echo "  profile  - Build and run with CPU profiling enabled"
 	echo "  explain  - Build and run with explanation mode and profiling"
 	echo "  graph    - Generate and open CPU profile visualization"
 	echo "  docs     - Start local godoc server on port 6060"
 	echo "  test     - Run tests for all samples"
 	echo "  bench    - Run benchmarks for all samples"
-	echo "  lint     - Run golangci-lint on all packages"
 	echo "  clean    - Remove all build artifacts"
 
 build:
@@ -46,9 +45,6 @@ test:
 bench:
 	go test -bench=. -benchtime=10x -cpu=4 ./cmd/lambda
 
-lint:
-	go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@latest run ./...
-
 clean:
 	rm -f ${BINARY_NAME}
 	rm -f program.out

README.md

@@ -1,6 +1,6 @@
 # lambda
 
-Making a lambda calculus runtime in Go.
+Making a lambda calculus interpreter in Go.
 
 ## Things to talk about
 

cmd/lambda/lambda.go

@@ -7,7 +7,7 @@ import (
 	"git.maximhutz.com/max/lambda/internal/config"
 	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/normalorder"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
@@ -34,34 +34,34 @@ func main() {
 	logger.Info("compiled λ expression", "tree", compiled.String())
 
 	// Create reducer with the compiled expression.
-	runtime := normalorder.NewRuntime(compiled)
+	reducer := lambda.NewNormalOrderReducer(&compiled)
 
 	// If the user selected to track CPU performance, attach a profiler.
 	if options.Profile != "" {
-		plugins.NewPerformance(options.Profile, runtime)
+		plugins.NewPerformance(options.Profile, reducer)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
 	// observer.
 	if options.Explanation {
-		plugins.NewExplanation(runtime)
+		plugins.NewExplanation(reducer)
 	}
 
 	// If the user opted to track statistics, attach a tracker.
 	if options.Statistics {
-		plugins.NewStatistics(runtime)
+		plugins.NewStatistics(reducer)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		plugins.NewLogs(logger, runtime)
+		plugins.NewLogs(logger, reducer)
 	}
 
 	// Run reduction.
-	runtime.Run()
+	reducer.Reduce()
 
 	// Return the final reduced result.
-	result := runtime.Expression().String()
+	result := reducer.Expression().String()
 	err = options.Destination.Write(result)
 	cli.HandleError(err)
 }

cmd/lambda/lambda_test.go

@@ -7,12 +7,12 @@ import (
 	"testing"
 
 	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/normalorder"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 	"github.com/stretchr/testify/assert"
 )
 
-// Helper function to run a single sample through the lambda runtime.
+// Helper function to run a single sample through the lambda interpreter.
 func runSample(samplePath string) (string, error) {
 	// Read the sample file.
 	input, err := os.ReadFile(samplePath)
@@ -30,8 +30,8 @@ func runSample(samplePath string) (string, error) {
 	compiled := convert.SaccharineToLambda(ast)
 
 	// Create and run the reducer.
-	reducer := normalorder.NewRuntime(compiled)
-	reducer.Run()
+	reducer := lambda.NewNormalOrderReducer(&compiled)
+	reducer.Reduce()
 
 	return reducer.Expression().String() + "\n", nil
 }

cmd/lambda/registry.go

@@ -1,26 +0,0 @@
-package main
-
-import (
-	"git.maximhutz.com/max/lambda/internal/cli"
-	"git.maximhutz.com/max/lambda/internal/registry"
-	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/engine/normalorder"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
-	"git.maximhutz.com/max/lambda/pkg/saccharine"
-)
-
-func MakeRegistry() *registry.Registry {
-	r := registry.New()
-
-	// Codecs
-	r.AddCodec(cli.ConvertCodec(convert.Saccharine2Lambda{}, "saccharine", "lambda"))
-
-	// Engines
-	r.AddEngine(cli.ConvertEngine(normalorder.Engine{}, "normalorder", "lambda"))
-
-	// Marshalers
-	r.AddMarshaler(cli.ConvertMarshaler(lambda.Marshaler{}, "lambda"))
-	r.AddMarshaler(cli.ConvertMarshaler(saccharine.Marshaler{}, "saccharine"))
-
-	return r
-}

internal/cli/codec.go

@@ -1,55 +0,0 @@
-package cli
-
-import (
-	"fmt"
-
-	"git.maximhutz.com/max/lambda/pkg/codec"
-)
-
-type Codec interface {
-	codec.Codec[Repr, Repr]
-
-	InType() string
-	OutType() string
-}
-
-type convertedCodec[T, U any] struct {
-	codec           codec.Codec[T, U]
-	inType, outType string
-}
-
-func (c convertedCodec[T, U]) Decode(r Repr) (Repr, error) {
-	u, ok := r.Data().(U)
-	if !ok {
-		return nil, fmt.Errorf("could not parse '%v' as '%s'", r, c.inType)
-	}
-
-	t, err := c.codec.Decode(u)
-	if err != nil {
-		return nil, err
-	}
-
-	return NewRepr(c.outType, t), nil
-}
-
-func (c convertedCodec[T, U]) Encode(r Repr) (Repr, error) {
-	t, ok := r.Data().(T)
-	if !ok {
-		return nil, fmt.Errorf("could not parse '%v' as '%s'", t, c.outType)
-	}
-
-	u, err := c.codec.Encode(t)
-	if err != nil {
-		return nil, err
-	}
-
-	return NewRepr(c.inType, u), nil
-}
-
-func (c convertedCodec[T, U]) InType() string { return c.inType }
-
-func (c convertedCodec[T, U]) OutType() string { return c.outType }
-
-func ConvertCodec[T, U any](e codec.Codec[T, U], inType, outType string) Codec {
-	return convertedCodec[T, U]{e, inType, outType}
-}

internal/cli/engine.go

@@ -1,49 +0,0 @@
-package cli
-
-import (
-	"fmt"
-
-	"git.maximhutz.com/max/lambda/pkg/engine"
-)
-
-type Engine interface {
-	engine.Engine[Repr]
-
-	Name() string
-	InType() string
-}
-
-type convertedEngine[T any] struct {
-	engine engine.Engine[T]
-	name   string
-	inType string
-}
-
-func (b convertedEngine[T]) InType() string { return b.inType }
-
-func (b convertedEngine[T]) Name() string { return b.name }
-
-func (b convertedEngine[T]) Get() (Repr, error) {
-	s, err := b.engine.Get()
-	if err != nil {
-		return nil, err
-	}
-
-	return NewRepr(b.inType, s), nil
-}
-
-func (b convertedEngine[T]) Set(r Repr) error {
-	if t, ok := r.Data().(T); ok {
-		return b.engine.Set(t)
-	}
-
-	return fmt.Errorf("Incorrent format '%s' for engine '%s'.", r.Id(), b.inType)
-}
-
-func (b convertedEngine[T]) Step(i int) bool {
-	return b.engine.Step(i)
-}
-
-func ConvertEngine[T any](e engine.Engine[T], name string, inType string) Engine {
-	return convertedEngine[T]{e, name, inType}
-}

internal/cli/marshaler.go

@@ -1,42 +0,0 @@
-package cli
-
-import (
-	"fmt"
-
-	"git.maximhutz.com/max/lambda/pkg/codec"
-)
-
-type Marshaler interface {
-	codec.Marshaler[Repr]
-
-	InType() string
-}
-
-type convertedMarshaler[T any] struct {
-	codec  codec.Marshaler[T]
-	inType string
-}
-
-func (c convertedMarshaler[T]) Decode(s string) (Repr, error) {
-	t, err := c.codec.Decode(s)
-	if err != nil {
-		return nil, err
-	}
-
-	return NewRepr(c.inType, t), nil
-}
-
-func (c convertedMarshaler[T]) Encode(r Repr) (string, error) {
-	t, ok := r.Data().(T)
-	if !ok {
-		return "", fmt.Errorf("could not parse '%v' as 'string'", t)
-	}
-
-	return c.codec.Encode(t)
-}
-
-func (c convertedMarshaler[T]) InType() string { return c.inType }
-
-func ConvertMarshaler[T any](e codec.Marshaler[T], inType string) Marshaler {
-	return convertedMarshaler[T]{e, inType}
-}

internal/cli/repr.go

@@ -1,21 +0,0 @@
-package cli
-
-type Repr interface {
-	// Id returns to name of the objects underlying representation. If is
-	// assumed that if two Repr objects have the same Id(), they share the same
-	// representation.
-	Id() string
-
-	Data() any
-}
-
-type baseRepr struct {
-	id   string
-	data any
-}
-
-func (r baseRepr) Id() string { return r.id }
-
-func (r baseRepr) Data() any { return r.data }
-
-func NewRepr(id string, data any) Repr { return baseRepr{id, data} }

internal/plugins/debug.go

@@ -0,0 +1,23 @@
+package plugins
+
+import (
+	"log/slog"
+
+	"git.maximhutz.com/max/lambda/pkg/reducer"
+)
+
+type Logs struct {
+	logger  *slog.Logger
+	reducer reducer.Reducer
+}
+
+func NewLogs(logger *slog.Logger, r reducer.Reducer) *Logs {
+	plugin := &Logs{logger, r}
+	r.On(reducer.StepEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Logs) Step() {
+	t.logger.Info("reduction", "tree", t.reducer.Expression().String())
+}

internal/plugins/explanation.go

@@ -0,0 +1,31 @@
+// Package "explanation" provides an observer to gather the reasoning during the
+// reduction, and present a thorough explanation to the user for each step.
+package plugins
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/pkg/reducer"
+)
+
+// Track the reductions made by a reduction process.
+type Explanation struct {
+	reducer reducer.Reducer
+}
+
+// Attaches a new explanation tracker to a reducer.
+func NewExplanation(r reducer.Reducer) *Explanation {
+	plugin := &Explanation{reducer: r}
+	r.On(reducer.StartEvent, plugin.Start)
+	r.On(reducer.StepEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Explanation) Start() {
+	fmt.Println(t.reducer.Expression().String())
+}
+
+func (t *Explanation) Step() {
+	fmt.Println("    =", t.reducer.Expression().String())
+}

internal/plugins/performance.go

@@ -0,0 +1,59 @@
+// Package "performance" provides a tracker to observer CPU performance during
+// execution.
+package plugins
+
+import (
+	"os"
+	"path/filepath"
+	"runtime/pprof"
+
+	"git.maximhutz.com/max/lambda/pkg/reducer"
+)
+
+// Observes a reduction process, and publishes a CPU performance profile on
+// completion.
+type Performance struct {
+	File        string
+	filePointer *os.File
+	Error       error
+}
+
+// Create a performance tracker that outputs a profile to "file".
+func NewPerformance(file string, process reducer.Reducer) *Performance {
+	plugin := &Performance{File: file}
+	process.On(reducer.StartEvent, plugin.Start)
+	process.On(reducer.StopEvent, plugin.Stop)
+
+	return plugin
+}
+
+// Begin profiling.
+func (t *Performance) Start() {
+	var absPath string
+
+	absPath, t.Error = filepath.Abs(t.File)
+	if t.Error != nil {
+		return
+	}
+
+	t.Error = os.MkdirAll(filepath.Dir(absPath), 0777)
+	if t.Error != nil {
+		return
+	}
+
+	t.filePointer, t.Error = os.Create(absPath)
+	if t.Error != nil {
+		return
+	}
+
+	t.Error = pprof.StartCPUProfile(t.filePointer)
+	if t.Error != nil {
+		return
+	}
+}
+
+// Stop profiling.
+func (t *Performance) Stop() {
+	pprof.StopCPUProfile()
+	t.filePointer.Close()
+}

internal/plugins/statistics.go

@@ -0,0 +1,44 @@
+package plugins
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"git.maximhutz.com/max/lambda/internal/statistics"
+	"git.maximhutz.com/max/lambda/pkg/reducer"
+)
+
+// An observer, to track reduction performance.
+type Statistics struct {
+	start time.Time
+	steps uint64
+}
+
+// Create a new reduction performance Statistics.
+func NewStatistics(r reducer.Reducer) *Statistics {
+	plugin := &Statistics{}
+	r.On(reducer.StartEvent, plugin.Start)
+	r.On(reducer.StepEvent, plugin.Step)
+	r.On(reducer.StopEvent, plugin.Stop)
+
+	return plugin
+}
+
+func (t *Statistics) Start() {
+	t.start = time.Now()
+	t.steps = 0
+}
+
+func (t *Statistics) Step() {
+	t.steps++
+}
+
+func (t *Statistics) Stop() {
+	results := statistics.Results{
+		StepsTaken:  t.steps,
+		TimeElapsed: uint64(time.Since(t.start).Milliseconds()),
+	}
+
+	fmt.Fprint(os.Stderr, results.String())
+}

internal/registry/registry.go

@@ -1,75 +0,0 @@
-package registry
-
-import (
-	"fmt"
-
-	"git.maximhutz.com/max/lambda/internal/cli"
-)
-
-type Registry struct {
-	marshalers map[string]cli.Marshaler
-	codecs     []cli.Codec
-	engines    map[string]cli.Engine
-}
-
-func New() *Registry {
-	return &Registry{
-		marshalers: map[string]cli.Marshaler{},
-		codecs:     []cli.Codec{},
-		engines:    map[string]cli.Engine{},
-	}
-}
-
-func (r *Registry) AddCodec(c cli.Codec) error {
-	r.codecs = append(r.codecs, c)
-	return nil
-}
-
-func (r *Registry) AddMarshaler(c cli.Marshaler) error {
-	if _, ok := r.marshalers[c.InType()]; ok {
-		return fmt.Errorf("marshaler for '%s' already registered", c.InType())
-	}
-
-	r.marshalers[c.InType()] = c
-	return nil
-}
-
-func (r *Registry) AddEngine(e cli.Engine) error {
-	if _, ok := r.engines[e.Name()]; ok {
-		return fmt.Errorf("engine '%s' already registered", e.Name())
-	}
-
-	r.engines[e.Name()] = e
-	return nil
-}
-
-func (r *Registry) GetEngine(name string) (cli.Engine, error) {
-	e, ok := r.engines[name]
-	if !ok {
-		return nil, fmt.Errorf("engine '%s' not found", name)
-	}
-
-	return e, nil
-}
-
-func (r *Registry) ConvertTo(repr cli.Repr, outType string) (cli.Repr, error) {
-	panic("")
-}
-
-func (r *Registry) Marshal(repr cli.Repr) (string, error) {
-	m, ok := r.marshalers[repr.Id()]
-	if !ok {
-		return "", fmt.Errorf("no marshaler for '%s'", repr.Id())
-	}
-
-	return m.Encode(repr)
-}
-
-func (r *Registry) Unmarshal(s string, outType string) (cli.Repr, error) {
-	m, ok := r.marshalers[outType]
-	if !ok {
-		return nil, fmt.Errorf("no marshaler for '%s'", outType)
-	}
-
-	return m.Decode(s)
-}

internal/statistics/statistics.go

@@ -0,0 +1,28 @@
+// Package "statistics" provides a way to observer reduction speed during
+// execution.
+package statistics
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Statistics for a specific reduction.
+type Results struct {
+	StepsTaken  uint64 // Number of steps taken during execution.
+	TimeElapsed uint64 // The time (ms) taken for execution to complete.
+}
+
+// Returns the average number of operations per second of the execution.
+func (r Results) OpsPerSecond() float32 {
+	return float32(r.StepsTaken) / (float32(r.TimeElapsed) / 1000)
+}
+
+// Format the results as a string.
+func (r Results) String() string {
+	builder := strings.Builder{}
+	fmt.Fprintln(&builder, "Time Spent:", r.TimeElapsed, "ms")
+	fmt.Fprintln(&builder, "Steps:", r.StepsTaken)
+	fmt.Fprintln(&builder, "Speed:", r.OpsPerSecond(), "ops")
+	return builder.String()
+}

pkg/codec/codec.go

@@ -1,8 +0,0 @@
-package codec
-
-type Codec[T, U any] interface {
-	Encode(T) (U, error)
-	Decode(U) (T, error)
-}
-
-type Marshaler[T any] = Codec[T, string]

pkg/convert/saccharine_to_lambda.go

@@ -3,24 +3,23 @@ package convert
 import (
 	"fmt"
 
-	"git.maximhutz.com/max/lambda/pkg/codec"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
-func encodeAtom(n *saccharine.Atom) lambda.Expression {
+func convertAtom(n *saccharine.Atom) lambda.Expression {
 	return lambda.NewVariable(n.Name)
 }
 
-func encodeAbstraction(n *saccharine.Abstraction) lambda.Expression {
-	result := encodeExpression(n.Body)
+func convertAbstraction(n *saccharine.Abstraction) lambda.Expression {
+	result := SaccharineToLambda(n.Body)
 
 	parameters := n.Parameters
 
 	// If the function has no parameters, it is a thunk. Lambda calculus still
 	// requires _some_ parameter exists, so generate one.
 	if len(parameters) == 0 {
-		freeVars := result.GetFree()
+		freeVars := lambda.GetFreeVariables(result)
 		freshName := lambda.GenerateFreshName(freeVars)
 		parameters = append(parameters, freshName)
 	}
@@ -32,13 +31,13 @@ func encodeAbstraction(n *saccharine.Abstraction) lambda.Expression {
 	return result
 }
 
-func encodeApplication(n *saccharine.Application) lambda.Expression {
-	result := encodeExpression(n.Abstraction)
+func convertApplication(n *saccharine.Application) lambda.Expression {
+	result := SaccharineToLambda(n.Abstraction)
 
 	arguments := []lambda.Expression{}
 	for _, argument := range n.Arguments {
-		encodeedArgument := encodeExpression(argument)
-		arguments = append(arguments, encodeedArgument)
+		convertedArgument := SaccharineToLambda(argument)
+		arguments = append(arguments, convertedArgument)
 	}
 
 	for _, argument := range arguments {
@@ -52,9 +51,9 @@ func reduceLet(s *saccharine.LetStatement, e lambda.Expression) lambda.Expressio
 	var value lambda.Expression
 
 	if len(s.Parameters) == 0 {
-		value = encodeExpression(s.Body)
+		value = SaccharineToLambda(s.Body)
 	} else {
-		value = encodeAbstraction(saccharine.NewAbstraction(s.Parameters, s.Body))
+		value = convertAbstraction(saccharine.NewAbstraction(s.Parameters, s.Body))
 	}
 
 	return lambda.NewApplication(
@@ -64,11 +63,11 @@ func reduceLet(s *saccharine.LetStatement, e lambda.Expression) lambda.Expressio
 }
 
 func reduceDeclare(s *saccharine.DeclareStatement, e lambda.Expression) lambda.Expression {
-	freshVar := lambda.GenerateFreshName(e.GetFree())
+	freshVar := lambda.GenerateFreshName(lambda.GetFreeVariables(e))
 
 	return lambda.NewApplication(
 		lambda.NewAbstraction(freshVar, e),
-		encodeExpression(s.Value),
+		SaccharineToLambda(s.Value),
 	)
 }
 
@@ -83,8 +82,8 @@ func reduceStatement(s saccharine.Statement, e lambda.Expression) lambda.Express
 	}
 }
 
-func encodeClause(n *saccharine.Clause) lambda.Expression {
-	result := encodeExpression(n.Returns)
+func convertClause(n *saccharine.Clause) lambda.Expression {
+	result := SaccharineToLambda(n.Returns)
 
 	for i := len(n.Statements) - 1; i >= 0; i-- {
 		result = reduceStatement(n.Statements[i], result)
@@ -93,46 +92,17 @@ func encodeClause(n *saccharine.Clause) lambda.Expression {
 	return result
 }
 
-func encodeExpression(s saccharine.Expression) lambda.Expression {
-	switch s := s.(type) {
+func SaccharineToLambda(n saccharine.Expression) lambda.Expression {
+	switch n := n.(type) {
 	case *saccharine.Atom:
-		return encodeAtom(s)
+		return convertAtom(n)
 	case *saccharine.Abstraction:
-		return encodeAbstraction(s)
+		return convertAbstraction(n)
 	case *saccharine.Application:
-		return encodeApplication(s)
+		return convertApplication(n)
 	case *saccharine.Clause:
-		return encodeClause(s)
+		return convertClause(n)
 	default:
-		panic(fmt.Errorf("unknown expression type: %T", s))
+		panic(fmt.Errorf("unknown expression type: %T", n))
 	}
 }
-
-func decodeExression(l lambda.Expression) saccharine.Expression {
-	switch l := l.(type) {
-	case lambda.Variable:
-		return saccharine.NewAtom(l.Name())
-	case lambda.Abstraction:
-		return saccharine.NewAbstraction(
-			[]string{l.Parameter()},
-			decodeExression(l.Body()))
-	case lambda.Application:
-		return saccharine.NewApplication(
-			decodeExression(l.Abstraction()),
-			[]saccharine.Expression{decodeExression(l.Argument())})
-	default:
-		panic(fmt.Errorf("unknown expression type: %T", l))
-	}
-}
-
-type Saccharine2Lambda struct{}
-
-func (c Saccharine2Lambda) Decode(l lambda.Expression) (saccharine.Expression, error) {
-	return decodeExression(l), nil
-}
-
-func (c Saccharine2Lambda) Encode(s saccharine.Expression) (lambda.Expression, error) {
-	return encodeExpression(s), nil
-}
-
-var _ codec.Codec[saccharine.Expression, lambda.Expression] = (*Saccharine2Lambda)(nil)

pkg/emitter/emitter.go

@@ -9,7 +9,7 @@ type Emitter[E comparable] interface {
 }
 
 type BaseEmitter[E comparable] struct {
-	listeners map[E]set.Set[Listener[E]]
+	listeners map[E]*set.Set[Listener[E]]
 }
 
 func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
@@ -41,6 +41,6 @@ func (e *BaseEmitter[E]) Emit(event E) {
 
 func New[E comparable]() *BaseEmitter[E] {
 	return &BaseEmitter[E]{
-		listeners: map[E]set.Set[Listener[E]]{},
+		listeners: map[E]*set.Set[Listener[E]]{},
 	}
 }

pkg/engine/engine.go

@@ -1,7 +0,0 @@
-package engine
-
-type Engine[T any] interface {
-	Get() (T, error)
-	Set(T) error
-	Step(int) bool
-}

pkg/engine/normalorder/engine.go

@@ -1,34 +0,0 @@
-package normalorder
-
-import (
-	"git.maximhutz.com/max/lambda/pkg/engine"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
-)
-
-type Engine struct {
-	expr lambda.Expression
-}
-
-func (e Engine) Get() (lambda.Expression, error) {
-	return e.expr, nil
-}
-
-func (e Engine) Set(l lambda.Expression) error {
-	e.expr = l
-	return nil
-}
-
-func (e Engine) Step(i int) bool {
-	var reduced bool
-
-	for range i {
-		e.expr, reduced = ReduceOnce(e.expr)
-		if !reduced {
-			return false
-		}
-	}
-
-	return true
-}
-
-var _ engine.Engine[lambda.Expression] = (*Engine)(nil)

pkg/engine/normalorder/reduce_one.go

@@ -1,34 +0,0 @@
-package normalorder
-
-import "git.maximhutz.com/max/lambda/pkg/lambda"
-
-func ReduceOnce(e lambda.Expression) (lambda.Expression, bool) {
-	switch e := e.(type) {
-	case lambda.Abstraction:
-		body, reduced := ReduceOnce(e.Body())
-		if reduced {
-			return lambda.NewAbstraction(e.Parameter(), body), true
-		}
-		return e, false
-
-	case lambda.Application:
-		if fn, fnOk := e.Abstraction().(lambda.Abstraction); fnOk {
-			return fn.Body().Substitute(fn.Parameter(), e.Argument()), true
-		}
-
-		abs, reduced := ReduceOnce(e.Abstraction())
-		if reduced {
-			return lambda.NewApplication(abs, e.Argument()), true
-		}
-
-		arg, reduced := ReduceOnce(e.Argument())
-		if reduced {
-			return lambda.NewApplication(e.Abstraction(), arg), true
-		}
-
-		return e, false
-
-	default:
-		return e, false
-	}
-}

pkg/expr/expr.go

@@ -0,0 +1,15 @@
+// Package expr provides the abstract Expression interface for all evaluatable
+// expression types in the lambda interpreter.
+package expr
+
+import (
+	"fmt"
+)
+
+// Expression is the base interface for all evaluatable expression types.
+// Different evaluation modes (lambda calculus, SKI combinators, typed lambda
+// calculus, etc.) implement this interface with their own concrete types.
+type Expression interface {
+	// The expression should have a human-readable representation.
+	fmt.Stringer
+}

pkg/lambda/expression.go

@@ -1,32 +1,13 @@
 package lambda
 
 import (
-	"fmt"
-
-	"git.maximhutz.com/max/lambda/pkg/set"
+	"git.maximhutz.com/max/lambda/pkg/expr"
 )
 
 // Expression is the interface for all lambda calculus expression types.
 // It embeds the general expr.Expression interface for cross-mode compatibility.
 type Expression interface {
-	fmt.Stringer
-
-	// Substitute replaces all free occurrences of the target variable with the
-	// replacement expression. Alpha-renaming is performed automatically to
-	// avoid variable capture.
-	Substitute(target string, replacement Expression) Expression
-
-	// GetFree returns the set of all free variable names in the expression.
-	// This function does not mutate the input expression.
-	// The returned set is newly allocated and can be modified by the caller.
-	GetFree() set.Set[string]
-
-	// Rename replaces all occurrences of the target variable name with the new name.
-	Rename(target string, newName string) Expression
-
-	// IsFree returns true if the variable name n occurs free in the expression.
-	// This function does not mutate the input expression.
-	IsFree(n string) bool
+	expr.Expression
 }
 
 /** ------------------------------------------------------------------------- */
@@ -36,22 +17,22 @@ type Abstraction struct {
 	body      Expression
 }
 
-var _ Expression = Abstraction{}
+var _ Expression = (*Abstraction)(nil)
 
-func (a Abstraction) Parameter() string {
+func (a *Abstraction) Parameter() string {
 	return a.parameter
 }
 
-func (a Abstraction) Body() Expression {
+func (a *Abstraction) Body() Expression {
 	return a.body
 }
 
-func (a Abstraction) String() string {
+func (a *Abstraction) String() string {
 	return "\\" + a.parameter + "." + a.body.String()
 }
 
-func NewAbstraction(parameter string, body Expression) Abstraction {
-	return Abstraction{parameter, body}
+func NewAbstraction(parameter string, body Expression) *Abstraction {
+	return &Abstraction{parameter, body}
 }
 
 /** ------------------------------------------------------------------------- */
@@ -61,40 +42,40 @@ type Application struct {
 	argument    Expression
 }
 
-var _ Expression = Application{}
+var _ Expression = (*Application)(nil)
 
-func (a Application) Abstraction() Expression {
+func (a *Application) Abstraction() Expression {
 	return a.abstraction
 }
 
-func (a Application) Argument() Expression {
+func (a *Application) Argument() Expression {
 	return a.argument
 }
 
-func (a Application) String() string {
+func (a *Application) String() string {
 	return "(" + a.abstraction.String() + " " + a.argument.String() + ")"
 }
 
-func NewApplication(abstraction Expression, argument Expression) Application {
-	return Application{abstraction, argument}
+func NewApplication(abstraction Expression, argument Expression) *Application {
+	return &Application{abstraction, argument}
 }
 
 /** ------------------------------------------------------------------------- */
 
 type Variable struct {
-	name string
+	value string
 }
 
-var _ Expression = Variable{}
+var _ Expression = (*Variable)(nil)
 
-func (v Variable) Name() string {
-	return v.name
+func (v *Variable) Value() string {
+	return v.value
 }
 
-func (v Variable) String() string {
-	return v.name
+func (v *Variable) String() string {
+	return v.value
 }
 
-func NewVariable(name string) Variable {
-	return Variable{name}
+func NewVariable(name string) *Variable {
+	return &Variable{name}
 }

pkg/lambda/generate_name.go

@@ -6,9 +6,7 @@ import (
 	"git.maximhutz.com/max/lambda/pkg/set"
 )
 
-// GenerateFreshName generates a variable name that is not in the used set.
-// This function does not mutate the used set.
-func GenerateFreshName(used set.Set[string]) string {
+func GenerateFreshName(used *set.Set[string]) string {
 	for i := uint64(0); ; i++ {
 		attempt := "_" + string(strconv.AppendUint(nil, i, 10))
 

pkg/lambda/get_free_variables.go

@@ -2,18 +2,19 @@ package lambda
 
 import "git.maximhutz.com/max/lambda/pkg/set"
 
-func (e Variable) GetFree() set.Set[string] {
-	return set.New(e.Name())
-}
-
-func (e Abstraction) GetFree() set.Set[string] {
-	vars := e.Body().GetFree()
-	vars.Remove(e.Parameter())
-	return vars
-}
-
-func (e Application) GetFree() set.Set[string] {
-	vars := e.Abstraction().GetFree()
-	vars.Merge(e.Argument().GetFree())
-	return vars
+func GetFreeVariables(e Expression) *set.Set[string] {
+	switch e := e.(type) {
+	case *Variable:
+		return set.New(e.value)
+	case *Abstraction:
+		vars := GetFreeVariables(e.body)
+		vars.Remove(e.parameter)
+		return vars
+	case *Application:
+		vars := GetFreeVariables(e.abstraction)
+		vars.Merge(GetFreeVariables(e.argument))
+		return vars
+	default:
+		return nil
+	}
 }

pkg/lambda/is_free_variable.go

@@ -1,12 +1,14 @@
 package lambda
 
-func (e Variable) IsFree(n string) bool {
-	return e.Name() == n
-}
-
-func (e Abstraction) IsFree(n string) bool {
-	return e.Parameter() != n && e.Body().IsFree(n)
-}
-func (e Application) IsFree(n string) bool {
-	return e.Abstraction().IsFree(n) || e.Argument().IsFree(n)
+func IsFreeVariable(n string, e Expression) bool {
+	switch e := e.(type) {
+	case *Variable:
+		return e.value == n
+	case *Abstraction:
+		return e.parameter != n && IsFreeVariable(n, e.body)
+	case *Application:
+		return IsFreeVariable(n, e.abstraction) || IsFreeVariable(n, e.argument)
+	default:
+		return false
+	}
 }

pkg/lambda/iterator.go

@@ -0,0 +1,68 @@
+package lambda
+
+type Iterator struct {
+	trace []*Expression
+}
+
+func NewIterator(expr *Expression) *Iterator {
+	return &Iterator{[]*Expression{expr}}
+}
+
+func (i *Iterator) Done() bool {
+	return len(i.trace) == 0
+}
+
+func (i *Iterator) Current() *Expression {
+	if i.Done() {
+		return nil
+	}
+
+	return i.trace[len(i.trace)-1]
+}
+
+func (i *Iterator) Parent() *Expression {
+	if len(i.trace) < 2 {
+		return nil
+	}
+
+	return i.trace[len(i.trace)-2]
+}
+
+func (i *Iterator) Swap(with Expression) {
+	current := i.Current()
+	if current != nil {
+		*current = with
+	}
+}
+
+func (i *Iterator) Back() bool {
+	if i.Done() {
+		return false
+	}
+
+	i.trace = i.trace[:len(i.trace)-1]
+	return true
+}
+
+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/lambda/marshaler.go

@@ -1,19 +0,0 @@
-package lambda
-
-import (
-	"fmt"
-
-	"git.maximhutz.com/max/lambda/pkg/codec"
-)
-
-type Marshaler struct{}
-
-func (m Marshaler) Decode(string) (Expression, error) {
-	return nil, fmt.Errorf("unimplemented")
-}
-
-func (m Marshaler) Encode(e Expression) (string, error) {
-	return e.String(), nil
-}
-
-var _ codec.Marshaler[Expression] = (*Marshaler)(nil)

pkg/lambda/reducer.go

@@ -0,0 +1,61 @@
+package lambda
+
+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 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
+}
+
+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
+	}
+}
+
+// Reduce performs normal order reduction on a lambda expression.
+// The expression must be a lambda.Expression; other types are returned unchanged.
+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(Substitute(fn.body, fn.parameter, arg))
+			r.Emit(reducer.StepEvent)
+
+			if _, _, ok := isViable(it.Parent()); ok {
+				it.Back()
+			}
+		}
+	}
+
+	r.Emit(reducer.StopEvent)
+}

pkg/lambda/rename.go

@@ -1,28 +1,38 @@
 package lambda
 
-// Rename replaces all occurrences of the target variable name with the new name.
-func (e Variable) Rename(target string, newName string) Expression {
-	if e.Name() == target {
-		return NewVariable(newName)
+func Rename(expr Expression, target string, newName string) Expression {
+	switch e := expr.(type) {
+	case *Variable:
+		if e.value == target {
+			return NewVariable(newName)
+		}
+		return e
+
+	case *Abstraction:
+		newParam := e.parameter
+		if e.parameter == target {
+			newParam = newName
+		}
+
+		newBody := Rename(e.body, target, newName)
+
+		if newParam == e.parameter && newBody == e.body {
+			return e
+		}
+
+		return NewAbstraction(newParam, newBody)
+
+	case *Application:
+		newAbs := Rename(e.abstraction, target, newName)
+		newArg := Rename(e.argument, target, newName)
+
+		if newAbs == e.abstraction && newArg == e.argument {
+			return e
+		}
+
+		return NewApplication(newAbs, newArg)
+
+	default:
+		return expr
 	}
-
-	return e
-}
-
-func (e Abstraction) Rename(target string, newName string) Expression {
-	newParam := e.Parameter()
-	if e.Parameter() == target {
-		newParam = newName
-	}
-
-	newBody := e.Body().Rename(target, newName)
-
-	return NewAbstraction(newParam, newBody)
-}
-
-func (e Application) Rename(target string, newName string) Expression {
-	newAbs := e.Abstraction().Rename(target, newName)
-	newArg := e.Argument().Rename(target, newName)
-
-	return NewApplication(newAbs, newArg)
 }

pkg/lambda/substitute.go

@@ -1,35 +1,46 @@
 package lambda
 
-func (e Variable) Substitute(target string, replacement Expression) Expression {
-	if e.Name() == target {
-		return replacement
-	}
-
-	return e
-}
-
-func (e Abstraction) Substitute(target string, replacement Expression) Expression {
-	if e.Parameter() == target {
+func Substitute(expr Expression, target string, replacement Expression) Expression {
+	switch e := expr.(type) {
+	case *Variable:
+		if e.value == target {
+			return replacement
+		}
 		return e
+
+	case *Abstraction:
+		if e.parameter == target {
+			return e
+		}
+
+		body := e.body
+		param := e.parameter
+		if IsFreeVariable(param, replacement) {
+			freeVars := GetFreeVariables(replacement)
+			freeVars.Merge(GetFreeVariables(body))
+			freshVar := GenerateFreshName(freeVars)
+			body = Rename(body, param, freshVar)
+			param = freshVar
+		}
+
+		newBody := Substitute(body, target, replacement)
+		if newBody == body && param == e.parameter {
+			return e
+		}
+
+		return NewAbstraction(param, newBody)
+
+	case *Application:
+		newAbs := Substitute(e.abstraction, target, replacement)
+		newArg := Substitute(e.argument, target, replacement)
+
+		if newAbs == e.abstraction && newArg == e.argument {
+			return e
+		}
+
+		return NewApplication(newAbs, newArg)
+
+	default:
+		return expr
 	}
-
-	body := e.Body()
-	param := e.Parameter()
-	if replacement.IsFree(param) {
-		freeVars := replacement.GetFree()
-		freeVars.Merge(body.GetFree())
-		freshVar := GenerateFreshName(freeVars)
-		body = body.Rename(param, freshVar)
-		param = freshVar
-	}
-
-	newBody := body.Substitute(target, replacement)
-	return NewAbstraction(param, newBody)
-}
-
-func (e Application) Substitute(target string, replacement Expression) Expression {
-	abs := e.Abstraction().Substitute(target, replacement)
-	arg := e.Argument().Substitute(target, replacement)
-
-	return NewApplication(abs, arg)
 }

pkg/reducer/events.go

@@ -0,0 +1,13 @@
+package reducer
+
+// Event represents lifecycle events during reduction.
+type Event int
+
+const (
+	// StartEvent is emitted before reduction begins.
+	StartEvent Event = iota
+	// StepEvent is emitted after each reduction step.
+	StepEvent
+	// StopEvent is emitted after reduction completes.
+	StopEvent
+)

pkg/reducer/reducer.go

@@ -0,0 +1,27 @@
+// Package reducer provides the abstract Reducer interface for all expression
+// reduction strategies.
+package reducer
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"git.maximhutz.com/max/lambda/pkg/expr"
+)
+
+// Reducer defines the interface for expression reduction strategies.
+// Different evaluation modes (normal order, applicative order, SKI combinators,
+// etc.) implement this interface with their own reduction logic.
+//
+// Reducers also implement the Emitter interface to allow plugins to observe
+// reduction lifecycle events (Start, Step, Stop).
+type Reducer interface {
+	emitter.Emitter[Event]
+
+	// Reduce performs all reduction steps on the expression.
+	// Emits StartEvent before reduction, StepEvent after each step, and
+	// StopEvent after completion.
+	// Returns the final reduced expression.
+	Reduce()
+
+	// Expression returns the current expression state.
+	Expression() expr.Expression
+}

pkg/saccharine/marshaler.go

@@ -1,24 +0,0 @@
-// Package "saccharine" provides a simple language built on top of λ-calculus,
-// to facilitate productive coding using it.
-package saccharine
-
-import (
-	"git.maximhutz.com/max/lambda/pkg/codec"
-)
-
-type Marshaler struct{}
-
-func (m Marshaler) Decode(s string) (Expression, error) {
-	tokens, err := scan(s)
-	if err != nil {
-		return nil, err
-	}
-
-	return parse(tokens)
-}
-
-func (m Marshaler) Encode(e Expression) (string, error) {
-	return stringifyExpression(e), nil
-}
-
-var _ codec.Marshaler[Expression] = (*Marshaler)(nil)

pkg/saccharine/parse.go

@@ -5,17 +5,18 @@ import (
 	"fmt"
 
 	"git.maximhutz.com/max/lambda/pkg/iterator"
+	"git.maximhutz.com/max/lambda/pkg/saccharine/token"
 	"git.maximhutz.com/max/lambda/pkg/trace"
 )
 
-type TokenIterator = iterator.Iterator[Token]
+type TokenIterator = iterator.Iterator[token.Token]
 
-func parseRawToken(i *TokenIterator, expected TokenType) (*Token, error) {
-	return iterator.Do(i, func(i *TokenIterator) (*Token, error) {
+func parseRawToken(i *TokenIterator, expected token.Type) (*token.Token, error) {
+	return iterator.Do(i, func(i *TokenIterator) (*token.Token, error) {
 		if tok, err := i.Next(); err != nil {
 			return nil, err
 		} else if tok.Type != expected {
-			return nil, fmt.Errorf("expected token %v, got %v'", expected.Name(), tok.Value)
+			return nil, fmt.Errorf("expected token %v, got %v'", token.Name(expected), tok.Value)
 		} else {
 			return &tok, nil
 		}
@@ -24,14 +25,14 @@ func parseRawToken(i *TokenIterator, expected TokenType) (*Token, error) {
 
 func passSoftBreaks(i *TokenIterator) {
 	for {
-		if _, err := parseRawToken(i, TokenSoftBreak); err != nil {
+		if _, err := parseRawToken(i, token.SoftBreak); err != nil {
 			return
 		}
 	}
 }
 
-func parseToken(i *TokenIterator, expected TokenType, ignoreSoftBreaks bool) (*Token, error) {
-	return iterator.Do(i, func(i *TokenIterator) (*Token, error) {
+func parseToken(i *TokenIterator, expected token.Type, ignoreSoftBreaks bool) (*token.Token, error) {
+	return iterator.Do(i, func(i *TokenIterator) (*token.Token, error) {
 		if ignoreSoftBreaks {
 			passSoftBreaks(i)
 		}
@@ -41,17 +42,17 @@ func parseToken(i *TokenIterator, expected TokenType, ignoreSoftBreaks bool) (*T
 }
 
 func parseString(i *TokenIterator) (string, error) {
-	if tok, err := parseToken(i, TokenAtom, true); err != nil {
+	if tok, err := parseToken(i, token.Atom, true); err != nil {
 		return "", trace.Wrap(err, "no variable (col %d)", i.Index())
 	} else {
 		return tok.Value, nil
 	}
 }
 
-func parseBreak(i *TokenIterator) (*Token, error) {
-	if tok, softErr := parseRawToken(i, TokenSoftBreak); softErr == nil {
+func parseBreak(i *TokenIterator) (*token.Token, error) {
+	if tok, softErr := parseRawToken(i, token.SoftBreak); softErr == nil {
 		return tok, nil
-	} else if tok, hardErr := parseRawToken(i, TokenHardBreak); hardErr == nil {
+	} else if tok, hardErr := parseRawToken(i, token.HardBreak); hardErr == nil {
 		return tok, nil
 	} else {
 		return nil, errors.Join(softErr, hardErr)
@@ -75,11 +76,11 @@ func parseList[U any](i *TokenIterator, fn func(*TokenIterator) (U, error), mini
 
 func parseAbstraction(i *TokenIterator) (*Abstraction, error) {
 	return iterator.Do(i, func(i *TokenIterator) (*Abstraction, error) {
-		if _, err := parseToken(i, TokenSlash, true); err != nil {
+		if _, err := parseToken(i, token.Slash, true); err != nil {
 			return nil, trace.Wrap(err, "no function slash (col %d)", i.MustGet().Column)
 		} else if parameters, err := parseList(i, parseString, 0); err != nil {
 			return nil, err
-		} else if _, err = parseToken(i, TokenDot, true); err != nil {
+		} else if _, err = parseToken(i, token.Dot, true); err != nil {
 			return nil, trace.Wrap(err, "no function dot (col %d)", i.MustGet().Column)
 		} else if body, err := parseExpression(i); err != nil {
 			return nil, err
@@ -91,11 +92,11 @@ func parseAbstraction(i *TokenIterator) (*Abstraction, error) {
 
 func parseApplication(i *TokenIterator) (*Application, error) {
 	return iterator.Do(i, func(i *TokenIterator) (*Application, error) {
-		if _, err := parseToken(i, TokenOpenParen, true); err != nil {
+		if _, err := parseToken(i, token.OpenParen, true); err != nil {
 			return nil, trace.Wrap(err, "no openning brackets (col %d)", i.MustGet().Column)
 		} else if expressions, err := parseList(i, parseExpression, 1); err != nil {
 			return nil, err
-		} else if _, err := parseToken(i, TokenCloseParen, true); err != nil {
+		} else if _, err := parseToken(i, token.CloseParen, true); err != nil {
 			return nil, trace.Wrap(err, "no closing brackets (col %d)", i.MustGet().Column)
 		} else {
 			return NewApplication(expressions[0], expressions[1:]), nil
@@ -104,7 +105,7 @@ func parseApplication(i *TokenIterator) (*Application, error) {
 }
 
 func parseAtom(i *TokenIterator) (*Atom, error) {
-	if tok, err := parseToken(i, TokenAtom, true); err != nil {
+	if tok, err := parseToken(i, token.Atom, true); err != nil {
 		return nil, trace.Wrap(err, "no variable (col %d)", i.Index())
 	} else {
 		return NewAtom(tok.Value), nil
@@ -132,7 +133,7 @@ func parseStatements(i *TokenIterator) ([]Statement, error) {
 
 func parseClause(i *TokenIterator, braces bool) (*Clause, error) {
 	if braces {
-		if _, err := parseToken(i, TokenOpenBrace, true); err != nil {
+		if _, err := parseToken(i, token.OpenBrace, true); err != nil {
 			return nil, err
 		}
 	}
@@ -151,7 +152,7 @@ func parseClause(i *TokenIterator, braces bool) (*Clause, error) {
 	}
 
 	if braces {
-		if _, err := parseToken(i, TokenCloseBrace, true); err != nil {
+		if _, err := parseToken(i, token.CloseBrace, true); err != nil {
 			return nil, err
 		}
 	}
@@ -164,13 +165,13 @@ func parseExpression(i *TokenIterator) (Expression, error) {
 		passSoftBreaks(i)
 
 		switch peek := i.MustGet(); peek.Type {
-		case TokenOpenParen:
+		case token.OpenParen:
 			return parseApplication(i)
-		case TokenSlash:
+		case token.Slash:
 			return parseAbstraction(i)
-		case TokenAtom:
+		case token.Atom:
 			return parseAtom(i)
-		case TokenOpenBrace:
+		case token.OpenBrace:
 			return parseClause(i, true)
 		default:
 			return nil, fmt.Errorf("expected expression, got '%v' (col %d)", peek.Value, peek.Column)
@@ -182,7 +183,7 @@ func parseLet(i *TokenIterator) (*LetStatement, error) {
 	return iterator.Do(i, func(i *TokenIterator) (*LetStatement, error) {
 		if parameters, err := parseList(i, parseString, 1); err != nil {
 			return nil, err
-		} else if _, err := parseToken(i, TokenAssign, true); err != nil {
+		} else if _, err := parseToken(i, token.Assign, true); err != nil {
 			return nil, err
 		} else if body, err := parseExpression(i); err != nil {
 			return nil, err
@@ -211,7 +212,7 @@ func parseStatement(i *TokenIterator) (Statement, error) {
 }
 
 // Given a list of tokens, attempt to parse it into an syntax tree.
-func parse(tokens []Token) (Expression, error) {
+func parse(tokens []token.Token) (Expression, error) {
 	i := iterator.Of(tokens)
 
 	exp, err := parseClause(i, false)

pkg/saccharine/saccharine.go

@@ -0,0 +1,22 @@
+// Package "saccharine" provides a simple language built on top of λ-calculus,
+// to facilitate productive coding using it.
+package saccharine
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/saccharine/token"
+)
+
+// Convert a piece of valid saccharine code into an expression.
+func Parse(code string) (Expression, error) {
+	tokens, err := token.Parse(code)
+	if err != nil {
+		return nil, err
+	}
+
+	return parse(tokens)
+}
+
+// Convert a parsed saccharine expression back into source code.
+func Stringify(expression Expression) string {
+	return stringifyExpression(expression)
+}

pkg/saccharine/token.go

@@ -1,91 +0,0 @@
-package saccharine
-
-import "fmt"
-
-// All tokens in the pseudo-lambda language.
-type TokenType int
-
-const (
-	TokenOpenParen  TokenType = iota // Denotes the '(' token.
-	TokenCloseParen                  // Denotes the ')' token.
-	TokenOpenBrace                   // Denotes the '{' token.
-	TokenCloseBrace                  // Denotes the '}' token.
-	TokenHardBreak                   // Denotes the ';' token.
-	TokenAssign                      // Denotes the ':=' token.
-	TokenAtom                        // Denotes an alpha-numeric variable.
-	TokenSlash                       // Denotes the '/' token.
-	TokenDot                         // Denotes the '.' token.
-	TokenSoftBreak                   // Denotes a new-line.
-)
-
-// A representation of a token in source code.
-type Token struct {
-	Column int       // Where the token begins in the source text.
-	Type   TokenType // What type the token is.
-	Value  string    // The value of the token.
-}
-
-func NewTokenOpenParen(column int) *Token {
-	return &Token{Type: TokenOpenParen, Column: column, Value: "("}
-}
-
-func NewTokenCloseParen(column int) *Token {
-	return &Token{Type: TokenCloseParen, Column: column, Value: ")"}
-}
-
-func NewTokenOpenBrace(column int) *Token {
-	return &Token{Type: TokenOpenBrace, Column: column, Value: "{"}
-}
-
-func NewTokenCloseBrace(column int) *Token {
-	return &Token{Type: TokenCloseBrace, Column: column, Value: "}"}
-}
-
-func NewTokenDot(column int) *Token {
-	return &Token{Type: TokenDot, Column: column, Value: "."}
-}
-
-func NewTokenHardBreak(column int) *Token {
-	return &Token{Type: TokenHardBreak, Column: column, Value: ";"}
-}
-
-func NewTokenAssign(column int) *Token {
-	return &Token{Type: TokenAssign, Column: column, Value: ":="}
-}
-
-func NewTokenSlash(column int) *Token {
-	return &Token{Type: TokenSlash, Column: column, Value: "\\"}
-}
-
-func NewTokenAtom(name string, column int) *Token {
-	return &Token{Type: TokenAtom, Column: column, Value: name}
-}
-
-func NewTokenSoftBreak(column int) *Token {
-	return &Token{Type: TokenSoftBreak, Column: column, Value: "\\n"}
-}
-
-func (t TokenType) Name() string {
-	switch t {
-	case TokenOpenParen:
-		return "("
-	case TokenCloseParen:
-		return ")"
-	case TokenSlash:
-		return "\\"
-	case TokenDot:
-		return "."
-	case TokenAtom:
-		return "ATOM"
-	case TokenSoftBreak:
-		return "\\n"
-	case TokenHardBreak:
-		return ";"
-	default:
-		panic(fmt.Errorf("unknown token type %v", t))
-	}
-}
-
-func (t Token) Name() string {
-	return t.Type.Name()
-}

pkg/saccharine/token/parse.go

@@ -1,4 +1,4 @@
-package saccharine
+package token
 
 import (
 	"errors"
@@ -14,7 +14,7 @@ func isVariable(r rune) bool {
 	return unicode.IsLetter(r) || unicode.IsNumber(r)
 }
 
-func scanRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
+func parseRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
 	i2 := i.Copy()
 
 	if r, err := i2.Next(); err != nil {
@@ -27,7 +27,7 @@ func scanRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error
 	}
 }
 
-func scanCharacter(i *iterator.Iterator[rune], expected rune) (rune, error) {
+func parseCharacter(i *iterator.Iterator[rune], expected rune) (rune, error) {
 	i2 := i.Copy()
 
 	if r, err := i2.Next(); err != nil {
@@ -42,7 +42,7 @@ func scanCharacter(i *iterator.Iterator[rune], expected rune) (rune, error) {
 
 // Pulls the next token from an iterator over runes. If it cannot, it will
 // return nil. If an error occurs, it will return that.
-func scanToken(i *iterator.Iterator[rune]) (*Token, error) {
+func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 	index := i.Index()
 
 	if i.Done() {
@@ -56,27 +56,27 @@ func scanToken(i *iterator.Iterator[rune]) (*Token, error) {
 
 	switch {
 	case letter == '(':
-		return NewTokenOpenParen(index), nil
+		return NewOpenParen(index), nil
 	case letter == ')':
-		return NewTokenCloseParen(index), nil
+		return NewCloseParen(index), nil
 	case letter == '.':
-		return NewTokenDot(index), nil
+		return NewDot(index), nil
 	case letter == '\\':
-		return NewTokenSlash(index), nil
+		return NewSlash(index), nil
 	case letter == '\n':
-		return NewTokenSoftBreak(index), nil
+		return NewSoftBreak(index), nil
 	case letter == '{':
-		return NewTokenOpenBrace(index), nil
+		return NewOpenBrace(index), nil
 	case letter == '}':
-		return NewTokenCloseBrace(index), nil
+		return NewCloseBrace(index), nil
 	case letter == ':':
-		if _, err := scanCharacter(i, '='); err != nil {
+		if _, err := parseCharacter(i, '='); err != nil {
 			return nil, err
 		} else {
-			return NewTokenAssign(index), nil
+			return NewAssign(index), nil
 		}
 	case letter == ';':
-		return NewTokenHardBreak(index), nil
+		return NewHardBreak(index), nil
 	case letter == '#':
 		// Skip everything until the next newline or EOF.
 		for !i.Done() {
@@ -98,27 +98,27 @@ func scanToken(i *iterator.Iterator[rune]) (*Token, error) {
 		atom := []rune{letter}
 
 		for {
-			if r, err := scanRune(i, isVariable); err != nil {
+			if r, err := parseRune(i, isVariable); err != nil {
 				break
 			} else {
 				atom = append(atom, r)
 			}
 		}
 
-		return NewTokenAtom(string(atom), index), nil
+		return NewAtom(string(atom), index), nil
 	}
 
 	return nil, fmt.Errorf("unknown character '%v'", string(letter))
 }
 
-// scan a string into tokens.
-func scan(input string) ([]Token, error) {
+// Parse a string into tokens.
+func Parse(input string) ([]Token, error) {
 	i := iterator.Of([]rune(input))
 	tokens := []Token{}
 	errorList := []error{}
 
 	for !i.Done() {
-		token, err := scanToken(i)
+		token, err := getToken(i)
 		if err != nil {
 			errorList = append(errorList, err)
 		} else if token != nil {

pkg/saccharine/token/token.go

@@ -0,0 +1,91 @@
+package token
+
+import "fmt"
+
+// All tokens in the pseudo-lambda language.
+type Type int
+
+const (
+	OpenParen  Type = iota // Denotes the '(' token.
+	CloseParen             // Denotes the ')' token.
+	OpenBrace              // Denotes the '{' token.
+	CloseBrace             // Denotes the '}' token.
+	HardBreak              // Denotes the ';' token.
+	Assign                 // Denotes the ':=' token.
+	Atom                   // Denotes an alpha-numeric variable.
+	Slash                  // Denotes the '/' token.
+	Dot                    // Denotes the '.' token.
+	SoftBreak              // Denotes a new-line.
+)
+
+// A representation of a token in source code.
+type Token struct {
+	Column int    // Where the token begins in the source text.
+	Type   Type   // What type the token is.
+	Value  string // The value of the token.
+}
+
+func NewOpenParen(column int) *Token {
+	return &Token{Type: OpenParen, Column: column, Value: "("}
+}
+
+func NewCloseParen(column int) *Token {
+	return &Token{Type: CloseParen, Column: column, Value: ")"}
+}
+
+func NewOpenBrace(column int) *Token {
+	return &Token{Type: OpenBrace, Column: column, Value: "{"}
+}
+
+func NewCloseBrace(column int) *Token {
+	return &Token{Type: CloseBrace, Column: column, Value: "}"}
+}
+
+func NewDot(column int) *Token {
+	return &Token{Type: Dot, Column: column, Value: "."}
+}
+
+func NewHardBreak(column int) *Token {
+	return &Token{Type: HardBreak, Column: column, Value: ";"}
+}
+
+func NewAssign(column int) *Token {
+	return &Token{Type: Assign, Column: column, Value: ":="}
+}
+
+func NewSlash(column int) *Token {
+	return &Token{Type: Slash, Column: column, Value: "\\"}
+}
+
+func NewAtom(name string, column int) *Token {
+	return &Token{Type: Atom, Column: column, Value: name}
+}
+
+func NewSoftBreak(column int) *Token {
+	return &Token{Type: SoftBreak, Column: column, Value: "\\n"}
+}
+
+func Name(typ Type) string {
+	switch typ {
+	case OpenParen:
+		return "("
+	case CloseParen:
+		return ")"
+	case Slash:
+		return "\\"
+	case Dot:
+		return "."
+	case Atom:
+		return "ATOM"
+	case SoftBreak:
+		return "\\n"
+	case HardBreak:
+		return ";"
+	default:
+		panic(fmt.Errorf("unknown token type %v", typ))
+	}
+}
+
+func (t Token) Name() string {
+	return Name(t.Type)
+}

pkg/set/set.go

@@ -4,9 +4,9 @@ import "iter"
 
 type Set[T comparable] map[T]bool
 
-func (s Set[T]) Add(items ...T) {
+func (s *Set[T]) Add(items ...T) {
 	for _, item := range items {
-		s[item] = true
+		(*s)[item] = true
 	}
 }
 
@@ -14,14 +14,14 @@ func (s Set[T]) Has(item T) bool {
 	return s[item]
 }
 
-func (s Set[T]) Remove(items ...T) {
+func (s *Set[T]) Remove(items ...T) {
 	for _, item := range items {
-		delete(s, item)
+		delete(*s, item)
 	}
 }
 
-func (s Set[T]) Merge(o Set[T]) {
-	for item := range o {
+func (s *Set[T]) Merge(o *Set[T]) {
+	for item := range *o {
 		s.Add(item)
 	}
 }
@@ -46,8 +46,8 @@ func (s Set[T]) Items() iter.Seq[T] {
 	}
 }
 
-func New[T comparable](items ...T) Set[T] {
-	result := Set[T]{}
+func New[T comparable](items ...T) *Set[T] {
+	result := &Set[T]{}
 
 	for _, item := range items {
 		result.Add(item)

tests/church_6^6.test

@@ -0,0 +1,8 @@
+0 := \f.\x.x
+inc n := \f x.(f (n f x))
+exp n m := (m n)
+print n := (n F X)
+
+N := (inc (inc (inc (inc (inc (inc 0))))))
+
+(print (exp N N))