feat: add test target to Makefile.

Added make test directive that runs tests without benchmarks.
Updated help text to include the new test target.

.gitea/ISSUE_TEMPLATE/bug.md

@@ -1,58 +0,0 @@
----
-name: "Bug Report"
-about: "Report a bug or unexpected behavior in the lambda interpreter."
-title: "fix: "
-ref: "main"
-assignees: []
-labels:
-  - bug
----
-
-## Context
-
-<!--
-Describe what you were trying to do when you encountered the bug.
-Explain what you expected to happen.
--->
-
-## Current Behavior
-
-<!--
-Describe what actually happened.
-Be specific about the incorrect behavior or error.
--->
-
-## Steps to Reproduce
-
-<!--
-Provide step-by-step instructions to reproduce the issue.
-Include any relevant code, commands, or input.
--->
-
-1.
-2.
-3.
-
-## Expected Behavior
-
-<!--
-Describe what should happen instead.
--->
-
-## Environment
-
-<!--
-Provide relevant information about your environment.
--->
-
-- Lambda version:
-- Go version:
-- Operating system:
-
-## Additional Context
-
-<!--
-Add any other context about the problem.
-Include error messages, logs, or screenshots if applicable.
-If none exist, omit this section.
--->

.gitea/ISSUE_TEMPLATE/feature.md

@@ -1,44 +0,0 @@
----
-name: "Feature Request"
-about: "Suggest a new feature or enhancement for the lambda interpreter."
-title: "feat: "
-ref: "main"
-assignees: []
-labels:
-  - enhancement
----
-
-## Context
-
-<!--
-Describe the problem or limitation you're encountering.
-Explain why this feature would be valuable.
--->
-
-## Proposed Solution
-
-<!--
-Describe your proposed solution or enhancement.
-Be specific about what you want to see implemented.
--->
-
-## Alternatives Considered
-
-<!--
-List any alternative solutions or approaches you've considered.
-If none exist, omit this section.
--->
-
-## Acceptance Criteria
-
-<!--
-List clear, testable criteria that define when this feature is complete.
-Use bullet points starting with •
--->
-
-## Additional Context
-
-<!--
-Add any other context, screenshots, or examples about the feature request.
-If none exist, omit this section.
--->

.gitea/ISSUE_TEMPLATE/general.md

@@ -1,37 +0,0 @@
----
-name: "General Issue"
-about: "Create an issue that doesn't fit other templates."
-title: ""
-ref: "main"
-assignees: []
-labels: []
----
-
-## Context
-
-<!--
-Describe the background and context for this issue.
-Explain why this issue exists.
--->
-
-## Description
-
-<!--
-Provide a detailed description of what needs to be done.
-Be clear and specific about the requirements.
--->
-
-## Acceptance Criteria
-
-<!--
-List clear, testable criteria that define when this issue is complete.
-Use bullet points starting with •
-If none exist, omit this section.
--->
-
-## Additional Context
-
-<!--
-Add any other relevant information, links, or references.
-If none exist, omit this section.
--->

CLAUDE.md

@@ -37,31 +37,6 @@ Use format: `<type>/<description>` with kebab-case.
 
 DO NOT advertise Claude.
 
-## Issue Management
-
-Use the `tea` CLI (Gitea command-line tool) for issue operations.
-
-**Common commands**:
-
-- `tea issues list` - List all issues.
-- `tea issues <number>` - View details of a specific issue.
-- `tea issues create --title "<title>" --body "<description>"` - Create a new issue.
-- `tea issues close <number>` - Close an issue.
-
-**Reading issues**: Use `tea issues <number>` to read the full details of an issue before starting work.
-
-## Issue Workflow
-
-When working on an issue:
-
-1. Read the issue using `tea issues <number>` to understand requirements.
-2. Create a feature branch following the branch naming convention.
-3. Make commits following the conventional commit format.
-4. Submit a pull request when ready.
-
-**Important**: Never commit directly to `main`.
-All work must be done in a feature branch and submitted via pull request.
-
 ## Pull Request Management
 
 Use the `tea` CLI (Gitea command-line tool) for PR operations instead of `gh`.
@@ -77,29 +52,3 @@ Use the `tea` CLI (Gitea command-line tool) for PR operations instead of `gh`.
 **Note**: Use `--description` (not `--body`) for PR body content.
 
 **Creating PRs**: Always create PRs in a branch other than `main`, to the `main` branch unless specified otherwise. ALWAYS FOLLOW THE PR TEMPLATE, EXACTLY.
-
-**Linking issues**: When a PR solves an issue, reference the issue in both the commit message and PR description using `Closes #<number>`.
-This automatically links and closes the issue when the PR is merged.
-
-### Updating PRs
-
-When pushing additional changes to an existing PR, add a comment summarizing the new commits.
-This keeps reviewers informed of what changed since the initial PR description.
-
-Use the `tea` CLI to add comments to pull requests:
-
-```bash
-tea comment <number> "Comment text"
-```
-
-#### Examples
-
-```bash
-# Add a comment to PR #42
-tea comment 42 "Updated implementation based on feedback"
-
-# Add a multi-line comment
-tea comment 42 "Summary of changes:
-- Fixed bug in reducer
-- Added new tests"
-```

cmd/lambda/lambda.go

@@ -5,11 +5,12 @@ import (
 
 	"git.maximhutz.com/max/lambda/internal/cli"
 	"git.maximhutz.com/max/lambda/internal/config"
-	"git.maximhutz.com/max/lambda/internal/plugins"
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/internal/explanation"
+	"git.maximhutz.com/max/lambda/internal/performance"
+	"git.maximhutz.com/max/lambda/internal/statistics"
 	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/debruijn"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
-	"git.maximhutz.com/max/lambda/pkg/reducer"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
@@ -33,53 +34,44 @@ func main() {
 
 	// Compile expression to lambda calculus.
 	compiled := convert.SaccharineToLambda(ast)
-	logger.Info("compiled λ expression", "tree", compiled.String())
+	logger.Info("compiled λ expression", "tree", lambda.Stringify(compiled))
 
-	// Create reducer based on the selected interpreter.
+	// Create reduction engine.
-	var red reducer.Reducer
+	process := engine.New(options, &compiled)
-	switch options.Interpreter {
-	case config.DeBruijnInterpreter:
-		dbExpr := convert.LambdaToDeBruijn(compiled)
-		logger.Info("converted to De Bruijn", "tree", dbExpr.String())
-		red = debruijn.NewNormalOrderReducer(&dbExpr)
-	default:
-		red = lambda.NewNormalOrderReducer(&compiled)
-	}
 
-	// If the user selected to track CPU performance, attach a profiler.
+	// If the user selected to track CPU performance, attach a profiler to the
+	// process.
 	if options.Profile != "" {
-		plugins.NewPerformance(options.Profile, red)
+		profiler := performance.Track(options.Profile)
+		process.On("start", profiler.Start)
+		process.On("end", profiler.End)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
-	// observer.
+	// observer here.
 	if options.Explanation {
-		plugins.NewExplanation(red)
+		explanation.Track(process)
 	}
 
-	// If the user opted to track statistics, attach a tracker.
+	// If the user opted to track statistics, attach a tracker here, too.
 	if options.Statistics {
-		plugins.NewStatistics(red)
+		statistics := statistics.Track()
+		process.On("start", statistics.Start)
+		process.On("step", statistics.Step)
+		process.On("end", statistics.End)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		plugins.NewLogs(logger, red)
+		process.On("step", func() {
+			logger.Info("reduction", "tree", lambda.Stringify(compiled))
+		})
 	}
 
-	// Run reduction.
+	process.Run()
-	red.Reduce()
 
 	// Return the final reduced result.
-	// For De Bruijn, convert back to lambda for consistent output.
+	result := lambda.Stringify(compiled)
-	var result string
-	if options.Interpreter == config.DeBruijnInterpreter {
-		dbExpr := red.Expression().(debruijn.Expression)
-		lambdaExpr := convert.DeBruijnToLambda(dbExpr)
-		result = lambdaExpr.String()
-	} else {
-		result = red.Expression().String()
-	}
 	err = options.Destination.Write(result)
 	cli.HandleError(err)
 }

cmd/lambda/lambda_test.go

@@ -6,8 +6,9 @@ import (
 	"strings"
 	"testing"
 
+	"git.maximhutz.com/max/lambda/internal/config"
+	"git.maximhutz.com/max/lambda/internal/engine"
 	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/debruijn"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 	"github.com/stretchr/testify/assert"
@@ -30,43 +31,24 @@ func runSample(samplePath string) (string, error) {
 	// Compile expression to lambda calculus.
 	compiled := convert.SaccharineToLambda(ast)
 
-	// Create and run the reducer.
+	// Create minimal config for benchmarking.
-	reducer := lambda.NewNormalOrderReducer(&compiled)
+	cfg := &config.Config{
-	reducer.Reduce()
+		Source:      config.StringSource{Data: ""},
-
+		Destination: config.StdoutDestination{},
-	return reducer.Expression().String() + "\n", nil
+		Profile:     "",
-}
+		Explanation: false,
-
+		Statistics:  false,
-// Helper function to run a single sample through the De Bruijn interpreter.
+		Verbose:     false,
-func runSampleDeBruijn(samplePath string) (string, error) {
-	// Read the sample file.
-	input, err := os.ReadFile(samplePath)
-	if err != nil {
-		return "", err
 	}
 
-	// Parse code into syntax tree.
+	// Create and run the engine.
-	ast, err := saccharine.Parse(string(input))
+	process := engine.New(cfg, &compiled)
-	if err != nil {
+	process.Run()
-		return "", err
-	}
 
-	// Compile expression to lambda calculus.
+	return lambda.Stringify(compiled) + "\n", nil
-	compiled := convert.SaccharineToLambda(ast)
-
-	// Convert to De Bruijn and run reducer.
-	dbExpr := convert.LambdaToDeBruijn(compiled)
-	reducer := debruijn.NewNormalOrderReducer(&dbExpr)
-	reducer.Reduce()
-
-	// Convert back to lambda for output.
-	result := reducer.Expression().(debruijn.Expression)
-	lambdaResult := convert.DeBruijnToLambda(result)
-
-	return lambdaResult.String() + "\n", nil
 }
 
-// Test that all samples produce expected output with lambda interpreter.
+// Test that all samples produce expected output.
 func TestSamplesValidity(t *testing.T) {
 	// Discover all .test files in the tests directory.
 	testFiles, err := filepath.Glob("../../tests/*.test")
@@ -95,35 +77,6 @@ func TestSamplesValidity(t *testing.T) {
 	}
 }
 
-// Test that all samples produce expected output with De Bruijn interpreter.
-func TestSamplesValidityDeBruijn(t *testing.T) {
-	// Discover all .test files in the tests directory.
-	testFiles, err := filepath.Glob("../../tests/*.test")
-	assert.NoError(t, err, "Failed to read tests directory.")
-	assert.NotEmpty(t, testFiles, "No '*.test' files found in directory.")
-
-	for _, testPath := range testFiles {
-		// Build expected file path.
-		expectedPath := strings.TrimSuffix(testPath, filepath.Ext(testPath)) + ".expected"
-
-		name := strings.TrimSuffix(filepath.Base(testPath), filepath.Ext(testPath))
-
-		t.Run(name, func(t *testing.T) {
-			// Run the sample and capture output.
-			actual, err := runSampleDeBruijn(testPath)
-			assert.NoError(t, err, "Failed to run sample.")
-
-			// Read expected output.
-			expectedBytes, err := os.ReadFile(expectedPath)
-			assert.NoError(t, err, "Failed to read expected output.")
-			expected := string(expectedBytes)
-
-			// Compare outputs.
-			assert.Equal(t, expected, actual, "Output does not match expected.")
-		})
-	}
-}
-
 // Benchmark all samples using sub-benchmarks.
 func BenchmarkSamples(b *testing.B) {
 	// Discover all .test files in the tests directory.
@@ -142,22 +95,3 @@ func BenchmarkSamples(b *testing.B) {
 		})
 	}
 }
-
-// Benchmark all samples using De Bruijn interpreter.
-func BenchmarkSamplesDeBruijn(b *testing.B) {
-	// Discover all .test files in the tests directory.
-	testFiles, err := filepath.Glob("../../tests/*.test")
-	assert.NoError(b, err, "Failed to read tests directory.")
-	assert.NotEmpty(b, testFiles, "No '*.test' files found in directory.")
-
-	for _, path := range testFiles {
-		name := strings.TrimSuffix(filepath.Base(path), filepath.Ext(path))
-
-		b.Run(name, func(b *testing.B) {
-			for b.Loop() {
-				_, err := runSampleDeBruijn(path)
-				assert.NoError(b, err, "Failed to run sample.")
-			}
-		})
-	}
-}

internal/config/config.go

@@ -1,14 +1,6 @@
 // Package "config" parses ad handles the user settings given to the program.
 package config
 
-// Interpreter specifies the reduction engine to use.
-type Interpreter string
-
-const (
-	LambdaInterpreter   Interpreter = "lambda"
-	DeBruijnInterpreter Interpreter = "debruijn"
-)
-
 // Configuration settings for the program.
 type Config struct {
 	Source      Source      // The source code given to the program.
@@ -17,5 +9,4 @@ type Config struct {
 	Explanation bool        // Whether or not to print an explanation of the reduction.
 	Profile     string      // If not nil, print a CPU profile during execution.
 	Statistics  bool        // Whether or not to print statistics.
-	Interpreter Interpreter // The interpreter engine to use.
 }

internal/config/parse_from_args.go

@@ -14,20 +14,8 @@ func FromArgs() (*Config, error) {
 	profile := flag.String("p", "", "CPU profiling. If an output file is defined, the program will profile its execution and dump its results into it.")
 	file := flag.String("f", "", "File. If set, read source from the specified file.")
 	output := flag.String("o", "", "Output. If set, write result to the specified file. Use '-' for stdout (default).")
-	interpreter := flag.String("i", "lambda", "Interpreter. The reduction engine to use: 'lambda' or 'debruijn'.")
 	flag.Parse()
 
-	// Validate interpreter flag.
-	var interpType Interpreter
-	switch *interpreter {
-	case "lambda":
-		interpType = LambdaInterpreter
-	case "debruijn":
-		interpType = DeBruijnInterpreter
-	default:
-		return nil, fmt.Errorf("invalid interpreter: %s (must be 'lambda' or 'debruijn')", *interpreter)
-	}
-
 	// Parse source type.
 	var source Source
 	if *file != "" {
@@ -64,6 +52,5 @@ func FromArgs() (*Config, error) {
 		Explanation: *explanation,
 		Profile:     *profile,
 		Statistics:  *statistics,
-		Interpreter: interpType,
 	}, nil
 }

internal/engine/engine.go

@@ -0,0 +1,32 @@
+// Package "engine" provides an extensible interface for users to interfact with
+// λ-calculus.
+package engine
+
+import (
+	"git.maximhutz.com/max/lambda/internal/config"
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+// A process for reducing one λ-expression.
+type Engine struct {
+	Config     *config.Config
+	Expression *lambda.Expression
+	emitter.Emitter
+}
+
+// Create a new engine, given an unreduced λ-expression.
+func New(config *config.Config, expression *lambda.Expression) *Engine {
+	return &Engine{Config: config, Expression: expression}
+}
+
+// Begin the reduction process.
+func (e Engine) Run() {
+	e.Emit("start")
+
+	lambda.ReduceAll(e.Expression, func() {
+		e.Emit("step")
+	})
+
+	e.Emit("end")
+}

internal/explanation/tracker.go

@@ -0,0 +1,32 @@
+// Package "explanation" provides a observer to gather the reasoning during the
+// reduction, and present a thorough explanation to the user for each step.
+package explanation
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+// Track the reductions made by a reduction proess.
+type Tracker struct {
+	process *engine.Engine
+}
+
+// Attaches a new explanation tracker to a process.
+func Track(process *engine.Engine) *Tracker {
+	tracker := &Tracker{process: process}
+	process.On("start", tracker.Start)
+	process.On("step", tracker.Step)
+
+	return tracker
+}
+
+func (t *Tracker) Start() {
+	fmt.Println(lambda.Stringify(*t.process.Expression))
+}
+
+func (t *Tracker) Step() {
+	fmt.Println("    =", lambda.Stringify(*t.process.Expression))
+}

internal/performance/tracker.go

@@ -1,34 +1,28 @@
 // Package "performance" provides a tracker to observer CPU performance during
 // execution.
-package plugins
+package performance
 
 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 {
+type Tracker 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 {
+func Track(file string) *Tracker {
-	plugin := &Performance{File: file}
+	return &Tracker{File: file}
-	process.On(reducer.StartEvent, plugin.Start)
-	process.On(reducer.StopEvent, plugin.Stop)
-
-	return plugin
 }
 
 // Begin profiling.
-func (t *Performance) Start() {
+func (t *Tracker) Start() {
 	var absPath string
 
 	absPath, t.Error = filepath.Abs(t.File)
@@ -53,7 +47,7 @@ func (t *Performance) Start() {
 }
 
 // Stop profiling.
-func (t *Performance) Stop() {
+func (t *Tracker) End() {
 	pprof.StopCPUProfile()
 	t.filePointer.Close()
 }

internal/plugins/debug.go

@@ -1,23 +0,0 @@
-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

@@ -1,31 +0,0 @@
-// 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/statistics.go

@@ -1,44 +0,0 @@
-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/statistics/tracker.go

@@ -0,0 +1,36 @@
+package statistics
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+// An observer, to track reduction performance.
+type Tracker struct {
+	start time.Time
+	steps uint64
+}
+
+// Create a new reduction performance tracker.
+func Track() *Tracker {
+	return &Tracker{}
+}
+
+func (t *Tracker) Start() {
+	t.start = time.Now()
+	t.steps = 0
+}
+
+func (t *Tracker) Step() {
+	t.steps++
+}
+
+func (t *Tracker) End() {
+	results := Results{
+		StepsTaken:  t.steps,
+		TimeElapsed: uint64(time.Since(t.start).Milliseconds()),
+	}
+
+	fmt.Fprint(os.Stderr, results.String())
+}

pkg/convert/debruijn_to_lambda.go

@@ -1,82 +0,0 @@
-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

@@ -1,44 +0,0 @@
-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

@@ -1,119 +0,0 @@
-// 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

@@ -1,76 +0,0 @@
-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

@@ -1,66 +0,0 @@
-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
-	}
-}
-
-// 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 {
-			// 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.
-			it.Swap(Shift(substituted, -1, 0))
-
-			r.Emit(reducer.StepEvent)
-
-			if _, _, ok := isViable(it.Parent()); ok {
-				it.Back()
-			}
-		}
-	}
-
-	r.Emit(reducer.StopEvent)
-}

pkg/debruijn/shift.go

@@ -1,32 +0,0 @@
-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

@@ -1,35 +0,0 @@
-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

@@ -1,34 +0,0 @@
-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
-	}
-}

pkg/emitter/emitter.go

@@ -2,45 +2,53 @@ package emitter
 
 import "git.maximhutz.com/max/lambda/pkg/set"
 
-type Emitter[E comparable] interface {
+type Observer struct {
-	On(E, func()) Listener[E]
+	fn      func()
-	Off(Listener[E])
+	message string
-	Emit(E)
+	emitter *Emitter
 }
 
-type BaseEmitter[E comparable] struct {
+type Emitter struct {
-	listeners map[E]*set.Set[Listener[E]]
+	listeners map[string]*set.Set[*Observer]
 }
 
-func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
+func Ignore[T any](fn func()) func(T) {
-	if e.listeners[kind] == nil {
+	return func(T) { fn() }
-		e.listeners[kind] = set.New[Listener[E]]()
+}
+
+func (e *Emitter) On(message string, fn func()) *Observer {
+	observer := &Observer{
+		fn:      fn,
+		message: message,
+		emitter: e,
 	}
 
-	listener := &BaseListener[E]{kind, fn}
+	if e.listeners == nil {
-	e.listeners[kind].Add(listener)
+		e.listeners = map[string]*set.Set[*Observer]{}
-	return listener
-}
-
-func (e *BaseEmitter[E]) Off(listener Listener[E]) {
-	kind := listener.Kind()
-	if e.listeners[kind] != nil {
-		e.listeners[kind].Remove(listener)
-	}
-}
-
-func (e *BaseEmitter[E]) Emit(event E) {
-	if e.listeners[event] == nil {
-		e.listeners[event] = set.New[Listener[E]]()
 	}
 
-	for listener := range e.listeners[event].Items() {
+	if e.listeners[message] == nil {
-		listener.Run()
+		e.listeners[message] = set.New[*Observer]()
 	}
+
+	e.listeners[message].Add(observer)
+	return observer
 }
 
-func New[E comparable]() *BaseEmitter[E] {
+func (o *Observer) Off() {
-	return &BaseEmitter[E]{
+	if o.emitter.listeners[o.message] == nil {
-		listeners: map[E]*set.Set[Listener[E]]{},
+		return
+	}
+
+	o.emitter.listeners[o.message].Remove(o)
+}
+
+func (e *Emitter) Emit(message string) {
+	if e.listeners[message] == nil {
+		return
+	}
+
+	for listener := range *e.listeners[message] {
+		listener.fn()
 	}
 }

pkg/emitter/listener.go

@@ -1,19 +0,0 @@
-package emitter
-
-type Listener[E comparable] interface {
-	Kind() E
-	Run()
-}
-
-type BaseListener[E comparable] struct {
-	kind E
-	fn   func()
-}
-
-func (l BaseListener[E]) Kind() E {
-	return l.kind
-}
-
-func (l BaseListener[E]) Run() {
-	l.fn()
-}

pkg/expr/expr.go

@@ -1,11 +0,0 @@
-// Package expr provides the abstract Expression interface for all evaluatable
-// expression types in the lambda interpreter.
-package expr
-
-// 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 {
-	// String returns a human-readable representation of the expression.
-	String() string
-}

pkg/lambda/expression.go

@@ -1,11 +1,6 @@
 package lambda
 
-import "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 {
-	expr.Expression
 	Accept(Visitor)
 }
 
@@ -28,10 +23,6 @@ func (a *Abstraction) Accept(v Visitor) {
 	v.VisitAbstraction(a)
 }
 
-func (a *Abstraction) String() string {
-	return Stringify(a)
-}
-
 func NewAbstraction(parameter string, body Expression) *Abstraction {
 	return &Abstraction{parameter: parameter, body: body}
 }
@@ -55,10 +46,6 @@ func (a *Application) Accept(v Visitor) {
 	v.VisitApplication(a)
 }
 
-func (a *Application) String() string {
-	return Stringify(a)
-}
-
 func NewApplication(abstraction Expression, argument Expression) *Application {
 	return &Application{abstraction: abstraction, argument: argument}
 }
@@ -77,10 +64,6 @@ func (v *Variable) Accept(visitor Visitor) {
 	visitor.VisitVariable(v)
 }
 
-func (v *Variable) String() string {
-	return Stringify(v)
-}
-
 func NewVariable(name string) *Variable {
 	return &Variable{value: name}
 }

pkg/lambda/reduce.go

@@ -0,0 +1,30 @@
+package lambda
+
+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
+	}
+}
+
+func ReduceAll(e *Expression, step func()) {
+	it := NewIterator(e)
+
+	for !it.Done() {
+		if fn, arg, ok := IsViable(it.Current()); !ok {
+			it.Next()
+		} else {
+			it.Swap(Substitute(fn.body, fn.parameter, arg))
+			step()
+
+			if _, _, ok := IsViable(it.Parent()); ok {
+				it.Back()
+			}
+		}
+	}
+}

pkg/lambda/reducer.go

@@ -1,61 +0,0 @@
-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/reducer/events.go

@@ -1,13 +0,0 @@
-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

@@ -1,27 +0,0 @@
-// 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/token/parse.go

@@ -77,21 +77,6 @@ func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 		}
 	case letter == ';':
 		return NewHardBreak(index), nil
-	case letter == '#':
-		// Skip everything until the next newline or EOF.
-		for !i.Done() {
-			r, err := i.Next()
-			if err != nil {
-				return nil, trace.Wrap(err, "error while parsing comment")
-			}
-
-			if r == '\n' {
-				// Put the newline back so it can be processed as a soft break.
-				i.Back()
-				break
-			}
-		}
-		return nil, nil
 	case unicode.IsSpace(letter):
 		return nil, nil
 	case isVariable(letter):

pkg/set/set.go

@@ -1,7 +1,5 @@
 package set
 
-import "iter"
-
 type Set[T comparable] map[T]bool
 
 func (s *Set[T]) Add(items ...T) {
@@ -36,16 +34,6 @@ func (s Set[T]) ToList() []T {
 	return list
 }
 
-func (s Set[T]) Items() iter.Seq[T] {
-	return func(yield func(T) bool) {
-		for item := range s {
-			if !yield(item) {
-				return
-			}
-		}
-	}
-}
-
 func New[T comparable](items ...T) *Set[T] {
 	result := &Set[T]{}
 

tests/comments.expected

@@ -1 +0,0 @@
-VALUE

tests/comments.test

@@ -1,17 +0,0 @@
-# This is a full-line comment at the start
-# The following defines the identity function
-identity := \x.x # This is an end-of-line comment
-
-# Define a simple function that applies a function twice
-twice := \f.\x.(f
-    # Comments can be anywhere!
-(f x))
-
-# Test that comments don't interfere with expressions
-result := (twice identity VALUE) # Should just return VALUE
-
-# Multiple comments in a row
-# can appear anywhere
-# without breaking the code
-
-result # Final comment at the end