docs: remove makefile-improvements.md in favor of CLAUDE.md

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
feat: explicitly set help as default target
2026-01-10 17:15:14 -05:00 · 2026-01-10 16:38:37 -05:00 · 2026-01-10 16:38:19 -05:00 · 2026-01-10 16:36:13 -05:00 · 2026-01-10 16:34:07 -05:00 · 2026-01-10 16:29:25 -05:00
65 changed files with 345 additions and 1800 deletions
--- a/.gitea/ISSUE_TEMPLATE/bug.md
+++ b/.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.
-->
--- a/.gitea/ISSUE_TEMPLATE/feature.md
+++ b/.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.
-->
--- a/.gitea/ISSUE_TEMPLATE/general.md
+++ b/.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.
-->
--- a/.gitea/PULL_REQUEST_TEMPLATE.md
+++ b/.gitea/PULL_REQUEST_TEMPLATE.md
@@ -1,37 +0,0 @@
---
-name: "Default Template"
-about: "The default template for `lambda`."
-title: "<type>: <description>"
-ref: "main"
-assignees: []
-labels: []
---
-
-## Description
-
-<!--
-First, describe the context for the PR.
-Then, explain why the PR exists.
-Finally, in concise, sentence-long bullets, explain each change.
-->
-
-### Decisions
-
-<!--
-List any major architectural decisions here.
-If none exist, omit this section.
-->
-
-## Benefits
-
-<!--
-List any major benefits here.
-How would this PR improve the codebase/product?
-->
-
-## Checklist
-
- [ ] Code follows conventional commit format.
- [ ] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [ ] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).
--- a/.gitignore
+++ b/.gitignore
@@ -3,13 +3,18 @@
 # https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore
 #
 # Binaries for programs and plugins
-/lambda
 *.exe
 *.exe~
 *.dll
 *.so
 *.dylib

+# Build artifacts
+lambda
+
+# Test binary, built with `go test -c`
+*.test
+
 # Output of the go coverage tool, specifically when used with LiteIDE
 *.out

--- a/.golangci.yml
+++ b/.golangci.yml
@@ -1,230 +1,83 @@
---
-# golangci-lint configuration file made by @ccoVeille
-# Source: https://github.com/ccoVeille/golangci-lint-config-examples/
-# Author: @ccoVeille
-# License: MIT
-# Variant: 03-safe
-# Version: v2.0.0
-#
 version: "2"

 formatters:
  enable:
-    # format the code
    - gofmt
-    # format the block of imports
    - gci

  settings:
-    # format the code with Go standard library
    gofmt:
-      # simplify the code
-      # https://pkg.go.dev/cmd/gofmt#hdr-The_simplify_command
      simplify: true
      rewrite-rules:
-        # replace `interface{}` with `any` in the code on format
        - pattern: 'interface{}'
          replacement: 'any'
-    
-    # make sure imports are always in a deterministic order
-    # https://github.com/daixiang0/gci/
-    gci:  # define the section orders for imports
+
+    gci:
      sections:
-        # Standard section: captures all standard packages.
        - standard
-        # Default section: catchall that is not standard or custom
        - default
-        # linters that related to local tool, so they should be separated
        - localmodule

 linters:
  exclusions:
-    # these presets where present in the v1 version of golangci-lint
-    # it's interesting to keep them when migrating, but removing them should be the goal
    presets:
-      # exclude check on comments format in godoc
-      # These are common false positives in poor code
-      # you should not use this on recent code you write from scratch
-      # 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
-
-      # Common false positives
-      # feel free to remove this if you don't have any false positives
-      # More information: https://golangci-lint.run/usage/false-positives/#common-false-positives
      - common-false-positives
-
-      # Legacy preset is not recommended anymore
-      # More information: https://golangci-lint.run/usage/false-positives/#legacy
      - legacy
-
-      # std-error-handling is a set of rules that avoid reporting unhandled errors on common functions/methods
-      # More information: https://golangci-lint.run/usage/false-positives/#std-error-handling
      - std-error-handling

-  # some linters are enabled by default
-  # https://golangci-lint.run/usage/linters/
-  #
-  # enable some extra linters
  enable:
-    # Errcheck is a program for checking for unchecked errors in Go code.
    - errcheck
-
-    # Vet examines Go source code and reports suspicious constructs.
    - govet
-
-    # Detects when assignments to existing variables are not used.
    - ineffassign
-
-    # It's a set of rules from staticcheck. See https://staticcheck.io/
    - staticcheck
-
-    # Checks Go code for unused constants, variables, functions and types.
    - unused
-
-    # Fast, configurable, extensible, flexible, and beautiful linter for Go.
-    # Drop-in replacement of golint.
    - revive
-
-    # make sure to use t.Helper() when needed
    - thelper
-
-    # mirror suggests rewrites to avoid unnecessary []byte/string conversion
    - mirror
-
-    # detect the possibility to use variables/constants from the Go standard library.
    - usestdlibvars
-
-    # Finds commonly misspelled English words.
    - misspell
-
-    # Checks for duplicate words in the source code.
    - dupword
-
-    # linter to detect errors invalid key values count
    - loggercheck
-
-    # detect when a package or method could be replaced by one from the standard library
    - exptostd
-
-    # detects nested contexts in loops or function literals
    - fatcontext
-
-    # Reports uses of functions with replacement inside the testing package.
    - usetesting

  settings:
    revive:
      rules:
-        # these are the default revive rules
-        # you can remove the whole "rules" node if you want
-        # BUT
-        # ! /!\ they all need to be present when you want to add more rules than the default ones
-        # otherwise, you won't have the default rules, but only the ones you define in the "rules" node
-
-        # Blank import should be only in a main or test package, or have a comment justifying it.
        - name: blank-imports
-
-        # context.Context() should be the first parameter of a function when provided as argument.
        - name: context-as-argument
          arguments:
            - allowTypesBefore: "*testing.T"
-
-        # Basic types should not be used as a key in `context.WithValue`
        - name: context-keys-type
-
-        # Importing with `.` makes the programs much harder to understand
        - name: dot-imports
-
-        # Empty blocks make code less readable and could be a symptom of a bug or unfinished refactoring.
        - name: empty-block
-
-        # for better readability, variables of type `error` must be named with the prefix `err`.
        - name: error-naming
-
-        # for better readability, the errors should be last in the list of returned values by a function.
        - name: error-return
-
-        # for better readability, error messages should not be capitalized or end with punctuation or a newline.
        - name: error-strings
-
-        # report when replacing `errors.New(fmt.Sprintf())` with `fmt.Errorf()` is possible
        - name: errorf
-
-        # check naming and commenting conventions on exported symbols.
        - name: exported
          arguments:
-            # make error messages clearer
            - "sayRepetitiveInsteadOfStutters"
-
-        # incrementing an integer variable by 1 is recommended to be done using the `++` operator
        - name: increment-decrement
-
-        # highlights redundant else-blocks that can be eliminated from the code
-        # - name: indent-error-flow
-
-        # This rule suggests a shorter way of writing ranges that do not use the second value.
        - name: range
-
-        # receiver names in a method should reflect the struct name (p for Person, for example)
        - name: receiver-naming
-
-        # redefining built in names (true, false, append, make) can lead to bugs very difficult to detect.
        - name: redefines-builtin-id
-
-        # redundant else-blocks that can be eliminated from the code.
-        # - name: superfluous-else
-
-        # prevent confusing name for variables when using `time` package
        - name: time-naming
-
-        # warns when an exported function or method returns a value of an un-exported type.
        - name: unexported-return
-
-        # spots and proposes to remove unreachable code. also helps to spot errors
        - name: unreachable-code
-
-        # Functions or methods with unused parameters can be a symptom of an unfinished refactoring or a bug.
        - name: unused-parameter
-
-        # report when a variable declaration can be simplified
        - name: var-declaration
-
-        # warns when initialism, variable or package naming conventions are not followed.
        - name: var-naming

    misspell:
-      # Correct spellings using locale preferences for US or UK.
-      # Setting locale to US will correct the British spelling of 'colour' to 'color'.
-      # Default ("") is to use a neutral variety of English.
      locale: US
-
-      # List of words to ignore
-      # among the one defined in https://github.com/golangci/misspell/blob/master/words.go
      ignore-rules: []
-      #  - valor
-      #  - and
-
-      # Extra word corrections.
      extra-words: []
-      #  - typo: "whattever"
-      #    correction: "whatever"

 output:
-  # Order to use when sorting results.
-  # Possible values: `file`, `linter`, and `severity`.
-  #
-  # If the severity values are inside the following list, they are ordered in this order:
-  #   1. error
-  #   2. warning
-  #   3. high
-  #   4. medium
-  #   5. low
-  # Either they are sorted alphabetically.
-  #
-  # Default: ["file"]
  sort-order:
    - linter
    - severity
-    - file # filepath, line, and column.
+    - file
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -0,0 +1,3 @@
+{
+    "makefile.configureOnOpen": false
+}
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1,105 +0,0 @@
-# Guide To `lambda`
-
-## Documentation Style
-
-Use full sentences.
-Every sentence gets its own line in Markdown.
-Every sentence ends in a period.
-
-## Coding Style
-
-### Conventional Commits
-
-Use conventional commit format: `<type>: <description>`.
-
-**Types**: `feat`, `fix`, `docs`, `refactor`, `test`, `chore`, `perf`
-
-**Examples**:
-
- `feat: add explanation mode flag to CLI`
- `fix: correct variable renaming in nested abstractions`
- `docs: update Makefile documentation`
-
-DO NOT advertise Claude.
-
-### Branch Names
-
-Use format: `<type>/<description>` with kebab-case.
-
-**Types**: Same as commits: `feat`, `fix`, `docs`, `refactor`, `test`, `chore`, `perf`.
-
-**Examples**:
-
- `feat/explanation-mode`
- `fix/variable-renaming`
- `docs/makefile-improvements`
- `refactor/silent-directive`
-
-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`.
-
-**Common commands**:
-
- `tea pr create --title "<title>" --description "<body>"` - Create a new pull request.
- `tea pr list` - List pull requests.
- `tea pr checkout <number>` - Check out a PR locally.
- `tea pr close <number>` - Close a pull request.
- `tea pr merge <number>` - Merge a pull request.
-
-**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"
-```
--- a/51
+++ b/51
@@ -1,51 +1,40 @@
 BINARY_NAME=lambda
 TEST=simple

-.PHONY: help build run profile explain graph docs test bench clean
 .DEFAULT_GOAL := help
 .SILENT:
+.PHONY: help it run profile explain graph clean

 help:
 	echo "Available targets:"
-	echo "  build    - Build the lambda executable"
-	echo "  run      - Build and run the lambda interpreter (use TEST=<name> to specify sample)"
+	echo "  it       - Build the lambda binary"
+	echo "  run      - Build and run with sample input (default: simple.txt)"
 	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 "  clean    - Remove all build artifacts"
+	echo "  explain  - Run with explanation mode and profiling"
+	echo "  graph    - Generate CPU profile visualization"
+	echo "  clean    - Remove build artifacts"
+	echo ""
+	echo "Usage: make run TEST=<sample-name>"

-build:
+it:
 	go build -o ${BINARY_NAME} ./cmd/lambda
-	chmod +x ${BINARY_NAME}

-run: build
-	./${BINARY_NAME} -s -f ./tests/$(TEST).test -o program.out
+run: it
+	./lambda - < ./samples/$(TEST).txt > program.out

-profile: build
-	./${BINARY_NAME} -p profile/cpu.prof -f ./tests/$(TEST).test -o program.out
+profile: it
+	mkdir -p profile
+	./lambda -p profile/cpu.prof - < ./samples/$(TEST).txt > program.out

-explain: build
-	./${BINARY_NAME} -x -p profile/cpu.prof -f ./tests/$(TEST).test -o program.out > explain.out
+explain: it
+	mkdir -p profile
+	./lambda -x -p profile/cpu.prof - < ./samples/$(TEST).txt > program.out

-graph:
+graph: profile
 	go tool pprof -raw -output=profile/cpu.raw profile/cpu.prof
 	go tool pprof -svg profile/cpu.prof > profile/cpu.svg
-	echo ">>> View at 'file://$(PWD)/profile/cpu.svg'"
-
-docs:
-	echo ">>> View at 'http://localhost:6060/pkg/git.maximhutz.com/max/lambda/'"
-	go run golang.org/x/tools/cmd/godoc@latest -http=:6060
-
-test:
-	go test -v ./cmd/lambda
-
-bench:
-	go test -bench=. -benchtime=10x -cpu=4 ./cmd/lambda
+	echo "Profile graph saved to 'file://profile/cpu.svg'"

 clean:
-	rm -f ${BINARY_NAME}
-	rm -f program.out
+	rm -f ${BINARY_NAME} program.out
 	rm -rf profile/
--- a/cmd/lambda/lambda.go
+++ b/cmd/lambda/lambda.go
@@ -1,15 +1,17 @@
 package main

 import (
+	"fmt"
 	"os"

 	"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 +35,42 @@ 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.
-	var red reducer.Reducer
-	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)
-	}
+	// Create reduction engine.
+	process := engine.New(options, &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.
-	red.Reduce()
+	process.Run()

 	// Return the final reduced result.
-	// For De Bruijn, convert back to lambda for consistent output.
-	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)
+	fmt.Println(lambda.Stringify(compiled))
 }
--- a/cmd/lambda/lambda_test.go
+++ b/cmd/lambda/lambda_test.go
@@ -1,163 +0,0 @@
-package main
-
-import (
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-
-	"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"
-)
-
-// 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)
-	if err != nil {
-		return "", err
-	}
-
-	// Parse code into syntax tree.
-	ast, err := saccharine.Parse(string(input))
-	if err != nil {
-		return "", err
-	}
-
-	// Compile expression to lambda calculus.
-	compiled := convert.SaccharineToLambda(ast)
-
-	// Create and run the reducer.
-	reducer := lambda.NewNormalOrderReducer(&compiled)
-	reducer.Reduce()
-
-	return reducer.Expression().String() + "\n", nil
-}
-
-// Helper function to run a single sample through the De Bruijn interpreter.
-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.
-	ast, err := saccharine.Parse(string(input))
-	if err != nil {
-		return "", err
-	}
-
-	// Compile expression to lambda calculus.
-	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.
-func TestSamplesValidity(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 := runSample(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.")
-		})
-	}
-}
-
-// 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.
-	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 := runSample(path)
-				assert.NoError(b, err, "Failed to run sample.")
-			}
-		})
-	}
-}
-
-// 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.")
-			}
-		})
-	}
-}
--- a/go.mod
+++ b/go.mod
@@ -1,11 +1,3 @@
 module git.maximhutz.com/max/lambda

 go 1.25.5
-
-require github.com/stretchr/testify v1.11.1
-
-require (
-	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/pmezard/go-difflib v1.0.0 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
-)
--- a/go.sum
+++ b/go.sum
@@ -1,9 +0,0 @@
-github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
-github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
-github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
-gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -1,21 +1,11 @@
 // 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.
-	Destination Destination // The destination for the final result.
-	Verbose     bool        // Whether or not to print debug logs.
-	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.
+	Source      Source // The source code given to the program.
+	Verbose     bool   // Whether or not to print debug logs.
+	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.
 }
--- a/internal/config/destination.go
+++ b/internal/config/destination.go
@@ -1,27 +0,0 @@
-package config
-
-import (
-	"fmt"
-	"os"
-)
-
-// A method of writing output to the user.
-type Destination interface {
-	// Write data to this destination.
-	Write(data string) error
-}
-
-// A destination writing to stdout.
-type StdoutDestination struct{}
-
-func (d StdoutDestination) Write(data string) error {
-	fmt.Println(data)
-	return nil
-}
-
-// A destination writing to a file.
-type FileDestination struct{ Path string }
-
-func (d FileDestination) Write(data string) error {
-	return os.WriteFile(d.Path, []byte(data+"\n"), 0644)
-}
--- a/internal/config/parse_from_args.go
+++ b/internal/config/parse_from_args.go
@@ -12,58 +12,28 @@ func FromArgs() (*Config, error) {
 	explanation := flag.Bool("x", false, "Explanation. Whether or not to show all reduction steps.")
 	statistics := flag.Bool("s", false, "Statistics. If set, the process will print various statistics about the run.")
 	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)
+	// There must only be one input argument.
+	if flag.NArg() == 0 {
+		return nil, fmt.Errorf("no input given")
+	} else if flag.NArg() > 1 {
+		return nil, fmt.Errorf("more than 1 command-line argument")
 	}

 	// Parse source type.
 	var source Source
-	if *file != "" {
-		// File flag takes precedence.
-		if flag.NArg() > 0 {
-			return nil, fmt.Errorf("cannot specify both -f flag and positional argument")
-		}
-		source = FileSource{Path: *file}
-	} else if flag.NArg() == 0 {
-		return nil, fmt.Errorf("no input given")
-	} else if flag.NArg() > 1 {
-		return nil, fmt.Errorf("more than 1 command-line argument")
+	if flag.Arg(0) == "-" {
+		source = StdinSource{}
 	} else {
-		// Positional argument.
-		if flag.Arg(0) == "-" {
-			source = StdinSource{}
-		} else {
-			source = StringSource{Data: flag.Arg(0)}
-		}
-	}
-
-	// Parse destination type.
-	var destination Destination
-	if *output == "" || *output == "-" {
-		destination = StdoutDestination{}
-	} else {
-		destination = FileDestination{Path: *output}
+		source = StringSource{Data: flag.Arg(0)}
 	}

 	return &Config{
 		Source:      source,
-		Destination: destination,
 		Verbose:     *verbose,
 		Explanation: *explanation,
 		Profile:     *profile,
 		Statistics:  *statistics,
-		Interpreter: interpType,
 	}, nil
 }
--- a/internal/config/source.go
+++ b/internal/config/source.go
@@ -27,15 +27,3 @@ func (s StdinSource) Extract() (string, error) {

 	return string(data), nil
 }
-
-// A source reading from a file.
-type FileSource struct{ Path string }
-
-func (s FileSource) Extract() (string, error) {
-	data, err := os.ReadFile(s.Path)
-	if err != nil {
-		return "", err
-	}
-
-	return string(data), nil
-}
--- a/internal/engine/engine.go
+++ b/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")
+
+	for lambda.ReduceOnce(e.Expression) {
+		e.Emit("step")
+	}
+
+	e.Emit("end")
+}
--- a/internal/explanation/tracker.go
+++ b/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))
+}
--- a/internal/performance/tracker.go
+++ b/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 {
-	plugin := &Performance{File: file}
-	process.On(reducer.StartEvent, plugin.Start)
-	process.On(reducer.StopEvent, plugin.Stop)
-
-	return plugin
+func Track(file string) *Tracker {
+	return &Tracker{File: file}
 }

 // 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()
 }
--- a/internal/plugins/debug.go
+++ b/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())
-}
--- a/internal/plugins/explanation.go
+++ b/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())
-}
--- a/internal/plugins/statistics.go
+++ b/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())
-}
--- a/internal/statistics/tracker.go
+++ b/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())
+}
--- a/pkg/convert/debruijn_to_lambda.go
+++ b/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
-		}
-	}
-}
--- a/pkg/convert/lambda_to_debruijn.go
+++ b/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")
-	}
-}
--- a/pkg/debruijn/expression.go
+++ b/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)
-}
--- a/pkg/debruijn/iterator.go
+++ b/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{}
-	}
-}
--- a/pkg/debruijn/reducer.go
+++ b/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)
-}
--- a/pkg/debruijn/shift.go
+++ b/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
-	}
-}
--- a/pkg/debruijn/stringify.go
+++ b/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()
-}
--- a/pkg/debruijn/substitute.go
+++ b/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
-	}
-}
--- a/pkg/deltanet/node.go
+++ b/pkg/deltanet/node.go
@@ -1,7 +1,5 @@
 package deltanet

-/** ------------------------------------------------------------------------- */
-
 // A connection between exactly two nodes in a graph.
 type Edge struct {
 	A, B Node
@@ -28,8 +26,6 @@ func (e Edge) IsPrincipleEdge() bool {
 	return e.A.GetMainPort() == e && e.B.GetMainPort() == e
 }

-/** ------------------------------------------------------------------------- */
-
 type Node interface {
 	// Returns the principle port that the node is attached to.
 	GetMainPort() Edge
@@ -42,8 +38,6 @@ type Node interface {
 	GetLabel() string
 }

-/** ------------------------------------------------------------------------- */
-
 type EraserNode struct {
 	Main Edge
 }
@@ -52,8 +46,6 @@ func (n EraserNode) GetLabel() string    { return "Ⓧ" }
 func (n EraserNode) GetMainPort() Edge   { return n.Main }
 func (n EraserNode) GetAuxPorts() []Edge { return []Edge{} }

-/** ------------------------------------------------------------------------- */
-
 type ReplicatorNode struct {
 	Main   Edge
 	Level  uint
@@ -68,8 +60,6 @@ func (n ReplicatorNode) GetAuxPorts() []Edge { return n.Aux }
 // Returns the level of the replicator node.
 func (n ReplicatorNode) GetLevel() uint { return n.Level }

-/** ------------------------------------------------------------------------- */
-
 type FanNode struct {
 	Label       string
 	Main        Edge
@@ -80,8 +70,6 @@ func (n FanNode) GetLabel() string    { return n.Label }
 func (n FanNode) GetMainPort() Edge   { return n.Main }
 func (n FanNode) GetAuxPorts() []Edge { return []Edge{n.Left, n.Right} }

-/** ------------------------------------------------------------------------- */
-
 type TerminalNode struct {
 	Label string
 	Main  Edge
@@ -90,5 +78,3 @@ type TerminalNode struct {
 func (n TerminalNode) GetLabel() string    { return n.Label }
 func (n TerminalNode) GetMainPort() Edge   { return n.Main }
 func (n TerminalNode) GetAuxPorts() []Edge { return []Edge{} }
-
-/** ------------------------------------------------------------------------- */
--- a/pkg/emitter/emitter.go
+++ b/pkg/emitter/emitter.go
@@ -2,45 +2,53 @@ package emitter

 import "git.maximhutz.com/max/lambda/pkg/set"

-type Emitter[E comparable] interface {
-	On(E, func()) Listener[E]
-	Off(Listener[E])
-	Emit(E)
+type Observer struct {
+	fn      func()
+	message string
+	emitter *Emitter
 }

-type BaseEmitter[E comparable] struct {
-	listeners map[E]*set.Set[Listener[E]]
+type Emitter struct {
+	listeners map[string]*set.Set[*Observer]
 }

-func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
-	if e.listeners[kind] == nil {
-		e.listeners[kind] = set.New[Listener[E]]()
+func Ignore[T any](fn func()) func(T) {
+	return func(T) { fn() }
+}
+
+func (e *Emitter) On(message string, fn func()) *Observer {
+	observer := &Observer{
+		fn:      fn,
+		message: message,
+		emitter: e,
 	}

-	listener := &BaseListener[E]{kind, fn}
-	e.listeners[kind].Add(listener)
-	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]]()
+	if e.listeners == nil {
+		e.listeners = map[string]*set.Set[*Observer]{}
 	}

-	for listener := range e.listeners[event].Items() {
-		listener.Run()
+	if e.listeners[message] == nil {
+		e.listeners[message] = set.New[*Observer]()
 	}
+
+	e.listeners[message].Add(observer)
+	return observer
 }

-func New[E comparable]() *BaseEmitter[E] {
-	return &BaseEmitter[E]{
-		listeners: map[E]*set.Set[Listener[E]]{},
+func (o *Observer) Off() {
+	if o.emitter.listeners[o.message] == nil {
+		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()
 	}
 }
--- a/pkg/emitter/listener.go
+++ b/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()
-}
--- a/pkg/expr/expr.go
+++ b/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
-}
--- a/pkg/fifo/fifo.go
+++ b/pkg/fifo/fifo.go
@@ -0,0 +1,35 @@
+package fifo
+
+import "fmt"
+
+type FIFO[T any] []T
+
+func New[T any](items ...T) *FIFO[T] {
+	f := FIFO[T](items)
+	return &f
+}
+
+func (f *FIFO[T]) Push(item T) {
+	*f = append(*f, item)
+}
+
+func (f *FIFO[T]) Empty() bool {
+	return len(*f) == 0
+}
+
+func (f *FIFO[T]) MustPop() T {
+	var item T
+
+	*f, item = (*f)[:len(*f)-1], (*f)[len(*f)-1]
+	return item
+}
+
+func (f *FIFO[T]) Pop() (T, error) {
+	var item T
+
+	if f.Empty() {
+		return item, fmt.Errorf("stack is exhausted")
+	}
+
+	return f.MustPop(), nil
+}
--- a/pkg/lambda/expression.go
+++ b/pkg/lambda/expression.go
@@ -1,92 +1,60 @@
 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)
+	Copy() Expression
 }

-/** ------------------------------------------------------------------------- */
-
 type Abstraction struct {
-	parameter string
-	body      Expression
+	Parameter string
+	Body      Expression
 }

-func (a *Abstraction) Parameter() string {
-	return a.parameter
-}
-
-func (a *Abstraction) Body() Expression {
-	return a.body
+func (a *Abstraction) Copy() Expression {
+	return NewAbstraction(a.Parameter, a.Body.Copy())
 }

 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}
+	return &Abstraction{Parameter: parameter, Body: body}
 }

-/** ------------------------------------------------------------------------- */
-
 type Application struct {
-	abstraction Expression
-	argument    Expression
+	Abstraction Expression
+	Argument    Expression
 }

-func (a *Application) Abstraction() Expression {
-	return a.abstraction
-}
-
-func (a *Application) Argument() Expression {
-	return a.argument
+func (a *Application) Copy() Expression {
+	return NewApplication(a.Abstraction.Copy(), a.Argument.Copy())
 }

 func (a *Application) Accept(v Visitor) {
 	v.VisitApplication(a)
 }

-func (a *Application) String() string {
-	return Stringify(a)
+func NewApplication(function Expression, argument Expression) *Application {
+	return &Application{Abstraction: function, Argument: argument}
 }

-func NewApplication(abstraction Expression, argument Expression) *Application {
-	return &Application{abstraction: abstraction, argument: argument}
-}
-
-/** ------------------------------------------------------------------------- */
-
 type Variable struct {
-	value string
+	Value string
 }

-func (v *Variable) Value() string {
-	return v.value
+func (v *Variable) Copy() Expression {
+	return NewVariable(v.Value)
 }

 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}
+	return &Variable{Value: name}
 }

-/** ------------------------------------------------------------------------- */
-
 type Visitor interface {
 	VisitAbstraction(*Abstraction)
 	VisitApplication(*Application)
--- a/pkg/lambda/get_free_variables.go
+++ b/pkg/lambda/get_free_variables.go
@@ -5,14 +5,14 @@ import "git.maximhutz.com/max/lambda/pkg/set"
 func GetFreeVariables(e Expression) *set.Set[string] {
 	switch e := e.(type) {
 	case *Variable:
-		return set.New(e.value)
+		return set.New(e.Value)
 	case *Abstraction:
-		vars := GetFreeVariables(e.body)
-		vars.Remove(e.parameter)
+		vars := GetFreeVariables(e.Body)
+		vars.Remove(e.Parameter)
 		return vars
 	case *Application:
-		vars := GetFreeVariables(e.abstraction)
-		vars.Merge(GetFreeVariables(e.argument))
+		vars := GetFreeVariables(e.Abstraction)
+		vars.Merge(GetFreeVariables(e.Argument))
 		return vars
 	default:
 		return nil
--- a/pkg/lambda/is_free_variable.go
+++ b/pkg/lambda/is_free_variable.go
@@ -3,11 +3,11 @@ package lambda
 func IsFreeVariable(n string, e Expression) bool {
 	switch e := e.(type) {
 	case *Variable:
-		return e.value == n
+		return e.Value == n
 	case *Abstraction:
-		return e.parameter != n && IsFreeVariable(n, e.body)
+		return e.Parameter != n && IsFreeVariable(n, e.Body)
 	case *Application:
-		return IsFreeVariable(n, e.abstraction) || IsFreeVariable(n, e.argument)
+		return IsFreeVariable(n, e.Abstraction) || IsFreeVariable(n, e.Argument)
 	default:
 		return false
 	}
--- a/pkg/lambda/iterator.go
+++ b/pkg/lambda/iterator.go
@@ -1,68 +0,0 @@
-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{}
-	}
-}
--- a/pkg/lambda/reduce.go
+++ b/pkg/lambda/reduce.go
@@ -0,0 +1,27 @@
+package lambda
+
+import "git.maximhutz.com/max/lambda/pkg/fifo"
+
+func ReduceOnce(e *Expression) bool {
+	stack := fifo.New(e)
+
+	for !stack.Empty() {
+		top := stack.MustPop()
+
+		switch typed := (*top).(type) {
+		case *Abstraction:
+			stack.Push(&typed.Body)
+		case *Application:
+			if fn, fnOk := typed.Abstraction.(*Abstraction); fnOk {
+				Substitute(&fn.Body, fn.Parameter, typed.Argument)
+				*top = fn.Body
+				return true
+			}
+
+			stack.Push(&typed.Argument)
+			stack.Push(&typed.Abstraction)
+		}
+	}
+
+	return false
+}
--- a/pkg/lambda/reducer.go
+++ b/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)
-}
--- a/pkg/lambda/rename.go
+++ b/pkg/lambda/rename.go
@@ -1,38 +1,19 @@
 package lambda

-func Rename(expr Expression, target string, newName string) Expression {
-	switch e := expr.(type) {
+func Rename(e Expression, target string, substitute string) {
+	switch e := e.(type) {
 	case *Variable:
-		if e.value == target {
-			return NewVariable(newName)
+		if e.Value == target {
+			e.Value = substitute
 		}
-		return e
-
 	case *Abstraction:
-		newParam := e.parameter
-		if e.parameter == target {
-			newParam = newName
+		if e.Parameter == target {
+			e.Parameter = substitute
 		}

-		newBody := Rename(e.body, target, newName)
-
-		if newParam == e.parameter && newBody == e.body {
-			return e
-		}
-
-		return NewAbstraction(newParam, newBody)
-
+		Rename(e.Body, target, substitute)
 	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
+		Rename(e.Abstraction, target, substitute)
+		Rename(e.Argument, target, substitute)
 	}
 }
--- a/pkg/lambda/stringify.go
+++ b/pkg/lambda/stringify.go
@@ -7,21 +7,21 @@ type stringifyVisitor struct {
 }

 func (v *stringifyVisitor) VisitVariable(a *Variable) {
-	v.builder.WriteString(a.value)
+	v.builder.WriteString(a.Value)
 }

 func (v *stringifyVisitor) VisitAbstraction(f *Abstraction) {
 	v.builder.WriteRune('\\')
-	v.builder.WriteString(f.parameter)
+	v.builder.WriteString(f.Parameter)
 	v.builder.WriteRune('.')
-	f.body.Accept(v)
+	f.Body.Accept(v)
 }

 func (v *stringifyVisitor) VisitApplication(c *Application) {
 	v.builder.WriteRune('(')
-	c.abstraction.Accept(v)
+	c.Abstraction.Accept(v)
 	v.builder.WriteRune(' ')
-	c.argument.Accept(v)
+	c.Argument.Accept(v)
 	v.builder.WriteRune(')')
 }

--- a/pkg/lambda/substitute.go
+++ b/pkg/lambda/substitute.go
@@ -1,46 +1,27 @@
 package lambda

-func Substitute(expr Expression, target string, replacement Expression) Expression {
-	switch e := expr.(type) {
+func Substitute(e *Expression, target string, replacement Expression) {
+	switch typed := (*e).(type) {
 	case *Variable:
-		if e.value == target {
-			return replacement
+		if typed.Value == target {
+			*e = replacement.Copy()
 		}
-		return e
-
 	case *Abstraction:
-		if e.parameter == target {
-			return e
+		if typed.Parameter == target {
+			return
 		}

-		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
+		if IsFreeVariable(typed.Parameter, replacement) {
+			replacementFreeVars := GetFreeVariables(replacement)
+			used := GetFreeVariables(typed.Body)
+			used.Merge(replacementFreeVars)
+			freshVar := GenerateFreshName(used)
+			Rename(typed, typed.Parameter, freshVar)
 		}

-		newBody := Substitute(body, target, replacement)
-		if newBody == body && param == e.parameter {
-			return e
-		}
-
-		return NewAbstraction(param, newBody)
-
+		Substitute(&typed.Body, target, replacement)
 	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
+		Substitute(&typed.Abstraction, target, replacement)
+		Substitute(&typed.Argument, target, replacement)
 	}
 }
--- a/pkg/reducer/events.go
+++ b/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
-)
--- a/pkg/reducer/reducer.go
+++ b/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
-}
--- a/pkg/saccharine/expression.go
+++ b/pkg/saccharine/expression.go
@@ -4,8 +4,6 @@ type Expression interface {
 	IsExpression()
 }

-/** ------------------------------------------------------------------------- */
-
 type Abstraction struct {
 	Parameters []string
 	Body       Expression
@@ -30,8 +28,6 @@ func (Application) IsExpression() {}
 func (Atom) IsExpression()        {}
 func (Clause) IsExpression()      {}

-/** ------------------------------------------------------------------------- */
-
 func NewAbstraction(parameter []string, body Expression) *Abstraction {
 	return &Abstraction{Parameters: parameter, Body: body}
 }
--- a/pkg/saccharine/statement.go
+++ b/pkg/saccharine/statement.go
@@ -4,8 +4,6 @@ type Statement interface {
 	IsStatement()
 }

-/** ------------------------------------------------------------------------- */
-
 type LetStatement struct {
 	Name       string
 	Parameters []string
@@ -19,8 +17,6 @@ type DeclareStatement struct {
 func (LetStatement) IsStatement()     {}
 func (DeclareStatement) IsStatement() {}

-/** ------------------------------------------------------------------------- */
-
 func NewLet(name string, parameters []string, body Expression) *LetStatement {
 	return &LetStatement{Name: name, Parameters: parameters, Body: body}
 }
--- a/pkg/saccharine/token/parse.go
+++ b/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):
--- a/pkg/set/set.go
+++ b/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]{}

--- a/tests/church_5^5.test
+++ b/tests/church_5^5.test
@@ -1,8 +1,7 @@
 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 0)))))

-(print (exp N N))
+(exp N N)
--- a/tests/fast_list_2^30.test
+++ b/tests/fast_list_2^30.test
--- a/samples/saccharine.txt
+++ b/samples/saccharine.txt
--- a/samples/simple.txt
+++ b/samples/simple.txt
@@ -0,0 +1,16 @@
+(\0.
+  (\inc.
+    (\add.
+      (\mult.
+        (\exp.
+          (exp (inc (inc (inc (inc 0)))) (inc (inc (inc (inc (inc 0))))))
+          \n m.(m n)
+        )
+        \m n f.(m (n f))
+      )
+      \n m.(m inc n)
+    )
+    \n f x.(f (n f x))
+  )
+  \f x.x
+)
--- a/samples/thunk.txt
+++ b/samples/thunk.txt
--- a/tests/church_5^5.expected
+++ b/tests/church_5^5.expected
--- a/tests/church_6^6.expected
+++ b/tests/church_6^6.expected
--- a/tests/church_6^6.test
+++ b/tests/church_6^6.test
@@ -1,8 +0,0 @@
-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))
--- a/tests/comments.expected
+++ b/tests/comments.expected
@@ -1 +0,0 @@
-VALUE
--- a/tests/comments.test
+++ b/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
--- a/tests/fast_list_2^30.expected
+++ b/tests/fast_list_2^30.expected
@@ -1 +0,0 @@
-(0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (1 END)))))))))))))))))))))))))))))))
--- a/tests/list_2^30.expected
+++ b/tests/list_2^30.expected
@@ -1 +0,0 @@
-(0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (1 END)))))))))))))))))))))))))))))))
--- a/tests/thunk.expected
+++ b/tests/thunk.expected
@@ -1 +0,0 @@
-VALUE
Author	SHA1	Message	Date
M.V. Hutz	602d8fa074	docs: remove makefile-improvements.md in favor of CLAUDE.md Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 17:15:14 -05:00
M.V. Hutz	edfee89bad	feat: explicitly set help as default target Adds .DEFAULT_GOAL := help to make it clear that running 'make' with no arguments will display the help message. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:38:37 -05:00
M.V. Hutz	b3db983f62	fix: add lambda binary to .gitignore Adds the lambda binary to .gitignore to prevent accidentally committing the build artifact. Previously only *.exe was ignored, which didn't cover the Unix binary name. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:38:19 -05:00
M.V. Hutz	997794eaa5	docs: add documentation of Makefile improvements Documents all issues found and fixes applied to the Makefile, including both implemented changes and remaining suggestions. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:36:13 -05:00
M.V. Hutz	8f70bfbbdb	refactor: use .SILENT directive instead of @ prefixes Adds .SILENT directive to suppress command echoing for all targets, replacing individual @ prefixes. Also moves TEST variable to top with other variables for better organization. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:34:07 -05:00
M.V. Hutz	3158c35df2	fix: add profile dependency to graph target Makes graph target depend on profile to ensure cpu.prof exists before attempting to generate visualizations. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:29:25 -05:00
M.V. Hutz	bb48d0777b	fix: ensure profile directory exists before writing Creates profile directory in profile and explain targets to prevent errors on first run. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:29:09 -05:00
M.V. Hutz	24fdc1c17c	feat: add help target to document available commands Adds help target that displays all available Make targets and their descriptions, improving discoverability. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:28:48 -05:00
M.V. Hutz	7927df4660	feat: add clean target to remove build artifacts Adds standard clean target to remove binary, output files, and profile directory. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:28:29 -05:00
M.V. Hutz	e5ceeb2fcc	feat: add .PHONY declarations for all targets Declares all non-file targets as phony to prevent conflicts with files of the same name and improve Make's performance. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:28:08 -05:00
M.V. Hutz	e0b0b92a8a	refactor: remove redundant chmod +x command Go build already sets the executable bit on binaries, making the explicit chmod +x unnecessary. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:27:55 -05:00
M.V. Hutz	0d06fac919	fix: remove Windows .exe extension from binary name Changed BINARY_NAME from lambda.exe to lambda for Unix systems. The .exe extension is a Windows convention and is inappropriate for macOS/Linux builds. Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>	2026-01-10 16:27:42 -05:00
M.V. Hutz	dc9a1b2b7d	feat: add usage instructions to README and improve Makefile	2026-01-10 16:21:22 -05:00
M.V. Hutz	7e59d5cefa	style: remove YAML document separator	2026-01-10 16:12:30 -05:00
M.V. Hutz	9f06a5109f	style: remove comments from golangci config	2026-01-10 16:11:37 -05:00
M.V. Hutz	b2b2655c1e	style: remove decorative comment separators	2026-01-10 16:07:39 -05:00
				`@@ -1 +0,0 @@`
				`(0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (1 END)))))))))))))))))))))))))))))))`