docs: add project guide for Claude

Add CLAUDE.md with conventions for commits, branch naming, and PR management using tea CLI.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.gitea/ISSUE_TEMPLATE/bug.md

@@ -1,58 +0,0 @@
----
-name: "Bug Report"
-about: "Report a bug or unexpected behavior in the lambda runtime."
-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 runtime."
-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.
--->

.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).

.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
 

.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
+    gci:
-    # https://github.com/daixiang0/gci/
-    gci:  # define the section orders for imports
       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

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+    "makefile.configureOnOpen": false
+}

CLAUDE.md

@@ -1,12 +1,6 @@
 # Guide To `lambda`
 
-## Documentation Style
+## Coding Styles
-
-Use full sentences.
-Every sentence gets its own line in Markdown.
-Every sentence ends in a period.
-
-## Coding Style
 
 ### Conventional Commits
 
@@ -20,13 +14,11 @@ Use conventional commit format: `<type>: <description>`.
 - `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`.
+**Types**: Same as commits: `feat`, `fix`, `docs`, `refactor`, `test`, `chore`, `perf`
 
 **Examples**:
 
@@ -35,71 +27,16 @@ Use format: `<type>/<description>` with kebab-case.
 - `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 create` - Create a new pull request
-- `tea pr list` - List pull requests.
+- `tea pr list` - List pull requests
-- `tea pr checkout <number>` - Check out a PR locally.
+- `tea pr checkout <number>` - Check out a PR locally
-- `tea pr close <number>` - Close a pull request.
+- `tea pr close <number>` - Close a pull request
-- `tea pr merge <number>` - Merge 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 to the `main` branch unless specified otherwise
-
-**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"
-```

LICENSE

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

Makefile

@@ -1,51 +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 "  it       - Build the lambda binary"
-	echo "  run      - Build and run the lambda runtime (use TEST=<name> to specify sample)"
+	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 "  explain  - Run with explanation mode and profiling"
-	echo "  graph    - Generate and open CPU profile visualization"
+	echo "  graph    - Generate CPU profile visualization"
-	echo "  docs     - Start local godoc server on port 6060"
+	echo "  clean    - Remove build artifacts"
-	echo "  test     - Run tests for all samples"
+	echo ""
-	echo "  bench    - Run benchmarks for all samples"
+	echo "Usage: make run TEST=<sample-name>"
-	echo "  clean    - Remove all build artifacts"
 
-build:
+it:
 	go build -o ${BINARY_NAME} ./cmd/lambda
-	chmod +x ${BINARY_NAME}
 
-run: build
+run: it
-	./${BINARY_NAME} -s -f ./tests/$(TEST).test -o program.out
+	./lambda - < ./samples/$(TEST).txt > program.out
 
-profile: build
+profile: it
-	./${BINARY_NAME} -p profile/cpu.prof -f ./tests/$(TEST).test -o program.out
+	mkdir -p profile
+	./lambda -p profile/cpu.prof - < ./samples/$(TEST).txt > program.out
 
-explain: build
+explain: it
-	./${BINARY_NAME} -x -p profile/cpu.prof -f ./tests/$(TEST).test -o program.out > explain.out
+	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'"
+	echo "Profile graph saved to 'file://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
 
 clean:
-	rm -f ${BINARY_NAME}
+	rm -f ${BINARY_NAME} program.out
-	rm -f program.out
 	rm -rf profile/

README.md

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

cmd/lambda/lambda.go

@@ -1,13 +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/normalorder"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
@@ -31,37 +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 with the compiled expression.
+	// Create reduction engine.
-	runtime := normalorder.NewRuntime(compiled)
+	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, runtime)
+		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(runtime)
+		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(runtime)
+		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, runtime)
+		process.On("step", func() {
+			logger.Info("reduction", "tree", lambda.Stringify(compiled))
+		})
 	}
 
-	// Run reduction.
+	process.Run()
-	runtime.Run()
 
 	// Return the final reduced result.
-	result := runtime.Expression().String()
+	fmt.Println(lambda.Stringify(compiled))
-	err = options.Destination.Write(result)
-	cli.HandleError(err)
 }

cmd/lambda/lambda_test.go

@@ -1,85 +0,0 @@
-package main
-
-import (
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-
-	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/normalorder"
-	"git.maximhutz.com/max/lambda/pkg/saccharine"
-	"github.com/stretchr/testify/assert"
-)
-
-// Helper function to run a single sample through the lambda runtime.
-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 := normalorder.NewRuntime(compiled)
-	reducer.Run()
-
-	return reducer.Expression().String() + "\n", nil
-}
-
-// 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")
-	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.")
-		})
-	}
-}
-
-// 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.")
-			}
-		})
-	}
-}

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
-)

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=

internal/config/config.go

@@ -4,7 +4,6 @@ package config
 // 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.

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)
-}

internal/config/parse_from_args.go

@@ -12,42 +12,25 @@ 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).")
 	flag.Parse()
 
-	// Parse source type.
+	// There must only be one input argument.
-	var source Source
+	if flag.NArg() == 0 {
-	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")
-	} else {
+	}
-		// Positional argument.
+
+	// Parse source type.
+	var source Source
 	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}
-	}
 
 	return &Config{
 		Source:      source,
-		Destination: destination,
 		Verbose:     *verbose,
 		Explanation: *explanation,
 		Profile:     *profile,

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
-}

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")
+}

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/runtime"
 )
 
 // 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 runtime.Runtime) *Performance {
+func Track(file string) *Tracker {
-	plugin := &Performance{File: file}
+	return &Tracker{File: file}
-	process.On(runtime.StartEvent, plugin.Start)
-	process.On(runtime.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/runtime"
-)
-
-type Logs struct {
-	logger  *slog.Logger
-	reducer runtime.Runtime
-}
-
-func NewLogs(logger *slog.Logger, r runtime.Runtime) *Logs {
-	plugin := &Logs{logger, r}
-	r.On(runtime.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/runtime"
-)
-
-// Track the reductions made by a reduction process.
-type Explanation struct {
-	reducer runtime.Runtime
-}
-
-// Attaches a new explanation tracker to a reducer.
-func NewExplanation(r runtime.Runtime) *Explanation {
-	plugin := &Explanation{reducer: r}
-	r.On(runtime.StartEvent, plugin.Start)
-	r.On(runtime.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/runtime"
-)
-
-// An observer, to track reduction performance.
-type Statistics struct {
-	start time.Time
-	steps uint64
-}
-
-// Create a new reduction performance Statistics.
-func NewStatistics(r runtime.Runtime) *Statistics {
-	plugin := &Statistics{}
-	r.On(runtime.StartEvent, plugin.Start)
-	r.On(runtime.StepEvent, plugin.Step)
-	r.On(runtime.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())
+}

makefile-improvements.md

@@ -0,0 +1,114 @@
+# Makefile Improvements
+
+This document lists the issues identified in the original Makefile and the improvements that were implemented.
+
+## Issues Fixed
+
+### 1. Hardcoded `.exe` extension on Unix
+**Problem**: `BINARY_NAME=lambda.exe` used a Windows extension on macOS/Linux systems.
+
+**Solution**: Changed to `BINARY_NAME=lambda` since Unix executables don't use extensions.
+
+**Commit**: `0d06fac` - fix: remove Windows .exe extension from binary name
+
+---
+
+### 2. Redundant `chmod +x`
+**Problem**: The `chmod +x` command was unnecessary since `go build` already sets the executable bit.
+
+**Solution**: Removed the redundant `chmod +x ${BINARY_NAME}` line.
+
+**Commit**: `e0b0b92` - refactor: remove redundant chmod +x command
+
+---
+
+### 3. Missing `.PHONY` declarations
+**Problem**: Without `.PHONY`, if files named `run`, `graph`, etc. existed, Make would skip those targets.
+
+**Solution**: Added `.PHONY: help it run profile explain graph clean` declaration.
+
+**Commit**: `e5ceeb2` - feat: add .PHONY declarations for all targets
+
+---
+
+### 4. No `clean` target
+**Problem**: No standard way to remove build artifacts.
+
+**Solution**: Added `clean` target to remove binary, output files, and profile directory:
+```makefile
+clean:
+	rm -f ${BINARY_NAME} program.out
+	rm -rf profile/
+```
+
+**Commit**: `7927df4` - feat: add clean target to remove build artifacts
+
+---
+
+### 5. Missing `help` or default target
+**Problem**: Running `make` with no arguments was unclear about available targets.
+
+**Solution**: Added `help` target documenting all available commands and their usage.
+
+**Commit**: `24fdc1c` - feat: add help target to document available commands
+
+---
+
+### 6. Profile directory not created
+**Problem**: The `profile` and `explain` targets wrote to `profile/cpu.prof` but never created the directory, causing failures on first run.
+
+**Solution**: Added `mkdir -p profile` to both `profile` and `explain` targets.
+
+**Commit**: `bb48d07` - fix: ensure profile directory exists before writing
+
+---
+
+### 7. No dependency check on `graph`
+**Problem**: The `graph` target assumed `profile/cpu.prof` exists but didn't depend on `profile`.
+
+**Solution**: Changed `graph:` to `graph: profile` to ensure profiling runs first.
+
+**Commit**: `3158c35` - fix: add profile dependency to graph target
+
+---
+
+### 8. Verbose command prefixes
+**Problem**: Every command used `@` prefix individually to suppress output, cluttering the file.
+
+**Solution**: Added `.SILENT:` directive at the top and removed all `@` prefixes. Also moved `TEST` variable to top with other variables.
+
+**Commit**: `8f70bfb` - refactor: use .SILENT directive instead of @ prefixes
+
+---
+
+## Additional Suggestions (Not Yet Implemented)
+
+### 9. Missing `lambda` binary in `.gitignore`
+**Issue**: The `.gitignore` has `*.exe` but not the actual `lambda` binary name. Since we removed the `.exe` extension, the binary won't be ignored.
+
+**Recommendation**: Add `lambda` to `.gitignore`:
+```
+# Build artifacts
+lambda
+```
+
+---
+
+### 10. No explicit default target
+**Issue**: While `help` is currently the first target (and thus default), it's not explicitly declared.
+
+**Recommendation**: Add `.DEFAULT_GOAL = help` at the top for clarity:
+```makefile
+BINARY_NAME=lambda
+TEST=simple
+
+.DEFAULT_GOAL := help
+.SILENT:
+.PHONY: help it run profile explain graph clean
+```
+
+---
+
+## Summary
+
+The Makefile has been significantly improved with better organization, proper dependency management, directory creation, helpful documentation, and cleaner syntax. The remaining suggestions are minor quality-of-life improvements that can be addressed as needed.

pkg/convert/saccharine_to_lambda.go

@@ -19,7 +19,7 @@ func convertAbstraction(n *saccharine.Abstraction) lambda.Expression {
 	// If the function has no parameters, it is a thunk. Lambda calculus still
 	// requires _some_ parameter exists, so generate one.
 	if len(parameters) == 0 {
-		freeVars := result.GetFree()
+		freeVars := lambda.GetFreeVariables(result)
 		freshName := lambda.GenerateFreshName(freeVars)
 		parameters = append(parameters, freshName)
 	}
@@ -63,7 +63,7 @@ func reduceLet(s *saccharine.LetStatement, e lambda.Expression) lambda.Expressio
 }
 
 func reduceDeclare(s *saccharine.DeclareStatement, e lambda.Expression) lambda.Expression {
-	freshVar := lambda.GenerateFreshName(e.GetFree())
+	freshVar := lambda.GenerateFreshName(lambda.GetFreeVariables(e))
 
 	return lambda.NewApplication(
 		lambda.NewAbstraction(freshVar, e),

pkg/deltanet/deltanet.go

@@ -0,0 +1,6 @@
+// Package "deltanet" is a reduction strategy using ∆-nets.
+package deltanet
+
+type Graph struct {
+	Nodes []Node
+}

pkg/deltanet/node.go

@@ -0,0 +1,80 @@
+package deltanet
+
+// A connection between exactly two nodes in a graph.
+type Edge struct {
+	A, B Node
+}
+
+// Returns all nodes the edge is connected to.
+func (e Edge) GetConnections() []Node { return []Node{e.A, e.B} }
+
+// Determines if a node is connected via this edge.
+func (e Edge) IsConnected(n Node) bool { return e.A == n || e.B == n }
+
+// Swaps an edges connected with one node, for another.
+func (e *Edge) Swap(from Node, to Node) {
+	if e.A == from {
+		e.A = to
+	}
+	if e.B == from {
+		e.B = to
+	}
+}
+
+// Returns true if the edge is connected to each node via their pricniple ports.
+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
+
+	// Returns all auxiliary ports that the node has. These ports are guaranteed
+	// to be ordered clockwise, as they would appear graphically.
+	GetAuxPorts() []Edge
+
+	// Returns the label of the node. May be blank.
+	GetLabel() string
+}
+
+type EraserNode struct {
+	Main Edge
+}
+
+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
+	Aux    []Edge
+	Deltas []int
+}
+
+func (n ReplicatorNode) GetLabel() string    { return "" }
+func (n ReplicatorNode) GetMainPort() Edge   { return n.Main }
+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
+	Left, Right Edge
+}
+
+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
+}
+
+func (n TerminalNode) GetLabel() string    { return n.Label }
+func (n TerminalNode) GetMainPort() Edge   { return n.Main }
+func (n TerminalNode) GetAuxPorts() []Edge { return []Edge{} }

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]]()
 }
 
-	listener := &BaseListener[E]{kind, fn}
+func (e *Emitter) On(message string, fn func()) *Observer {
-	e.listeners[kind].Add(listener)
+	observer := &Observer{
-	return listener
+		fn:      fn,
+		message: message,
+		emitter: e,
 	}
 
-func (e *BaseEmitter[E]) Off(listener Listener[E]) {
+	if e.listeners == nil {
-	kind := listener.Kind()
+		e.listeners = map[string]*set.Set[*Observer]{}
-	if e.listeners[kind] != nil {
-		e.listeners[kind].Remove(listener)
-	}
 	}
 
-func (e *BaseEmitter[E]) Emit(event E) {
+	if e.listeners[message] == nil {
-	if e.listeners[event] == nil {
+		e.listeners[message] = set.New[*Observer]()
-		e.listeners[event] = set.New[Listener[E]]()
 	}
 
-	for listener := range e.listeners[event].Items() {
+	e.listeners[message].Add(observer)
-		listener.Run()
+	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

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

pkg/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
+}

pkg/lambda/expression.go

@@ -1,99 +1,62 @@
 package lambda
 
-import (
-	"git.maximhutz.com/max/lambda/pkg/expr"
-	"git.maximhutz.com/max/lambda/pkg/set"
-)
-
-// 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
-	// Substitute replaces all free occurrences of the target variable with the
-	// replacement expression. Alpha-renaming is performed automatically to
-	// avoid variable capture.
-	Substitute(target string, replacement Expression) Expression
-
-	// GetFree returns the set of all free variable names in the expression.
-	// This function does not mutate the input expression.
-	// The returned set is newly allocated and can be modified by the caller.
-	GetFree() set.Set[string]
-
-	// Rename replaces all occurrences of the target variable name with the new name.
-	Rename(target string, newName string) Expression
-
-	// IsFree returns true if the variable name n occurs free in the expression.
-	// This function does not mutate the input expression.
-	IsFree(n string) bool
 }
 
-/** ------------------------------------------------------------------------- */
-
 type Abstraction struct {
-	parameter string
+	Parameter string
-	body      Expression
+	Body      Expression
 }
 
-var _ Expression = Abstraction{}
+func (a *Abstraction) Copy() Expression {
-
+	return NewAbstraction(a.Parameter, a.Body.Copy())
-func (a Abstraction) Parameter() string {
-	return a.parameter
 }
 
-func (a Abstraction) Body() Expression {
+func (a *Abstraction) Accept(v Visitor) {
-	return a.body
+	v.VisitAbstraction(a)
 }
 
-func (a Abstraction) String() string {
+func NewAbstraction(parameter string, body Expression) *Abstraction {
-	return "\\" + a.parameter + "." + a.body.String()
+	return &Abstraction{Parameter: parameter, Body: body}
 }
 
-func NewAbstraction(parameter string, body Expression) Abstraction {
-	return Abstraction{parameter, body}
-}
-
-/** ------------------------------------------------------------------------- */
-
 type Application struct {
-	abstraction Expression
+	Abstraction Expression
-	argument    Expression
+	Argument    Expression
 }
 
-var _ Expression = Application{}
+func (a *Application) Copy() Expression {
-
+	return NewApplication(a.Abstraction.Copy(), a.Argument.Copy())
-func (a Application) Abstraction() Expression {
-	return a.abstraction
 }
 
-func (a Application) Argument() Expression {
+func (a *Application) Accept(v Visitor) {
-	return a.argument
+	v.VisitApplication(a)
 }
 
-func (a Application) String() string {
+func NewApplication(function Expression, argument Expression) *Application {
-	return "(" + a.abstraction.String() + " " + a.argument.String() + ")"
+	return &Application{Abstraction: function, Argument: argument}
 }
 
-func NewApplication(abstraction Expression, argument Expression) Application {
-	return Application{abstraction, argument}
-}
-
-/** ------------------------------------------------------------------------- */
-
 type Variable struct {
-	name string
+	Value string
 }
 
-var _ Expression = Variable{}
+func (v *Variable) Copy() Expression {
-
+	return NewVariable(v.Value)
-func (v Variable) Name() string {
-	return v.name
 }
 
-func (v Variable) String() string {
+func (v *Variable) Accept(visitor Visitor) {
-	return v.name
+	visitor.VisitVariable(v)
 }
 
-func NewVariable(name string) Variable {
+func NewVariable(name string) *Variable {
-	return Variable{name}
+	return &Variable{Value: name}
+}
+
+type Visitor interface {
+	VisitAbstraction(*Abstraction)
+	VisitApplication(*Application)
+	VisitVariable(*Variable)
 }

pkg/lambda/generate_name.go

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

pkg/lambda/get_free_variables.go

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

pkg/lambda/is_free_variable.go

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

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
+}

pkg/lambda/rename.go

@@ -1,28 +1,19 @@
 package lambda
 
-// Rename replaces all occurrences of the target variable name with the new name.
+func Rename(e Expression, target string, substitute string) {
-func (e Variable) Rename(target string, newName string) Expression {
+	switch e := e.(type) {
-	if e.Name() == target {
+	case *Variable:
-		return NewVariable(newName)
+		if e.Value == target {
+			e.Value = substitute
+		}
+	case *Abstraction:
+		if e.Parameter == target {
+			e.Parameter = substitute
 		}
 
-	return e
+		Rename(e.Body, target, substitute)
+	case *Application:
+		Rename(e.Abstraction, target, substitute)
+		Rename(e.Argument, target, substitute)
 	}
-
-func (e Abstraction) Rename(target string, newName string) Expression {
-	newParam := e.Parameter()
-	if e.Parameter() == target {
-		newParam = newName
-	}
-
-	newBody := e.Body().Rename(target, newName)
-
-	return NewAbstraction(newParam, newBody)
-}
-
-func (e Application) Rename(target string, newName string) Expression {
-	newAbs := e.Abstraction().Rename(target, newName)
-	newArg := e.Argument().Rename(target, newName)
-
-	return NewApplication(newAbs, newArg)
 }

pkg/lambda/stringify.go

@@ -0,0 +1,32 @@
+package lambda
+
+import "strings"
+
+type stringifyVisitor struct {
+	builder strings.Builder
+}
+
+func (v *stringifyVisitor) VisitVariable(a *Variable) {
+	v.builder.WriteString(a.Value)
+}
+
+func (v *stringifyVisitor) VisitAbstraction(f *Abstraction) {
+	v.builder.WriteRune('\\')
+	v.builder.WriteString(f.Parameter)
+	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(')')
+}
+
+func Stringify(e Expression) string {
+	b := &stringifyVisitor{builder: strings.Builder{}}
+	e.Accept(b)
+	return b.builder.String()
+}

pkg/lambda/substitute.go

@@ -1,35 +1,27 @@
 package lambda
 
-func (e Variable) Substitute(target string, replacement Expression) Expression {
+func Substitute(e *Expression, target string, replacement Expression) {
-	if e.Name() == target {
+	switch typed := (*e).(type) {
-		return replacement
+	case *Variable:
+		if typed.Value == target {
+			*e = replacement.Copy()
+		}
+	case *Abstraction:
+		if typed.Parameter == target {
+			return
 		}
 
-	return e
+		if IsFreeVariable(typed.Parameter, replacement) {
+			replacementFreeVars := GetFreeVariables(replacement)
+			used := GetFreeVariables(typed.Body)
+			used.Merge(replacementFreeVars)
+			freshVar := GenerateFreshName(used)
+			Rename(typed, typed.Parameter, freshVar)
 		}
 
-func (e Abstraction) Substitute(target string, replacement Expression) Expression {
+		Substitute(&typed.Body, target, replacement)
-	if e.Parameter() == target {
+	case *Application:
-		return e
+		Substitute(&typed.Abstraction, target, replacement)
+		Substitute(&typed.Argument, target, replacement)
 	}
-
-	body := e.Body()
-	param := e.Parameter()
-	if replacement.IsFree(param) {
-		freeVars := replacement.GetFree()
-		freeVars.Merge(body.GetFree())
-		freshVar := GenerateFreshName(freeVars)
-		body = body.Rename(param, freshVar)
-		param = freshVar
-	}
-
-	newBody := body.Substitute(target, replacement)
-	return NewAbstraction(param, newBody)
-}
-
-func (e Application) Substitute(target string, replacement Expression) Expression {
-	abs := e.Abstraction().Substitute(target, replacement)
-	arg := e.Argument().Substitute(target, replacement)
-
-	return NewApplication(abs, arg)
 }

pkg/normalorder/reduce_once.go

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

pkg/normalorder/runtime.go

@@ -1,46 +0,0 @@
-package normalorder
-
-import (
-	"git.maximhutz.com/max/lambda/pkg/emitter"
-	"git.maximhutz.com/max/lambda/pkg/expr"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
-	"git.maximhutz.com/max/lambda/pkg/runtime"
-)
-
-// NormalOrderReducer implements normal order (leftmost-outermost) reduction
-// for lambda calculus expressions.
-type Runtime struct {
-	emitter.BaseEmitter[runtime.Event]
-	expression lambda.Expression
-}
-
-// NewNormalOrderReducer creates a new normal order reducer.
-func NewRuntime(expression lambda.Expression) *Runtime {
-	return &Runtime{
-		BaseEmitter: *emitter.New[runtime.Event](),
-		expression:  expression,
-	}
-}
-
-// Expression returns the current expression state.
-func (r *Runtime) Expression() expr.Expression {
-	return r.expression
-}
-
-func (r *Runtime) Step() bool {
-	result, done := ReduceOnce(r.expression)
-	r.expression = result
-	return !done
-}
-
-// Reduce performs normal order reduction on a lambda expression.
-// The expression must be a lambda.Expression; other types are returned unchanged.
-func (r *Runtime) Run() {
-	r.Emit(runtime.StartEvent)
-
-	for !r.Step() {
-		r.Emit(runtime.StepEvent)
-	}
-
-	r.Emit(runtime.StopEvent)
-}

pkg/runtime/events.go

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

pkg/runtime/runtime.go

@@ -1,27 +0,0 @@
-// Package runtime provides the abstract Reducer interface for all expression
-// reduction strategies.
-package runtime
-
-import (
-	"git.maximhutz.com/max/lambda/pkg/emitter"
-	"git.maximhutz.com/max/lambda/pkg/expr"
-)
-
-// Runtime 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.
-//
-// Runtimes also implement the Emitter interface to allow plugins to observe
-// reduction lifecycle events (Start, Step, Stop).
-type Runtime interface {
-	emitter.Emitter[Event]
-
-	// Run a single step. Returns whether the runtime is complete or not.
-	Step() bool
-
-	// Run until completion.
-	Run()
-
-	// Copy the state of the runtime.
-	Expression() expr.Expression
-}

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}
 }

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}
 }

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

@@ -1,12 +1,10 @@
 package set
 
-import "iter"
-
 type Set[T comparable] map[T]bool
 
-func (s Set[T]) Add(items ...T) {
+func (s *Set[T]) Add(items ...T) {
 	for _, item := range items {
-		s[item] = true
+		(*s)[item] = true
 	}
 }
 
@@ -14,14 +12,14 @@ func (s Set[T]) Has(item T) bool {
 	return s[item]
 }
 
-func (s Set[T]) Remove(items ...T) {
+func (s *Set[T]) Remove(items ...T) {
 	for _, item := range items {
-		delete(s, item)
+		delete(*s, item)
 	}
 }
 
-func (s Set[T]) Merge(o Set[T]) {
+func (s *Set[T]) Merge(o *Set[T]) {
-	for item := range o {
+	for item := range *o {
 		s.Add(item)
 	}
 }
@@ -36,18 +34,8 @@ func (s Set[T]) ToList() []T {
 	return list
 }
 
-func (s Set[T]) Items() iter.Seq[T] {
+func New[T comparable](items ...T) *Set[T] {
-	return func(yield func(T) bool) {
+	result := &Set[T]{}
-		for item := range s {
-			if !yield(item) {
-				return
-			}
-		}
-	}
-}
-
-func New[T comparable](items ...T) Set[T] {
-	result := Set[T]{}
 
 	for _, item := range items {
 		result.Add(item)

samples/church.txt

@@ -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)

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
+)

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

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)))))))))))))))))))))))))))))))

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)))))))))))))))))))))))))))))))

tests/thunk.expected

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