style: scanner not lexer

## Description

`GenerateFreshName` used a global `ticker` variable but never incremented it inside the loop.
This caused an infinite loop if the first generated name (`_0`) was already in the used set.

- Remove global `ticker` variable.
- Use loop variable `i` directly to generate candidate names.

## Benefits

- Fixes infinite loop bug when generated name collides with used set.
- Removes unnecessary global state.
- Simpler and more predictable behavior.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #40
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

.gitea/ISSUE_TEMPLATE/bug.md

@@ -0,0 +1,58 @@
+---
+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

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

@@ -0,0 +1,37 @@
+---
+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.
+-->

.gitignore

@@ -3,16 +3,13 @@
 # https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore
 #
 # Binaries for programs and plugins
-lambda
+/lambda
 *.exe
 *.exe~
 *.dll
 *.so
 *.dylib
 
-# Test binary, built with `go test -c`
-*.test
-
 # Output of the go coverage tool, specifically when used with LiteIDE
 *.out
 

CLAUDE.md

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

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 interpreter 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 runtime 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

@@ -1,45 +1,30 @@
 BINARY_NAME=lambda
-TEST=simple
 
-.PHONY: help build run profile explain graph docs bench clean
+.PHONY: help build docs test bench clean
 .DEFAULT_GOAL := help
 .SILENT:
 
 help:
 	echo "Available targets:"
 	echo "  build    - Build the lambda executable"
-	echo "  run      - Build and run the lambda interpreter (use TEST=<name> to specify sample)"
-	echo "  profile  - Build and run with CPU profiling enabled"
-	echo "  explain  - Build and run with explanation mode and profiling"
-	echo "  graph    - Generate and open CPU profile visualization"
 	echo "  docs     - Start local godoc server on port 6060"
-	echo "  bench    - Run benchmarks for all samples"
+	echo "  test     - Run tests"
+	echo "  bench    - Run benchmarks"
 	echo "  clean    - Remove all build artifacts"
 
 build:
 	go build -o ${BINARY_NAME} ./cmd/lambda
 	chmod +x ${BINARY_NAME}
 
-run: build
-	./${BINARY_NAME} -f ./samples/$(TEST).txt -o program.out
-
-profile: build
-	./${BINARY_NAME} -p profile/cpu.prof -f ./samples/$(TEST).txt -o program.out
-
-explain: build
-	./${BINARY_NAME} -x -p profile/cpu.prof -f ./samples/$(TEST).txt -o program.out
-
-graph:
-	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 ./...
+
 bench:
-	go test -bench=. -benchtime=10x -cpu=4 ./cmd/lambda
+	go test -bench=. -benchtime=10x -cpu=4 ./...
 
 clean:
 	rm -f ${BINARY_NAME}

README.md

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

cmd/lambda/benchmark_test.go

@@ -1,70 +0,0 @@
-package main
-
-import (
-	"os"
-	"testing"
-
-	"git.maximhutz.com/max/lambda/internal/config"
-	"git.maximhutz.com/max/lambda/internal/engine"
-	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
-	"git.maximhutz.com/max/lambda/pkg/saccharine"
-)
-
-// Helper function to run a single sample through the lambda interpreter.
-func runSample(samplePath 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 minimal config for benchmarking.
-	cfg := &config.Config{
-		Source:      config.StringSource{Data: ""},
-		Destination: config.StdoutDestination{},
-		Profile:     "",
-		Explanation: false,
-		Statistics:  false,
-		Verbose:     false,
-	}
-
-	// Create and run the engine.
-	process := engine.New(cfg, &compiled)
-	process.Run()
-
-	// Get final result (to ensure it's not optimized away).
-	_ = lambda.Stringify(compiled)
-
-	return nil
-}
-
-// Benchmark all samples using sub-benchmarks.
-func BenchmarkSamples(b *testing.B) {
-	samples := map[string]string{
-		"Church":     "../../samples/church.txt",
-		"Fast":       "../../samples/fast.txt",
-		"Saccharine": "../../samples/saccharine.txt",
-		"Simple":     "../../samples/simple.txt",
-		"Thunk":      "../../samples/thunk.txt",
-	}
-
-	for name, path := range samples {
-		b.Run(name, func(b *testing.B) {
-			for b.Loop() {
-				if err := runSample(path); err != nil {
-					b.Fatal(err)
-				}
-			}
-		})
-	}
-}

cmd/lambda/convert.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var convertCmd = &cobra.Command{
+	Use:   "convert [input] [output]",
+	Short: "Convert between lambda calculus representations",
+	Long: `Convert lambda calculus expressions between different representations.
+
+The format is inferred from file extensions when possible.
+Use --from and --to flags to override format detection.`,
+	Example: `  lambda convert input.sch output.dbi
+  lambda convert input.txt output.txt --from saccharine --to lambda`,
+	Args: cobra.ExactArgs(2),
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Fprintln(os.Stderr, "not implemented")
+		os.Exit(1)
+	},
+}
+
+func init() {
+	convertCmd.Flags().String("from", "", "source format")
+	convertCmd.Flags().String("to", "", "target format")
+	rootCmd.AddCommand(convertCmd)
+}

cmd/lambda/engine.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var engineCmd = &cobra.Command{
+	Use:   "engine",
+	Short: "List available evaluation engines",
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Fprintln(os.Stderr, "not implemented")
+		os.Exit(1)
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(engineCmd)
+}

cmd/lambda/lambda.go

@@ -1,77 +0,0 @@
-package main
-
-import (
-	"os"
-
-	"git.maximhutz.com/max/lambda/internal/cli"
-	"git.maximhutz.com/max/lambda/internal/config"
-	"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/lambda"
-	"git.maximhutz.com/max/lambda/pkg/saccharine"
-)
-
-func main() {
-	// Parse CLI arguments.
-	options, err := config.FromArgs()
-	cli.HandleError(err)
-
-	logger := options.GetLogger()
-	logger.Info("using program arguments", "args", os.Args)
-	logger.Info("parsed CLI options", "options", options)
-
-	// Get input.
-	input, err := options.Source.Extract()
-	cli.HandleError(err)
-
-	// Parse code into syntax tree.
-	ast, err := saccharine.Parse(input)
-	cli.HandleError(err)
-	logger.Info("parsed syntax tree", "tree", ast)
-
-	// Compile expression to lambda calculus.
-	compiled := convert.SaccharineToLambda(ast)
-	logger.Info("compiled λ expression", "tree", lambda.Stringify(compiled))
-
-	// Create reduction engine.
-	process := engine.New(options, &compiled)
-
-	// If the user selected to track CPU performance, attach a profiler to the
-	// process.
-	if options.Profile != "" {
-		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 here.
-	if options.Explanation {
-		explanation.Track(process)
-	}
-
-	// If the user opted to track statistics, attach a tracker here, too.
-	if options.Statistics {
-		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 {
-		process.On("step", func() {
-			logger.Info("reduction", "tree", lambda.Stringify(compiled))
-		})
-	}
-
-	process.Run()
-
-	// Return the final reduced result.
-	result := lambda.Stringify(compiled)
-	err = options.Destination.Write(result)
-	cli.HandleError(err)
-}

cmd/lambda/repl.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var replCmd = &cobra.Command{
+	Use:   "repl",
+	Short: "Start an interactive lambda calculus REPL",
+	Long: `Start an interactive Read-Eval-Print Loop for lambda calculus.
+
+Enter lambda expressions to evaluate them.
+Type 'exit' or 'quit' to leave the REPL.
+Press Ctrl+D to exit.`,
+	Example: `  lambda repl`,
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Fprintln(os.Stderr, "not implemented")
+		os.Exit(1)
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(replCmd)
+}

cmd/lambda/repr.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var reprCmd = &cobra.Command{
+	Use:   "repr",
+	Short: "List available representations",
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Fprintln(os.Stderr, "not implemented")
+		os.Exit(1)
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(reprCmd)
+}

cmd/lambda/root.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var rootCmd = &cobra.Command{
+	Use:   "lambda",
+	Short: "A lambda calculus interpreter and toolkit",
+	Long:  `Lambda is a CLI tool for working with lambda calculus expressions.`,
+}
+
+func main() {
+	if err := rootCmd.Execute(); err != nil {
+		os.Exit(1)
+	}
+}

cmd/lambda/run.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var runCmd = &cobra.Command{
+	Use:   "run [expression]",
+	Short: "Evaluate a lambda calculus expression",
+	Long: `Evaluate a lambda calculus expression and print the result.
+
+The expression can be provided as an argument, read from a file with --file,
+or piped through stdin.`,
+	Example: `  lambda run "\x.x"
+  echo "\x.x" | lambda run
+  lambda run --file program.sch
+  lambda run --file program.sch --engine krivine`,
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Fprintln(os.Stderr, "not implemented")
+		os.Exit(1)
+	},
+}
+
+func init() {
+	runCmd.Flags().StringP("file", "f", "", "read expression from file")
+	runCmd.Flags().StringP("engine", "e", "normal", "evaluation engine to use")
+	rootCmd.AddCommand(runCmd)
+}

cmd/lambda/trace.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var traceCmd = &cobra.Command{
+	Use:   "trace [file]",
+	Short: "Trace the evaluation of a lambda calculus expression",
+	Long: `Trace the step-by-step evaluation of a lambda calculus expression.
+
+This command shows each reduction step during evaluation.`,
+	Example: `  lambda trace program.sch
+  lambda trace program.sch --engine krivine`,
+	Args: cobra.ExactArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Fprintln(os.Stderr, "not implemented")
+		os.Exit(1)
+	},
+}
+
+func init() {
+	traceCmd.Flags().StringP("engine", "e", "normal", "evaluation engine to use")
+	rootCmd.AddCommand(traceCmd)
+}

go.mod

@@ -1,3 +1,14 @@
 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/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/spf13/cobra v1.10.2 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

@@ -0,0 +1,18 @@
+github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=
+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/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=
+github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
+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/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
+github.com/spf13/cobra v1.10.2 h1:DMTTonx5m65Ic0GOoRY2c16WCbHxOOw6xxezuLaBpcU=
+github.com/spf13/cobra v1.10.2/go.mod h1:7C1pvHqHw5A4vrJfjNwvOdzYu0Gml16OCs2GRiTUUS4=
+github.com/spf13/pflag v1.0.9 h1:9exaQaMOCwffKiiiYk6/BndUBv+iRViNW+4lEMi0PvY=
+github.com/spf13/pflag v1.0.9/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
+github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
+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/cli/exit.go

@@ -1,18 +0,0 @@
-// Package "cli" provides miscellaneous helper functions.
-package cli
-
-import (
-	"fmt"
-	"os"
-)
-
-// A helper function to handle errors in the program. If it is given an error,
-// the program will exist, and print the error.
-func HandleError(err error) {
-	if err == nil {
-		return
-	}
-
-	fmt.Fprintln(os.Stderr, "ERROR:", err)
-	os.Exit(1)
-}

internal/config/config.go

@@ -1,12 +0,0 @@
-// Package "config" parses ad handles the user settings given to the program.
-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.
-	Statistics  bool        // Whether or not to print statistics.
-}

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/get_logger.go

@@ -1,23 +0,0 @@
-package config
-
-import (
-	"log/slog"
-	"os"
-)
-
-// Returns a structured logger with the appropriate configurations.
-func (c Config) GetLogger() *slog.Logger {
-	// By default, only print out errors.
-	level := slog.LevelError
-
-	// If the user set the output to be "VERBOSE", return the debug logs.
-	if c.Verbose {
-		level = slog.LevelInfo
-	}
-
-	return slog.New(
-		slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
-			Level: level,
-		}),
-	)
-}

internal/config/parse_from_args.go

@@ -1,56 +0,0 @@
-package config
-
-import (
-	"flag"
-	"fmt"
-)
-
-// Extract the program configuration from the command-line arguments.
-func FromArgs() (*Config, error) {
-	// Relevant flags.
-	verbose := flag.Bool("v", false, "Verbosity. If set, the program will print logs.")
-	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.
-	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")
-	} 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}
-	}
-
-	return &Config{
-		Source:      source,
-		Destination: destination,
-		Verbose:     *verbose,
-		Explanation: *explanation,
-		Profile:     *profile,
-		Statistics:  *statistics,
-	}, nil
-}

internal/config/source.go

@@ -1,41 +0,0 @@
-package config
-
-import (
-	"io"
-	"os"
-)
-
-// A method of extracting input from the user.
-type Source interface {
-	// Fetch data from this source.
-	Extract() (string, error)
-}
-
-// A source defined by a string.
-type StringSource struct{ Data string }
-
-func (s StringSource) Extract() (string, error) { return s.Data, nil }
-
-// A source pulling from standard input.
-type StdinSource struct{}
-
-func (s StdinSource) Extract() (string, error) {
-	data, err := io.ReadAll(os.Stdin)
-	if err != nil {
-		return "", err
-	}
-
-	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

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

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

@@ -1,53 +0,0 @@
-// Package "performance" provides a tracker to observer CPU performance during
-// execution.
-package performance
-
-import (
-	"os"
-	"path/filepath"
-	"runtime/pprof"
-)
-
-// Observes a reduction process, and publishes a CPU performance profile on
-// completion.
-type Tracker struct {
-	File        string
-	filePointer *os.File
-	Error       error
-}
-
-// Create a performance tracker that outputs a profile to "file".
-func Track(file string) *Tracker {
-	return &Tracker{File: file}
-}
-
-// Begin profiling.
-func (t *Tracker) Start() {
-	var absPath string
-
-	absPath, t.Error = filepath.Abs(t.File)
-	if t.Error != nil {
-		return
-	}
-
-	t.Error = os.MkdirAll(filepath.Dir(absPath), 0777)
-	if t.Error != nil {
-		return
-	}
-
-	t.filePointer, t.Error = os.Create(absPath)
-	if t.Error != nil {
-		return
-	}
-
-	t.Error = pprof.StartCPUProfile(t.filePointer)
-	if t.Error != nil {
-		return
-	}
-}
-
-// Stop profiling.
-func (t *Tracker) End() {
-	pprof.StopCPUProfile()
-	t.filePointer.Close()
-}

internal/statistics/statistics.go

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

internal/statistics/tracker.go

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

pkg/convert/saccharine_to_lambda.go

@@ -3,11 +3,11 @@ package convert
 import (
 	"fmt"
 
-	"git.maximhutz.com/max/lambda/pkg/lambda"
-	"git.maximhutz.com/max/lambda/pkg/saccharine"
+	"git.maximhutz.com/max/lambda/pkg/repr/lambda"
+	"git.maximhutz.com/max/lambda/pkg/repr/saccharine"
 )
 
-func convertAtom(n *saccharine.Atom) lambda.Expression {
+func convertAtom(n *saccharine.Variable) lambda.Expression {
 	return lambda.NewVariable(n.Name)
 }
 
@@ -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 := lambda.GetFreeVariables(result)
+		freeVars := result.GetFree()
 		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(lambda.GetFreeVariables(e))
+	freshVar := lambda.GenerateFreshName(e.GetFree())
 
 	return lambda.NewApplication(
 		lambda.NewAbstraction(freshVar, e),
@@ -94,7 +94,7 @@ func convertClause(n *saccharine.Clause) lambda.Expression {
 
 func SaccharineToLambda(n saccharine.Expression) lambda.Expression {
 	switch n := n.(type) {
-	case *saccharine.Atom:
+	case *saccharine.Variable:
 		return convertAtom(n)
 	case *saccharine.Abstraction:
 		return convertAbstraction(n)

pkg/deltanet/deltanet.go

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

pkg/deltanet/node.go

@@ -1,94 +0,0 @@
-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

@@ -1,54 +0,0 @@
-package emitter
-
-import "git.maximhutz.com/max/lambda/pkg/set"
-
-type Observer struct {
-	fn      func()
-	message string
-	emitter *Emitter
-}
-
-type Emitter struct {
-	listeners map[string]*set.Set[*Observer]
-}
-
-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,
-	}
-
-	if e.listeners == nil {
-		e.listeners = map[string]*set.Set[*Observer]{}
-	}
-
-	if e.listeners[message] == nil {
-		e.listeners[message] = set.New[*Observer]()
-	}
-
-	e.listeners[message].Add(observer)
-	return observer
-}
-
-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()
-	}
-}

pkg/engine/engine.go

@@ -0,0 +1,23 @@
+package engine
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/pkg/repr"
+)
+
+type Engine[R repr.Repr] interface {
+	Run(trace bool, count int) (Trace, bool)
+	Step() uint64
+	State() (repr.Repr, error)
+
+	fmt.Stringer
+}
+
+type Trace interface {
+	Before() string
+	After() string
+	Step() uint64
+	TransitionName() string
+	TransitionNodes() string
+}

pkg/fifo/fifo.go

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

@@ -1,77 +0,0 @@
-package lambda
-
-type Expression interface {
-	Accept(Visitor)
-}
-
-/** ------------------------------------------------------------------------- */
-
-type Abstraction struct {
-	parameter string
-	body      Expression
-}
-
-func (a *Abstraction) Parameter() string {
-	return a.parameter
-}
-
-func (a *Abstraction) Body() Expression {
-	return a.body
-}
-
-func (a *Abstraction) Accept(v Visitor) {
-	v.VisitAbstraction(a)
-}
-
-func NewAbstraction(parameter string, body Expression) *Abstraction {
-	return &Abstraction{parameter: parameter, body: body}
-}
-
-/** ------------------------------------------------------------------------- */
-
-type Application struct {
-	abstraction Expression
-	argument    Expression
-}
-
-func (a *Application) Abstraction() Expression {
-	return a.abstraction
-}
-
-func (a *Application) Argument() Expression {
-	return a.argument
-}
-
-func (a *Application) Accept(v Visitor) {
-	v.VisitApplication(a)
-}
-
-func NewApplication(abstraction Expression, argument Expression) *Application {
-	return &Application{abstraction: abstraction, argument: argument}
-}
-
-/** ------------------------------------------------------------------------- */
-
-type Variable struct {
-	value string
-}
-
-func (v *Variable) Value() string {
-	return v.value
-}
-
-func (v *Variable) Accept(visitor Visitor) {
-	visitor.VisitVariable(v)
-}
-
-func NewVariable(name string) *Variable {
-	return &Variable{value: name}
-}
-
-/** ------------------------------------------------------------------------- */
-
-type Visitor interface {
-	VisitAbstraction(*Abstraction)
-	VisitApplication(*Application)
-	VisitVariable(*Variable)
-}

pkg/lambda/get_free_variables.go

@@ -1,20 +0,0 @@
-package lambda
-
-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)
-	case *Abstraction:
-		vars := GetFreeVariables(e.body)
-		vars.Remove(e.parameter)
-		return vars
-	case *Application:
-		vars := GetFreeVariables(e.abstraction)
-		vars.Merge(GetFreeVariables(e.argument))
-		return vars
-	default:
-		return nil
-	}
-}

pkg/lambda/is_free_variable.go

@@ -1,14 +0,0 @@
-package lambda
-
-func IsFreeVariable(n string, e Expression) bool {
-	switch e := e.(type) {
-	case *Variable:
-		return e.value == n
-	case *Abstraction:
-		return e.parameter != n && IsFreeVariable(n, e.body)
-	case *Application:
-		return IsFreeVariable(n, e.abstraction) || IsFreeVariable(n, e.argument)
-	default:
-		return false
-	}
-}

pkg/lambda/reduce.go

@@ -1,27 +0,0 @@
-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 {
-				reduced := Substitute(fn.body, fn.parameter, typed.argument)
-				*top = reduced
-				return true
-			}
-
-			stack.Push(&typed.argument)
-			stack.Push(&typed.abstraction)
-		}
-	}
-
-	return false
-}

pkg/lambda/rename.go

@@ -1,38 +0,0 @@
-package lambda
-
-func Rename(expr Expression, target string, newName string) Expression {
-	switch e := expr.(type) {
-	case *Variable:
-		if e.value == target {
-			return NewVariable(newName)
-		}
-		return e
-
-	case *Abstraction:
-		newParam := e.parameter
-		if e.parameter == target {
-			newParam = newName
-		}
-
-		newBody := Rename(e.body, target, newName)
-
-		if newParam == e.parameter && newBody == e.body {
-			return e
-		}
-
-		return NewAbstraction(newParam, newBody)
-
-	case *Application:
-		newAbs := Rename(e.abstraction, target, newName)
-		newArg := Rename(e.argument, target, newName)
-
-		if newAbs == e.abstraction && newArg == e.argument {
-			return e
-		}
-
-		return NewApplication(newAbs, newArg)
-
-	default:
-		return expr
-	}
-}

pkg/lambda/stringify.go

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

@@ -1,46 +0,0 @@
-package lambda
-
-func Substitute(expr Expression, target string, replacement Expression) Expression {
-	switch e := expr.(type) {
-	case *Variable:
-		if e.value == target {
-			return replacement
-		}
-		return e
-
-	case *Abstraction:
-		if e.parameter == target {
-			return e
-		}
-
-		body := e.body
-		param := e.parameter
-		if IsFreeVariable(param, replacement) {
-			freeVars := GetFreeVariables(replacement)
-			freeVars.Merge(GetFreeVariables(body))
-			freshVar := GenerateFreshName(freeVars)
-			body = Rename(body, param, freshVar)
-			param = freshVar
-		}
-
-		newBody := Substitute(body, target, replacement)
-		if newBody == body && param == e.parameter {
-			return e
-		}
-
-		return NewAbstraction(param, newBody)
-
-	case *Application:
-		newAbs := Substitute(e.abstraction, target, replacement)
-		newArg := Substitute(e.argument, target, replacement)
-
-		if newAbs == e.abstraction && newArg == e.argument {
-			return e
-		}
-
-		return NewApplication(newAbs, newArg)
-
-	default:
-		return expr
-	}
-}

pkg/repr/lambda/expression.go

@@ -0,0 +1,87 @@
+package lambda
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/repr"
+	"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 {
+	repr.Repr
+
+	// 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
+}
+
+var (
+	_ Expression = Abstraction{}
+	_ Expression = Application{}
+	_ Expression = Variable{}
+)
+
+/** ------------------------------------------------------------------------- */
+
+type Abstraction struct {
+	parameter string
+	body      Expression
+}
+
+func (a Abstraction) Parameter() string {
+	return a.parameter
+}
+
+func (a Abstraction) Body() Expression {
+	return a.body
+}
+
+func NewAbstraction(parameter string, body Expression) Abstraction {
+	return Abstraction{parameter, body}
+}
+
+/** ------------------------------------------------------------------------- */
+
+type Application struct {
+	abstraction Expression
+	argument    Expression
+}
+
+func (a Application) Abstraction() Expression {
+	return a.abstraction
+}
+
+func (a Application) Argument() Expression {
+	return a.argument
+}
+
+func NewApplication(abstraction Expression, argument Expression) Application {
+	return Application{abstraction, argument}
+}
+
+/** ------------------------------------------------------------------------- */
+
+type Variable struct {
+	name string
+}
+
+func (v Variable) Name() string {
+	return v.name
+}
+
+func NewVariable(name string) Variable {
+	return Variable{name}
+}

pkg/repr/lambda/generate_name.go

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

pkg/repr/lambda/get_free_variables.go

@@ -0,0 +1,19 @@
+package lambda
+
+import "git.maximhutz.com/max/lambda/pkg/set"
+
+func (e Variable) GetFree() set.Set[string] {
+	return set.New(e.Name())
+}
+
+func (e Abstraction) GetFree() set.Set[string] {
+	vars := e.Body().GetFree()
+	vars.Remove(e.Parameter())
+	return vars
+}
+
+func (e Application) GetFree() set.Set[string] {
+	vars := e.Abstraction().GetFree()
+	vars.Merge(e.Argument().GetFree())
+	return vars
+}

pkg/repr/lambda/is_free_variable.go

@@ -0,0 +1,12 @@
+package lambda
+
+func (e Variable) IsFree(n string) bool {
+	return e.Name() == n
+}
+
+func (e Abstraction) IsFree(n string) bool {
+	return e.Parameter() != n && e.Body().IsFree(n)
+}
+func (e Application) IsFree(n string) bool {
+	return e.Abstraction().IsFree(n) || e.Argument().IsFree(n)
+}

pkg/repr/lambda/rename.go

@@ -0,0 +1,28 @@
+package lambda
+
+// Rename replaces all occurrences of the target variable name with the new name.
+func (e Variable) Rename(target string, newName string) Expression {
+	if e.Name() == target {
+		return NewVariable(newName)
+	}
+
+	return e
+}
+
+func (e Abstraction) Rename(target string, newName string) Expression {
+	newParam := e.Parameter()
+	if e.Parameter() == target {
+		newParam = newName
+	}
+
+	newBody := e.Body().Rename(target, newName)
+
+	return NewAbstraction(newParam, newBody)
+}
+
+func (e Application) Rename(target string, newName string) Expression {
+	newAbs := e.Abstraction().Rename(target, newName)
+	newArg := e.Argument().Rename(target, newName)
+
+	return NewApplication(newAbs, newArg)
+}

pkg/repr/lambda/substitute.go

@@ -0,0 +1,35 @@
+package lambda
+
+func (e Variable) Substitute(target string, replacement Expression) Expression {
+	if e.Name() == target {
+		return replacement
+	}
+
+	return e
+}
+
+func (e Abstraction) Substitute(target string, replacement Expression) Expression {
+	if e.Parameter() == target {
+		return e
+	}
+
+	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/repr/repr.go

@@ -0,0 +1,4 @@
+package repr
+
+type Repr interface {
+}

pkg/repr/saccharine/expression.go

@@ -1,9 +1,20 @@
 package saccharine
 
+import (
+	"git.maximhutz.com/max/lambda/pkg/repr"
+)
+
 type Expression interface {
-	IsExpression()
+	repr.Repr
 }
 
+var (
+	_ Expression = Abstraction{}
+	_ Expression = Application{}
+	_ Expression = Variable{}
+	_ Expression = Clause{}
+)
+
 /** ------------------------------------------------------------------------- */
 
 type Abstraction struct {
@@ -11,37 +22,36 @@ type Abstraction struct {
 	Body       Expression
 }
 
-type Application struct {
-	Abstraction Expression
-	Arguments   []Expression
+func NewAbstraction(parameter []string, body Expression) *Abstraction {
+	return &Abstraction{Parameters: parameter, Body: body}
 }
 
-type Atom struct {
-	Name string
-}
-
-type Clause struct {
-	Statements []Statement
-	Returns    Expression
-}
-
-func (Abstraction) IsExpression() {}
-func (Application) IsExpression() {}
-func (Atom) IsExpression()        {}
-func (Clause) IsExpression()      {}
-
 /** ------------------------------------------------------------------------- */
 
-func NewAbstraction(parameter []string, body Expression) *Abstraction {
-	return &Abstraction{Parameters: parameter, Body: body}
+type Application struct {
+	Abstraction Expression
+	Arguments   []Expression
 }
 
 func NewApplication(abstraction Expression, arguments []Expression) *Application {
 	return &Application{Abstraction: abstraction, Arguments: arguments}
 }
 
-func NewAtom(name string) *Atom {
-	return &Atom{Name: name}
+/** ------------------------------------------------------------------------- */
+
+type Variable struct {
+	Name string
+}
+
+func NewVariable(name string) *Variable {
+	return &Variable{Name: name}
+}
+
+/** ------------------------------------------------------------------------- */
+
+type Clause struct {
+	Statements []Statement
+	Returns    Expression
 }
 
 func NewClause(statements []Statement, returns Expression) *Clause {

pkg/repr/saccharine/parse.go

@@ -5,18 +5,17 @@ import (
 	"fmt"
 
 	"git.maximhutz.com/max/lambda/pkg/iterator"
-	"git.maximhutz.com/max/lambda/pkg/saccharine/token"
 	"git.maximhutz.com/max/lambda/pkg/trace"
 )
 
-type TokenIterator = iterator.Iterator[token.Token]
+type TokenIterator = iterator.Iterator[Token]
 
-func parseRawToken(i *TokenIterator, expected token.Type) (*token.Token, error) {
-	return iterator.Do(i, func(i *TokenIterator) (*token.Token, error) {
+func parseRawToken(i *TokenIterator, expected Type) (*Token, error) {
+	return iterator.Do(i, func(i *TokenIterator) (*Token, error) {
 		if tok, err := i.Next(); err != nil {
 			return nil, err
 		} else if tok.Type != expected {
-			return nil, fmt.Errorf("expected token %v, got %v'", token.Name(expected), tok.Value)
+			return nil, fmt.Errorf("expected token %v, got %v'", Name(expected), tok.Value)
 		} else {
 			return &tok, nil
 		}
@@ -25,14 +24,14 @@ func parseRawToken(i *TokenIterator, expected token.Type) (*token.Token, error)
 
 func passSoftBreaks(i *TokenIterator) {
 	for {
-		if _, err := parseRawToken(i, token.SoftBreak); err != nil {
+		if _, err := parseRawToken(i, TokenSoftBreak); err != nil {
 			return
 		}
 	}
 }
 
-func parseToken(i *TokenIterator, expected token.Type, ignoreSoftBreaks bool) (*token.Token, error) {
-	return iterator.Do(i, func(i *TokenIterator) (*token.Token, error) {
+func parseToken(i *TokenIterator, expected Type, ignoreSoftBreaks bool) (*Token, error) {
+	return iterator.Do(i, func(i *TokenIterator) (*Token, error) {
 		if ignoreSoftBreaks {
 			passSoftBreaks(i)
 		}
@@ -42,17 +41,17 @@ func parseToken(i *TokenIterator, expected token.Type, ignoreSoftBreaks bool) (*
 }
 
 func parseString(i *TokenIterator) (string, error) {
-	if tok, err := parseToken(i, token.Atom, true); err != nil {
+	if tok, err := parseToken(i, TokenAtom, true); err != nil {
 		return "", trace.Wrap(err, "no variable (col %d)", i.Index())
 	} else {
 		return tok.Value, nil
 	}
 }
 
-func parseBreak(i *TokenIterator) (*token.Token, error) {
-	if tok, softErr := parseRawToken(i, token.SoftBreak); softErr == nil {
+func parseBreak(i *TokenIterator) (*Token, error) {
+	if tok, softErr := parseRawToken(i, TokenSoftBreak); softErr == nil {
 		return tok, nil
-	} else if tok, hardErr := parseRawToken(i, token.HardBreak); hardErr == nil {
+	} else if tok, hardErr := parseRawToken(i, TokenHardBreak); hardErr == nil {
 		return tok, nil
 	} else {
 		return nil, errors.Join(softErr, hardErr)
@@ -76,11 +75,11 @@ func parseList[U any](i *TokenIterator, fn func(*TokenIterator) (U, error), mini
 
 func parseAbstraction(i *TokenIterator) (*Abstraction, error) {
 	return iterator.Do(i, func(i *TokenIterator) (*Abstraction, error) {
-		if _, err := parseToken(i, token.Slash, true); err != nil {
+		if _, err := parseToken(i, TokenSlash, true); err != nil {
 			return nil, trace.Wrap(err, "no function slash (col %d)", i.MustGet().Column)
 		} else if parameters, err := parseList(i, parseString, 0); err != nil {
 			return nil, err
-		} else if _, err = parseToken(i, token.Dot, true); err != nil {
+		} else if _, err = parseToken(i, TokenDot, true); err != nil {
 			return nil, trace.Wrap(err, "no function dot (col %d)", i.MustGet().Column)
 		} else if body, err := parseExpression(i); err != nil {
 			return nil, err
@@ -92,11 +91,11 @@ func parseAbstraction(i *TokenIterator) (*Abstraction, error) {
 
 func parseApplication(i *TokenIterator) (*Application, error) {
 	return iterator.Do(i, func(i *TokenIterator) (*Application, error) {
-		if _, err := parseToken(i, token.OpenParen, true); err != nil {
+		if _, err := parseToken(i, TokenOpenParen, true); err != nil {
 			return nil, trace.Wrap(err, "no openning brackets (col %d)", i.MustGet().Column)
 		} else if expressions, err := parseList(i, parseExpression, 1); err != nil {
 			return nil, err
-		} else if _, err := parseToken(i, token.CloseParen, true); err != nil {
+		} else if _, err := parseToken(i, TokenCloseParen, true); err != nil {
 			return nil, trace.Wrap(err, "no closing brackets (col %d)", i.MustGet().Column)
 		} else {
 			return NewApplication(expressions[0], expressions[1:]), nil
@@ -104,11 +103,11 @@ func parseApplication(i *TokenIterator) (*Application, error) {
 	})
 }
 
-func parseAtom(i *TokenIterator) (*Atom, error) {
-	if tok, err := parseToken(i, token.Atom, true); err != nil {
+func parseAtom(i *TokenIterator) (*Variable, error) {
+	if tok, err := parseToken(i, TokenAtom, true); err != nil {
 		return nil, trace.Wrap(err, "no variable (col %d)", i.Index())
 	} else {
-		return NewAtom(tok.Value), nil
+		return NewVariable(tok.Value), nil
 	}
 }
 
@@ -133,7 +132,7 @@ func parseStatements(i *TokenIterator) ([]Statement, error) {
 
 func parseClause(i *TokenIterator, braces bool) (*Clause, error) {
 	if braces {
-		if _, err := parseToken(i, token.OpenBrace, true); err != nil {
+		if _, err := parseToken(i, TokenOpenBrace, true); err != nil {
 			return nil, err
 		}
 	}
@@ -152,7 +151,7 @@ func parseClause(i *TokenIterator, braces bool) (*Clause, error) {
 	}
 
 	if braces {
-		if _, err := parseToken(i, token.CloseBrace, true); err != nil {
+		if _, err := parseToken(i, TokenCloseBrace, true); err != nil {
 			return nil, err
 		}
 	}
@@ -165,13 +164,13 @@ func parseExpression(i *TokenIterator) (Expression, error) {
 		passSoftBreaks(i)
 
 		switch peek := i.MustGet(); peek.Type {
-		case token.OpenParen:
+		case TokenOpenParen:
 			return parseApplication(i)
-		case token.Slash:
+		case TokenSlash:
 			return parseAbstraction(i)
-		case token.Atom:
+		case TokenAtom:
 			return parseAtom(i)
-		case token.OpenBrace:
+		case TokenOpenBrace:
 			return parseClause(i, true)
 		default:
 			return nil, fmt.Errorf("expected expression, got '%v' (col %d)", peek.Value, peek.Column)
@@ -183,7 +182,7 @@ func parseLet(i *TokenIterator) (*LetStatement, error) {
 	return iterator.Do(i, func(i *TokenIterator) (*LetStatement, error) {
 		if parameters, err := parseList(i, parseString, 1); err != nil {
 			return nil, err
-		} else if _, err := parseToken(i, token.Assign, true); err != nil {
+		} else if _, err := parseToken(i, TokenAssign, true); err != nil {
 			return nil, err
 		} else if body, err := parseExpression(i); err != nil {
 			return nil, err
@@ -212,7 +211,7 @@ func parseStatement(i *TokenIterator) (Statement, error) {
 }
 
 // Given a list of tokens, attempt to parse it into an syntax tree.
-func parse(tokens []token.Token) (Expression, error) {
+func parse(tokens []Token) (Expression, error) {
 	i := iterator.Of(tokens)
 
 	exp, err := parseClause(i, false)
@@ -226,3 +225,13 @@ func parse(tokens []token.Token) (Expression, error) {
 
 	return exp, nil
 }
+
+// Convert a piece of valid saccharine code into an expression.
+func Parse(code string) (Expression, error) {
+	tokens, err := Scan(code)
+	if err != nil {
+		return nil, err
+	}
+
+	return parse(tokens)
+}

pkg/repr/saccharine/scan.go

@@ -1,4 +1,4 @@
-package token
+package saccharine
 
 import (
 	"errors"
@@ -14,7 +14,7 @@ func isVariable(r rune) bool {
 	return unicode.IsLetter(r) || unicode.IsNumber(r)
 }
 
-func parseRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
+func scanRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
 	i2 := i.Copy()
 
 	if r, err := i2.Next(); err != nil {
@@ -27,7 +27,7 @@ func parseRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, erro
 	}
 }
 
-func parseCharacter(i *iterator.Iterator[rune], expected rune) (rune, error) {
+func scanCharacter(i *iterator.Iterator[rune], expected rune) (rune, error) {
 	i2 := i.Copy()
 
 	if r, err := i2.Next(); err != nil {
@@ -42,7 +42,7 @@ func parseCharacter(i *iterator.Iterator[rune], expected rune) (rune, error) {
 
 // Pulls the next token from an iterator over runes. If it cannot, it will
 // return nil. If an error occurs, it will return that.
-func getToken(i *iterator.Iterator[rune]) (*Token, error) {
+func scanToken(i *iterator.Iterator[rune]) (*Token, error) {
 	index := i.Index()
 
 	if i.Done() {
@@ -70,20 +70,35 @@ func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 	case letter == '}':
 		return NewCloseBrace(index), nil
 	case letter == ':':
-		if _, err := parseCharacter(i, '='); err != nil {
+		if _, err := scanCharacter(i, '='); err != nil {
 			return nil, err
 		} else {
 			return NewAssign(index), nil
 		}
 	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):
 		atom := []rune{letter}
 
 		for {
-			if r, err := parseRune(i, isVariable); err != nil {
+			if r, err := scanRune(i, isVariable); err != nil {
 				break
 			} else {
 				atom = append(atom, r)
@@ -97,13 +112,13 @@ func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 }
 
 // Parse a string into tokens.
-func Parse(input string) ([]Token, error) {
+func Scan(input string) ([]Token, error) {
 	i := iterator.Of([]rune(input))
 	tokens := []Token{}
 	errorList := []error{}
 
 	for !i.Done() {
-		token, err := getToken(i)
+		token, err := scanToken(i)
 		if err != nil {
 			errorList = append(errorList, err)
 		} else if token != nil {

pkg/repr/saccharine/statement.go

@@ -1,7 +1,11 @@
 package saccharine
 
+import (
+	"git.maximhutz.com/max/lambda/pkg/repr"
+)
+
 type Statement interface {
-	IsStatement()
+	repr.Repr
 }
 
 /** ------------------------------------------------------------------------- */
@@ -12,17 +16,14 @@ type LetStatement struct {
 	Body       Expression
 }
 
-type DeclareStatement struct {
-	Value Expression
+func NewLet(name string, parameters []string, body Expression) *LetStatement {
+	return &LetStatement{Name: name, Parameters: parameters, Body: body}
 }
 
-func (LetStatement) IsStatement()     {}
-func (DeclareStatement) IsStatement() {}
-
 /** ------------------------------------------------------------------------- */
 
-func NewLet(name string, parameters []string, body Expression) *LetStatement {
-	return &LetStatement{Name: name, Parameters: parameters, Body: body}
+type DeclareStatement struct {
+	Value Expression
 }
 
 func NewDeclare(value Expression) *DeclareStatement {

pkg/repr/saccharine/stringify.go

@@ -5,30 +5,30 @@ import (
 	"strings"
 )
 
-func stringifyAtom(n *Atom) string {
+func stringifyAtom(n *Variable) string {
 	return n.Name
 }
 
 func stringifyAbstraction(n *Abstraction) string {
-	return "\\" + strings.Join(n.Parameters, " ") + "." + stringifyExpression(n.Body)
+	return "\\" + strings.Join(n.Parameters, " ") + "." + Stringify(n.Body)
 }
 
 func stringifyApplication(n *Application) string {
-	arguments := []string{stringifyExpression(n.Abstraction)}
+	arguments := []string{Stringify(n.Abstraction)}
 
 	for _, argument := range n.Arguments {
-		arguments = append(arguments, stringifyExpression(argument))
+		arguments = append(arguments, Stringify(argument))
 	}
 
 	return "(" + strings.Join(arguments, " ") + ")"
 }
 
 func stringifyLet(s *LetStatement) string {
-	return s.Name + " " + strings.Join(s.Parameters, " ") + " := " + stringifyExpression(s.Body)
+	return s.Name + " " + strings.Join(s.Parameters, " ") + " := " + Stringify(s.Body)
 }
 
 func stringifyDeclare(s *DeclareStatement) string {
-	return stringifyExpression(s.Value)
+	return Stringify(s.Value)
 }
 
 func stringifyStatement(s Statement) string {
@@ -49,13 +49,13 @@ func stringifyClause(n *Clause) string {
 		stmts += stringifyStatement(statement) + "; "
 	}
 
-	return "{ " + stmts + stringifyExpression(n.Returns) + " }"
+	return "{ " + stmts + Stringify(n.Returns) + " }"
 }
 
 // Convert an expression back into valid source code.
-func stringifyExpression(n Expression) string {
+func Stringify(n Expression) string {
 	switch n := n.(type) {
-	case *Atom:
+	case *Variable:
 		return stringifyAtom(n)
 	case *Abstraction:
 		return stringifyAbstraction(n)

pkg/repr/saccharine/token.go

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

pkg/saccharine/saccharine.go

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

pkg/saccharine/token/token.go

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

pkg/set/set.go

@@ -1,10 +1,12 @@
 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
 	}
 }
 
@@ -12,14 +14,14 @@ func (s Set[T]) Has(item T) bool {
 	return s[item]
 }
 
-func (s *Set[T]) Remove(items ...T) {
+func (s Set[T]) Remove(items ...T) {
 	for _, item := range items {
-		delete(*s, item)
+		delete(s, item)
 	}
 }
 
-func (s *Set[T]) Merge(o *Set[T]) {
-	for item := range *o {
+func (s Set[T]) Merge(o Set[T]) {
+	for item := range o {
 		s.Add(item)
 	}
 }
@@ -34,8 +36,18 @@ func (s Set[T]) ToList() []T {
 	return list
 }
 
-func New[T comparable](items ...T) *Set[T] {
-	result := &Set[T]{}
+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]{}
 
 	for _, item := range items {
 		result.Add(item)

samples/simple.txt

@@ -1,16 +0,0 @@
-(\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/church_5^5.test

@@ -1,7 +1,8 @@
 0 := \f.\x.x
 inc n := \f x.(f (n f x))
 exp n m := (m n)
+print n := (n F X)
 
 N := (inc (inc (inc (inc (inc 0)))))
 
-(exp N N)
+(print (exp N N))

tests/comments.expected

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

tests/comments.test

@@ -0,0 +1,17 @@
+# 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

@@ -0,0 +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 (1 END)))))))))))))))))))))))))))))))

tests/list_2^30.expected

@@ -0,0 +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 (1 END)))))))))))))))))))))))))))))))

tests/thunk.expected

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