refactor: rewrite CLI and internal architecture (#41)

## Description

The old architecture used a monolithic `main()` with a custom arg parser, an event-emitter-based runtime, and a plugin system for optional features.
This PR rewrites the CLI and internal architecture to be modular, extensible, and built around a registry of interchangeable components.

- Replace custom CLI arg parsing with Cobra subcommands (`convert`, `reduce`, `engine list`).
- Introduce a registry system (`internal/registry`) for marshalers, codecs, and engines, with BFS-based conversion path resolution.
- Add type-erased adapter layer (`internal/cli`) with `Repr`, `Engine`, `Process`, `Marshaler`, and `Conversion` interfaces wrapping generic `pkg/` types.
- Replace the event-emitter-based `Runtime` with a simpler `Engine`/`Process` model (`pkg/engine`).
- Add generic `Codec[T, U]` and `Marshaler[T]` interfaces (`pkg/codec`).
- Merge `saccharine/token` sub-package into `saccharine` and rename scanner functions from `parse*` to `scan*`.
- Make saccharine-to-lambda conversion bidirectional (encode and decode).
- Add `lambda.Marshaler` and `saccharine.Marshaler` implementing `codec.Marshaler`.
- Remove old infrastructure: `pkg/runtime`, `pkg/expr`, `internal/plugins`, `internal/statistics`.
- Add `make lint` target and update golangci-lint config.

### Decisions

- Cobra was chosen for the CLI framework to support nested subcommands and standard flag handling.
- The registry uses BFS to find conversion paths between representations, allowing multi-hop conversions without hardcoding routes.
- Type erasure via `cli.Repr` (wrapping `any`) enables the registry to work with heterogeneous types while keeping `pkg/` generics type-safe.
- The old plugin/event system was removed entirely rather than adapted, since the new `Process` model can support hooks differently in the future.

## Benefits

- Subcommands make the CLI self-documenting and easier to extend with new functionality.
- The registry pattern decouples representations, conversions, and engines, making it trivial to add new ones.
- BFS conversion routing means adding a single codec automatically enables transitive conversions.
- Simpler `Engine`/`Process` model reduces complexity compared to the event-emitter runtime.
- Consolidating the `token` sub-package reduces import depth and package sprawl.

## Checklist

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

Reviewed-on: #41
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

pkg/codec/codec.go

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

pkg/convert/saccharine_to_lambda.go

@@ -3,16 +3,17 @@ package convert
 import (
 	"fmt"
 
+	"git.maximhutz.com/max/lambda/pkg/codec"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
-func convertAtom(n *saccharine.Atom) lambda.Expression {
+func encodeAtom(n *saccharine.Atom) lambda.Expression {
 	return lambda.NewVariable(n.Name)
 }
 
-func convertAbstraction(n *saccharine.Abstraction) lambda.Expression {
-	result := SaccharineToLambda(n.Body)
+func encodeAbstraction(n *saccharine.Abstraction) lambda.Expression {
+	result := encodeExpression(n.Body)
 
 	parameters := n.Parameters
 
@@ -31,13 +32,13 @@ func convertAbstraction(n *saccharine.Abstraction) lambda.Expression {
 	return result
 }
 
-func convertApplication(n *saccharine.Application) lambda.Expression {
-	result := SaccharineToLambda(n.Abstraction)
+func encodeApplication(n *saccharine.Application) lambda.Expression {
+	result := encodeExpression(n.Abstraction)
 
 	arguments := []lambda.Expression{}
 	for _, argument := range n.Arguments {
-		convertedArgument := SaccharineToLambda(argument)
-		arguments = append(arguments, convertedArgument)
+		encodeedArgument := encodeExpression(argument)
+		arguments = append(arguments, encodeedArgument)
 	}
 
 	for _, argument := range arguments {
@@ -51,9 +52,9 @@ func reduceLet(s *saccharine.LetStatement, e lambda.Expression) lambda.Expressio
 	var value lambda.Expression
 
 	if len(s.Parameters) == 0 {
-		value = SaccharineToLambda(s.Body)
+		value = encodeExpression(s.Body)
 	} else {
-		value = convertAbstraction(saccharine.NewAbstraction(s.Parameters, s.Body))
+		value = encodeAbstraction(saccharine.NewAbstraction(s.Parameters, s.Body))
 	}
 
 	return lambda.NewApplication(
@@ -67,7 +68,7 @@ func reduceDeclare(s *saccharine.DeclareStatement, e lambda.Expression) lambda.E
 
 	return lambda.NewApplication(
 		lambda.NewAbstraction(freshVar, e),
-		SaccharineToLambda(s.Value),
+		encodeExpression(s.Value),
 	)
 }
 
@@ -82,8 +83,8 @@ func reduceStatement(s saccharine.Statement, e lambda.Expression) lambda.Express
 	}
 }
 
-func convertClause(n *saccharine.Clause) lambda.Expression {
-	result := SaccharineToLambda(n.Returns)
+func encodeClause(n *saccharine.Clause) lambda.Expression {
+	result := encodeExpression(n.Returns)
 
 	for i := len(n.Statements) - 1; i >= 0; i-- {
 		result = reduceStatement(n.Statements[i], result)
@@ -92,17 +93,46 @@ func convertClause(n *saccharine.Clause) lambda.Expression {
 	return result
 }
 
-func SaccharineToLambda(n saccharine.Expression) lambda.Expression {
-	switch n := n.(type) {
+func encodeExpression(s saccharine.Expression) lambda.Expression {
+	switch s := s.(type) {
 	case *saccharine.Atom:
-		return convertAtom(n)
+		return encodeAtom(s)
 	case *saccharine.Abstraction:
-		return convertAbstraction(n)
+		return encodeAbstraction(s)
 	case *saccharine.Application:
-		return convertApplication(n)
+		return encodeApplication(s)
 	case *saccharine.Clause:
-		return convertClause(n)
+		return encodeClause(s)
 	default:
-		panic(fmt.Errorf("unknown expression type: %T", n))
+		panic(fmt.Errorf("unknown expression type: %T", s))
 	}
 }
+
+func decodeExression(l lambda.Expression) saccharine.Expression {
+	switch l := l.(type) {
+	case lambda.Variable:
+		return saccharine.NewAtom(l.Name())
+	case lambda.Abstraction:
+		return saccharine.NewAbstraction(
+			[]string{l.Parameter()},
+			decodeExression(l.Body()))
+	case lambda.Application:
+		return saccharine.NewApplication(
+			decodeExression(l.Abstraction()),
+			[]saccharine.Expression{decodeExression(l.Argument())})
+	default:
+		panic(fmt.Errorf("unknown expression type: %T", l))
+	}
+}
+
+type Saccharine2Lambda struct{}
+
+func (c Saccharine2Lambda) Decode(l lambda.Expression) (saccharine.Expression, error) {
+	return decodeExression(l), nil
+}
+
+func (c Saccharine2Lambda) Encode(s saccharine.Expression) (lambda.Expression, error) {
+	return encodeExpression(s), nil
+}
+
+var _ codec.Codec[saccharine.Expression, lambda.Expression] = (*Saccharine2Lambda)(nil)

pkg/engine/engine.go

@@ -0,0 +1,11 @@
+package engine
+
+type Engine[T any] interface {
+	Load() Process[T]
+}
+
+type Process[T any] interface {
+	Get() (T, error)
+	Set(T) error
+	Step(int) bool
+}

pkg/engine/normalorder/engine.go

@@ -0,0 +1,42 @@
+package normalorder
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/engine"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+type Process struct {
+	expr lambda.Expression
+}
+
+func (e Process) Get() (lambda.Expression, error) {
+	return e.expr, nil
+}
+
+func (e *Process) Set(l lambda.Expression) error {
+	e.expr = l
+	return nil
+}
+
+func (e *Process) Step(i int) bool {
+	for range i {
+		next, reduced := ReduceOnce(e.expr)
+		if !reduced {
+			return false
+		}
+
+		e.expr = next
+	}
+
+	return true
+}
+
+type Engine struct {
+}
+
+func (e Engine) Load() engine.Process[lambda.Expression] {
+	return &Process{}
+}
+
+var _ engine.Process[lambda.Expression] = (*Process)(nil)
+var _ engine.Engine[lambda.Expression] = (*Engine)(nil)

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/lambda/expression.go

@@ -1,14 +1,15 @@
 package lambda
 
 import (
-	"git.maximhutz.com/max/lambda/pkg/expr"
+	"fmt"
+
 	"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
+	fmt.Stringer
 
 	// Substitute replaces all free occurrences of the target variable with the
 	// replacement expression. Alpha-renaming is performed automatically to

pkg/lambda/marshaler.go

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

pkg/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/marshaler.go

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

pkg/saccharine/parse.go

@@ -5,18 +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 TokenType) (*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'", expected.Name(), 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 TokenType, 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
@@ -105,7 +104,7 @@ func parseApplication(i *TokenIterator) (*Application, error) {
 }
 
 func parseAtom(i *TokenIterator) (*Atom, error) {
-	if tok, err := parseToken(i, token.Atom, true); err != nil {
+	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
@@ -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)

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/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() {
@@ -56,27 +56,27 @@ func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 
 	switch {
 	case letter == '(':
-		return NewOpenParen(index), nil
+		return NewTokenOpenParen(index), nil
 	case letter == ')':
-		return NewCloseParen(index), nil
+		return NewTokenCloseParen(index), nil
 	case letter == '.':
-		return NewDot(index), nil
+		return NewTokenDot(index), nil
 	case letter == '\\':
-		return NewSlash(index), nil
+		return NewTokenSlash(index), nil
 	case letter == '\n':
-		return NewSoftBreak(index), nil
+		return NewTokenSoftBreak(index), nil
 	case letter == '{':
-		return NewOpenBrace(index), nil
+		return NewTokenOpenBrace(index), nil
 	case letter == '}':
-		return NewCloseBrace(index), nil
+		return NewTokenCloseBrace(index), nil
 	case letter == ':':
-		if _, err := parseCharacter(i, '='); err != nil {
+		if _, err := scanCharacter(i, '='); err != nil {
 			return nil, err
 		} else {
-			return NewAssign(index), nil
+			return NewTokenAssign(index), nil
 		}
 	case letter == ';':
-		return NewHardBreak(index), nil
+		return NewTokenHardBreak(index), nil
 	case letter == '#':
 		// Skip everything until the next newline or EOF.
 		for !i.Done() {
@@ -98,27 +98,27 @@ func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 		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)
 			}
 		}
 
-		return NewAtom(string(atom), index), nil
+		return NewTokenAtom(string(atom), index), nil
 	}
 
 	return nil, fmt.Errorf("unknown character '%v'", string(letter))
 }
 
-// Parse a string into tokens.
-func Parse(input string) ([]Token, error) {
+// scan a string into tokens.
+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/saccharine/token.go

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

pkg/saccharine/token/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)
-}