feat: better recursive descent

.golangci.yml

@@ -162,7 +162,7 @@ linters:
         - name: increment-decrement
 
         # highlights redundant else-blocks that can be eliminated from the code
-        - name: indent-error-flow
+        # - name: indent-error-flow
 
         # This rule suggests a shorter way of writing ranges that do not use the second value.
         - name: range
@@ -174,7 +174,7 @@ linters:
         - name: redefines-builtin-id
 
         # redundant else-blocks that can be eliminated from the code.
-        - name: superfluous-else
+        # - name: superfluous-else
 
         # prevent confusing name for variables when using `time` package
         - name: time-naming

Makefile

@@ -5,4 +5,4 @@ it:
 	@ chmod +x ${BINARY_NAME}
 
 ex: it
-	@ ./lambda.exe - < ./samples/simple.txt
+	@ ./lambda.exe -v - < ./samples/simple.txt

README.md

@@ -1,3 +1,8 @@
 # lambda
 
 Making a lambda calculus interpreter in Go.
+
+## Things to talk about
+
+- Exhaustive sum types.
+- Recursive descent and left-recursion.

cmd/lambda/lambda.go

@@ -28,9 +28,9 @@ func main() {
 	logger.Info("Parsed tokens.", "tokens", tokens)
 
 	// Turn tokens into syntax tree.
-	expression, err := saccharine.GetTree(tokens)
+	expression, err := saccharine.Parse(tokens)
 	cli.HandleError(err)
-	logger.Info("Parsed syntax tree.", "tree", saccharine.Stringify(expression))
+	logger.Info("Parsed syntax tree.", "tree", expression)
 
 	// Reduce expression.
 	start := time.Now()

pkg/iterator/iterator.go

@@ -7,13 +7,13 @@ import "fmt"
 
 // An iterator over slices.
 type Iterator[T any] struct {
-	data  []T
+	items []T
 	index int
 }
 
 // Create a new iterator, over a set of items.
-func New[T any](items []T) *Iterator[T] {
+func Of[T any](items []T) *Iterator[T] {
-	return &Iterator[T]{data: items, index: 0}
+	return &Iterator[T]{items: items, index: 0}
 }
 
 // Returns the current position of the iterator.
@@ -21,50 +21,40 @@ func (i Iterator[T]) Index() int {
 	return i.index
 }
 
-// Returns true if the iterator has no more items to iterate over.
+func (i Iterator[T]) Copy() *Iterator[T] {
-func (i Iterator[T]) IsDone() bool {
+	return &Iterator[T]{items: i.items, index: i.index}
-	return i.index == len(i.data)
 }
 
-// Gets the next item in the slice, if one exists. Returns an error if there
+func (i *Iterator[T]) Sync(o *Iterator[T]) {
-// isn't one.
+	i.index = o.index
-func (i Iterator[T]) Peek() (T, error) {
+}
-	var null T
 
-	if i.IsDone() {
+// Create a new iterator, over a set of items.
+func (i Iterator[T]) Get() (T, error) {
+	var null T
+	if i.Done() {
 		return null, fmt.Errorf("iterator is exhausted")
 	}
 
-	return i.data[i.index], nil
+	return i.items[i.index], nil
-}
-
-// Moves the iterator pointer to the next item. Returns the current item. Fails
-// if there are no more items to iterate over.
-func (i *Iterator[T]) Pop() (T, error) {
-	val, err := i.Peek()
-	if err != nil {
-		return val, err
 }
 
+// Create a new iterator, over a set of items.
+func (i *Iterator[T]) Next() (T, error) {
+	item, err := i.Get()
+	if err == nil {
 		i.index++
-	return val, nil
 	}
 
-// Pop until the clause returns false.
+	return item, err
-func (i *Iterator[T]) PopWhile(fn func(T) bool) []T {
-	result := []T{}
-
-	for {
-		popped, err := i.Peek()
-		if err != nil || !fn(popped) {
-			break
 }
 
-		result = append(result, popped)
+// Create a new iterator, over a set of items.
-		if _, err := i.Pop(); err != nil {
+func (i *Iterator[T]) Back() {
-			break
+	i.index = max(i.index-1, 0)
-		}
 }
 
-	return result
+// Returns the current position of the iterator.
+func (i Iterator[T]) Done() bool {
+	return i.index == len(i.items)
 }

pkg/saccharine/ast.go

@@ -1,49 +0,0 @@
-package saccharine
-
-type Node interface {
-	Accept(Visitor)
-}
-
-/** ------------------------------------------------------------------------- */
-
-type Abstraction struct {
-	Parameters []string
-	Body       Node
-}
-
-type Application struct {
-	Abstraction Node
-	Arguments   []Node
-}
-
-type Variable struct {
-	Name string
-}
-
-/** ------------------------------------------------------------------------- */
-
-func NewAbstraction(parameter []string, body Node) *Abstraction {
-	return &Abstraction{Parameters: parameter, Body: body}
-}
-
-func NewApplication(abstraction Node, arguments []Node) *Application {
-	return &Application{Abstraction: abstraction, Arguments: arguments}
-}
-
-func NewVariable(name string) *Variable {
-	return &Variable{Name: name}
-}
-
-/** ------------------------------------------------------------------------- */
-
-func (a *Abstraction) Accept(x Visitor) { x.VisitAbstraction(a) }
-func (a *Application) Accept(x Visitor) { x.VisitApplication(a) }
-func (v *Variable) Accept(x Visitor)    { x.VisitVariable(v) }
-
-/** ------------------------------------------------------------------------- */
-
-type Visitor interface {
-	VisitAbstraction(*Abstraction)
-	VisitApplication(*Application)
-	VisitVariable(*Variable)
-}

pkg/saccharine/ast/node.go

@@ -0,0 +1,41 @@
+package ast
+
+type Expression interface {
+	IsExpression()
+}
+
+/** ------------------------------------------------------------------------- */
+
+type Abstraction struct {
+	Parameters []string
+	Body       Expression
+}
+
+type Application struct {
+	Abstraction Expression
+	Arguments   []Expression
+}
+
+type Atom struct {
+	Name string
+}
+
+/** ------------------------------------------------------------------------- */
+
+func NewAbstraction(parameter []string, body Expression) *Abstraction {
+	return &Abstraction{Parameters: parameter, Body: body}
+}
+
+func NewApplication(abstraction Expression, arguments []Expression) *Application {
+	return &Application{Abstraction: abstraction, Arguments: arguments}
+}
+
+func NewAtom(name string) *Atom {
+	return &Atom{Name: name}
+}
+
+/** ------------------------------------------------------------------------- */
+
+func (a Abstraction) IsExpression() {}
+func (a Application) IsExpression() {}
+func (v Atom) IsExpression()        {}

pkg/saccharine/ast/visit.go

@@ -0,0 +1,22 @@
+package ast
+
+import "fmt"
+
+type Visitor[T any] interface {
+	VisitAtom(*Atom) T
+	VisitAbstraction(*Abstraction) T
+	VisitApplication(*Application) T
+}
+
+func Visit[T any](visitor Visitor[T], node Expression) T {
+	switch node := node.(type) {
+	case *Atom:
+		return visitor.VisitAtom(node)
+	case *Abstraction:
+		return visitor.VisitAbstraction(node)
+	case *Application:
+		return visitor.VisitApplication(node)
+	default:
+		panic(fmt.Sprintf("unknown node %t", node))
+	}
+}

pkg/saccharine/parser.go

@@ -1,80 +1,132 @@
 package saccharine
 
 import (
+	"errors"
 	"fmt"
+	"log/slog"
 
 	"git.maximhutz.com/max/lambda/pkg/iterator"
+	"git.maximhutz.com/max/lambda/pkg/saccharine/ast"
+	"git.maximhutz.com/max/lambda/pkg/saccharine/token"
 )
 
-func isVariableToken(t Token) bool {
+type TokenIterator = iterator.Iterator[token.Token]
-	return t.Type == TokenVariable
+
+func parseToken(i *TokenIterator, expected token.Type) (*token.Token, error) {
+	i2 := i.Copy()
+
+	if tok, err := i2.Next(); err != nil {
+		return nil, err
+	} else if tok.Type != expected {
+		return nil, fmt.Errorf("expected token, got %v'", tok.Value)
+	} else {
+		i.Sync(i2)
+		return &tok, nil
+	}
 }
 
-func ParseExpression(i *iterator.Iterator[Token]) (Node, error) {
+func parseExpression(i *TokenIterator) (ast.Expression, error) {
-	token, err := i.Pop()
+	slog.Info("attempt exp", "index", i.Index())
-	if err != nil {
+	if abs, absErr := parseAbstraction(i); absErr == nil {
-		return nil, fmt.Errorf("could not get next token: %w", err)
+		slog.Info("got exp")
+		return abs, nil
+	} else if atm, atmErr := parseApplication(i); atmErr == nil {
+		slog.Info("got exp")
+		return atm, nil
+	} else if app, appErr := parseAtom(i); appErr == nil {
+		slog.Info("got exp")
+		return app, nil
+	} else {
+		slog.Info("fail exp")
+		return nil, errors.Join(absErr, appErr, atmErr)
+	}
 }
 
-	switch token.Type {
+func parseParameters(i *TokenIterator) ([]string, error) {
-	case TokenVariable:
+	slog.Info("parse param")
-		return NewVariable(token.Value), nil
+	i2 := i.Copy()
-	case TokenDot:
-		return nil, fmt.Errorf("token '.' found without a corresponding slash (column %d)", token.Index)
-	case TokenSlash:
-		tokens := i.PopWhile(isVariableToken)
 	variables := []string{}
 
-		for _, token := range tokens {
+	for {
-			variables = append(variables, token.Value)
+		if tok, err := parseToken(i2, token.Atom); err != nil {
+			break
+		} else {
+			variables = append(variables, tok.Value)
+		}
 	}
 
-		if dot, dotErr := i.Pop(); dotErr != nil {
+	slog.Info("got exp")
-			return nil, fmt.Errorf("could not find parameter terminator: %w", dotErr)
+	i.Sync(i2)
-		} else if dot.Type != TokenDot {
+	return variables, nil
-			return nil, fmt.Errorf("expected '.', got '%v' (column %d)", dot.Value, dot.Index)
 }
 
-		body, bodyErr := ParseExpression(i)
+func parseAbstraction(i *TokenIterator) (*ast.Abstraction, error) {
-		if bodyErr != nil {
+	slog.Info("attempt abs")
-			return nil, fmt.Errorf("could not parse function body: %w", bodyErr)
+	i2 := i.Copy()
+
+	if _, err := parseToken(i2, token.Slash); err != nil {
+		slog.Info("fail abs")
+		return nil, err
+	} else if parameters, err := parseParameters(i2); err != nil {
+		slog.Info("fail abs")
+		return nil, err
+	} else if _, err = parseToken(i2, token.Dot); err != nil {
+		slog.Info("fail abs")
+		return nil, err
+	} else if body, err := parseExpression(i2); err != nil {
+		slog.Info("fail abs")
+		return nil, err
+	} else {
+		slog.Info("got abs")
+		i.Sync(i2)
+		return ast.NewAbstraction(parameters, body), nil
+	}
 }
 
-		return NewAbstraction(variables, body), nil
+func parseApplication(i *TokenIterator) (*ast.Application, error) {
-	case TokenOpenParen:
+	slog.Info("attempt app")
-		fn, fnErr := ParseExpression(i)
+	i2 := i.Copy()
-		if fnErr != nil {
+	expressions := []ast.Expression{}
-			return nil, fmt.Errorf("could not parse call function: %w", fnErr)
-		}
 
-		args := []Node{}
+	if _, err := parseToken(i2, token.OpenParen); err != nil {
+		slog.Info("fail app")
+		return nil, err
+	}
 
 	for {
-			if next, nextErr := i.Peek(); nextErr == nil && next.Type == TokenCloseParen {
+		if exp, err := parseExpression(i2); err != nil {
 			break
+		} else {
+			expressions = append(expressions, exp)
+		}
 	}
 
-			arg, argErr := ParseExpression(i)
+	if _, err := parseToken(i2, token.CloseParen); err != nil {
-			if argErr != nil {
+		slog.Info("fail app")
-				return nil, fmt.Errorf("could not parse call argument: %w", argErr)
+		return nil, err
 	}
 
-			args = append(args, arg)
+	if len(expressions) == 0 {
+		slog.Info("fail app")
+		return nil, fmt.Errorf("application has no arguments")
 	}
 
-		closing, closingErr := i.Pop()
+	slog.Info("got app")
-		if closingErr != nil {
+	i.Sync(i2)
-			return nil, fmt.Errorf("could not parse call terminating parenthesis: %w", closingErr)
+	return ast.NewApplication(expressions[0], expressions[1:]), nil
-		} else if closing.Type != TokenCloseParen {
-			return nil, fmt.Errorf("expected call terminating parenthesis, got '%v' (column %v)", closing.Value, closing.Index)
 }
 
-		return NewApplication(fn, args), nil
+func parseAtom(i *TokenIterator) (*ast.Atom, error) {
+	slog.Info("attempt atm")
+	if tok, err := parseToken(i, token.Atom); err != nil {
+		slog.Info("fail atm")
+		return nil, err
+	} else {
+		slog.Info("got atm")
+		return ast.NewAtom(tok.Value), nil
+	}
 }
 
-	return nil, fmt.Errorf("unexpected token '%v' (column %d)", token.Value, token.Index)
+func Parse(i *TokenIterator) (ast.Expression, error) {
-}
+	return parseExpression(i)
-
-func GetTree(tokens []Token) (Node, error) {
-	return ParseExpression(iterator.New(tokens))
 }

pkg/saccharine/stringify.go

@@ -1,36 +1,31 @@
 package saccharine
 
-import "strings"
+import (
+	"strings"
 
-type stringifyVisitor struct {
+	"git.maximhutz.com/max/lambda/pkg/saccharine/ast"
-	builder strings.Builder
+)
+
+type stringifyVisitor struct{}
+
+func (v stringifyVisitor) VisitAtom(n *ast.Atom) string {
+	return n.Name
 }
 
-func (v *stringifyVisitor) VisitVariable(a *Variable) {
+func (v stringifyVisitor) VisitAbstraction(n *ast.Abstraction) string {
-	v.builder.WriteString(a.Name)
+	return "\\" + strings.Join(n.Parameters, " ") + "." + ast.Visit(v, n.Body)
 }
 
-func (v *stringifyVisitor) VisitAbstraction(f *Abstraction) {
+func (v stringifyVisitor) VisitApplication(n *ast.Application) string {
-	v.builder.WriteRune('\\')
+	arguments := []string{ast.Visit(v, n.Abstraction)}
-	v.builder.WriteString(strings.Join(f.Parameters, " "))
+
-	v.builder.WriteRune('.')
+	for _, argument := range n.Arguments {
-	f.Body.Accept(v)
+		arguments = append(arguments, ast.Visit(v, argument))
 	}
 
-func (v *stringifyVisitor) VisitApplication(c *Application) {
+	return "(" + strings.Join(arguments, " ") + ")"
-	v.builder.WriteRune('(')
-	c.Abstraction.Accept(v)
-
-	for _, argument := range c.Arguments {
-		v.builder.WriteRune(' ')
-		argument.Accept(v)
 }
 
-	v.builder.WriteRune(')')
+func Stringify(n ast.Expression) string {
-}
+	return ast.Visit(&stringifyVisitor{}, n)
-
-func Stringify(n Node) string {
-	b := &stringifyVisitor{}
-	n.Accept(b)
-	return b.builder.String()
 }

pkg/saccharine/token.go

@@ -1,27 +0,0 @@
-package saccharine
-
-// All tokens in the pseudo-lambda language.
-type TokenType int
-
-const (
-	// Denotes the '(' token.
-	TokenOpenParen TokenType = iota
-	// Denotes the ')' token.
-	TokenCloseParen
-	// Denotes an alpha-numeric variable.
-	TokenVariable
-	// Denotes the '/' token.
-	TokenSlash
-	// Denotes the '.' token.
-	TokenDot
-)
-
-// A representation of a token in source code.
-type Token struct {
-	// Where the token begins in the source text.
-	Index int
-	// What type the token is.
-	Type TokenType
-	// The value of the token.
-	Value string
-}

pkg/saccharine/token/token.go

@@ -0,0 +1,47 @@
+package token
+
+// All tokens in the pseudo-lambda language.
+type Type int
+
+const (
+	// Denotes the '(' token.
+	OpenParen Type = iota
+	// Denotes the ')' token.
+	CloseParen
+	// Denotes an alpha-numeric variable.
+	Atom
+	// Denotes the '/' token.
+	Slash
+	// Denotes the '.' token.
+	Dot
+)
+
+// A representation of a token in source code.
+type Token struct {
+	// Where the token begins in the source text.
+	Index int
+	// What type the token is.
+	Type Type
+	// The value of the token.
+	Value string
+}
+
+func NewOpenParen(index int) *Token {
+	return &Token{Type: OpenParen, Index: index, Value: "("}
+}
+
+func NewCloseParen(index int) *Token {
+	return &Token{Type: CloseParen, Index: index, Value: ")"}
+}
+
+func NewDot(index int) *Token {
+	return &Token{Type: Dot, Index: index, Value: "."}
+}
+
+func NewSlash(index int) *Token {
+	return &Token{Type: Slash, Index: index, Value: "\\"}
+}
+
+func NewAtom(name string, index int) *Token {
+	return &Token{Type: Atom, Index: index, Value: name}
+}

pkg/saccharine/tokenizer.go

@@ -3,9 +3,11 @@ package saccharine
 import (
 	"errors"
 	"fmt"
+	"log/slog"
 	"unicode"
 
 	"git.maximhutz.com/max/lambda/pkg/iterator"
+	"git.maximhutz.com/max/lambda/pkg/saccharine/token"
 )
 
 // isVariables determines whether a rune can be a valid variable.
@@ -13,60 +15,77 @@ func isVariable(r rune) bool {
 	return unicode.IsLetter(r) || unicode.IsNumber(r)
 }
 
+func parseRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
+	i2 := i.Copy()
+
+	if r, err := i2.Next(); err != nil {
+		return r, err
+	} else if !expected(r) {
+		return r, fmt.Errorf("got unexpected rune %v'", r)
+	} else {
+		i.Sync(i2)
+		return r, nil
+	}
+}
+
 // 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 getToken(i *iterator.Iterator[rune]) (*token.Token, error) {
 	index := i.Index()
 
-	if i.IsDone() {
+	if i.Done() {
 		return nil, nil
 	}
 
-	letter, err := i.Pop()
+	letter, err := i.Next()
 	if err != nil {
 		return nil, fmt.Errorf("cannot produce next token: %w", err)
 	}
 
 	switch {
 	case letter == '(':
-		// The opening deliminator of an application.
+		return token.NewOpenParen(index), nil
-		return &Token{Type: TokenOpenParen, Index: index, Value: string(letter)}, nil
 	case letter == ')':
-		// The terminator of an application.
+		return token.NewCloseParen(index), nil
-		return &Token{Type: TokenCloseParen, Index: index, Value: string(letter)}, nil
 	case letter == '.':
-		// The terminator of the parameters in an abstraction.
+		return token.NewDot(index), nil
-		return &Token{Type: TokenDot, Index: index, Value: string(letter)}, nil
 	case letter == '\\':
-		// The opening deliminator of an abstraction.
+		return token.NewSlash(index), nil
-		return &Token{Type: TokenSlash, Index: index, Value: string(letter)}, nil
 	case unicode.IsSpace(letter):
-		// If there is a space character, ignore it.
 		return nil, nil
 	case isVariable(letter):
-		rest := i.PopWhile(isVariable)
+		atom := []rune{letter}
-		atom := string(append([]rune{letter}, rest...))
 
-		return &Token{Index: index, Type: TokenVariable, Value: atom}, nil
+		for {
+			if r, err := parseRune(i, isVariable); err != nil {
+				break
+			} else {
+				atom = append(atom, r)
+			}
+		}
+
+		return token.NewAtom(string(atom), index), nil
 	}
 
 	return nil, fmt.Errorf("unknown character '%v'", letter)
 }
 
 // Parses a list of runes into tokens. All error encountered are returned, as well.
-func GetTokens(input []rune) ([]Token, error) {
+func GetTokens(input []rune) (*iterator.Iterator[token.Token], error) {
-	i := iterator.New(input)
+	i := iterator.Of(input)
-	tokens := []Token{}
+	tokens := []token.Token{}
 	errorList := []error{}
 
-	for !i.IsDone() {
+	for !i.Done() {
 		token, err := getToken(i)
 		if err != nil {
+			slog.Info("token error", "error", err)
 			errorList = append(errorList, err)
 		} else if token != nil {
+			slog.Info("token parsed", "token", token)
 			tokens = append(tokens, *token)
 		}
 	}
 
-	return tokens, errors.Join(errorList...)
+	return iterator.Of(tokens), errors.Join(errorList...)
 }