holy carp it parses

ast.go

@@ -0,0 +1,53 @@
+package main
+
+// todo: find something better than any here
+// we can't use generics on either the visitor itself or
+// each individual method because the adding it to the
+// visitor itself infects every expr you use with it
+// and methods cannot have generic parameters
+
+type exprVisitor interface {
+	visitBinaryExpr(b *binaryExpr) any
+	visitGroupingExpr(g *groupingExpr) any
+	visitLiteralExpr(g *literalExpr) any
+	visitUnaryExpr(g *unaryExpr) any
+}
+
+type expr interface {
+	accept(v exprVisitor) any
+}
+
+type binaryExpr struct {
+	Left     expr
+	Operator *token
+	Right    expr
+}
+
+func (b *binaryExpr) accept(v exprVisitor) any {
+	return v.visitBinaryExpr(b)
+}
+
+type groupingExpr struct {
+	Expr expr
+}
+
+func (g *groupingExpr) accept(v exprVisitor) any {
+	return v.visitGroupingExpr(g)
+}
+
+type literalExpr struct {
+	Value any
+}
+
+func (l *literalExpr) accept(v exprVisitor) any {
+	return v.visitLiteralExpr(l)
+}
+
+type unaryExpr struct {
+	Operator *token
+	Right    expr
+}
+
+func (u *unaryExpr) accept(v exprVisitor) any {
+	return v.visitUnaryExpr(u)
+}

astprinter.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+type astPrinter struct {
+	result string
+}
+
+// visitBinaryExpr implements exprVisitor.
+func (p *astPrinter) visitBinaryExpr(b *binaryExpr) any {
+	return p.parenthesize(b.Operator.Lexeme, b.Left, b.Right)
+}
+
+// visitGroupingExpr implements exprVisitor.
+func (p *astPrinter) visitGroupingExpr(g *groupingExpr) any {
+	return p.parenthesize("group", g.Expr)
+}
+
+// visitLiteralExpr implements exprVisitor.
+func (p *astPrinter) visitLiteralExpr(g *literalExpr) any {
+	switch t := g.Value.(type) {
+	case string:
+		return t
+	case bool:
+		if t {
+			return "true"
+		}
+		return "false"
+	case float64:
+		return strconv.FormatFloat(t, 'f', 3, 64)
+	case nil:
+		return "nil"
+	}
+
+	return fmt.Sprintf("%v", g.Value)
+}
+
+// visitUnaryExpr implements exprVisitor.
+func (p *astPrinter) visitUnaryExpr(g *unaryExpr) any {
+	return p.parenthesize(g.Operator.Lexeme, g.Right)
+}
+
+func (p *astPrinter) parenthesize(name string, expressions ...expr) string {
+	val := "(" + name
+
+	for _, e := range expressions {
+		exprStr, ok := (e.accept(p)).(string)
+		if !ok {
+			panic("badly implemented visitor")
+		}
+		val += " " + exprStr
+	}
+
+	val += ")"
+	return val
+
+}
+
+func (p *astPrinter) print(e expr) string {
+	str, ok := (e.accept(p)).(string)
+	if !ok {
+		panic("badly implemented visitor")
+	}
+	return str
+}

main.go

@@ -29,10 +29,10 @@ func runFile(filename string) {
 	bs, err := os.ReadFile(filename)
 	if err != nil {
 		fmt.Printf("unable to read file '%s':\n\t%s", filename, err.Error())
-		os.Exit(1)
+		os.Exit(64)
 	}
 
-	if run(string(bs)) {
+	if !run(string(bs)) {
 		os.Exit(65)
 	}
 }
@@ -59,20 +59,33 @@ func run(source string) bool {
 	s := newScanner(source)
 	tokens, ok := s.ScanTokens()
 	if !ok {
-		return true
-	}
-
-	for _, token := range tokens {
-		fmt.Println(token)
-	}
-
 		return false
 	}
 
+	p := newParser(tokens)
+	expr, err := p.Parse()
+	if err != nil {
+		return false
+	}
+
+	printer := &astPrinter{}
+	fmt.Println(printer.print(expr))
+
+	return true
+}
+
 func reportErr(line int, message string) {
 	report(line, "", message)
 }
 
+func reportSyntaxError(token *token, message string) {
+	if token.Type == tokenTypeEOF {
+		report(token.Line, "at EOF", message)
+	} else {
+		report(token.Line, "at \""+token.Lexeme+"\"", message)
+	}
+}
+
 func report(line int, where, message string) {
 	fmt.Printf("[line %d] Error%s: %s\n", line, where, message)
 }

parser.go

@@ -0,0 +1,201 @@
+package main
+
+type parseError interface {
+	Error() string
+	Token() *token
+}
+
+type syntaxError struct {
+	token   *token
+	message string
+}
+
+func newSyntaxError(token *token, message string) *syntaxError {
+	return &syntaxError{token, message}
+}
+
+func (s *syntaxError) Token() *token {
+	return s.token
+}
+
+func (s *syntaxError) Error() string {
+	return s.message
+}
+
+type operandFunc func() (expr, error)
+
+type parser struct {
+	tokens  []*token
+	current int
+}
+
+func newParser(tokens []*token) *parser {
+	return &parser{
+		tokens:  tokens,
+		current: 0,
+	}
+}
+
+func (p *parser) Parse() (expr, error) {
+	e, err := p.expression()
+	if err != nil {
+		return e, err
+	}
+
+	return e, err
+}
+
+// expression -> equality
+func (p *parser) expression() (expr, error) {
+	return p.equality()
+}
+
+func (p *parser) parseLeftAssocBinOps(operand operandFunc, tokenTypes ...tokenType) (expr, error) {
+	e, err := operand()
+	if err != nil {
+		return e, err
+	}
+
+	for p.match(tokenTypes...) {
+		op := p.previous()
+		r, err := operand()
+		if err != nil {
+			return e, err
+		}
+		e = &binaryExpr{e, op, r}
+	}
+
+	return e, nil
+}
+
+// eqality -> comparison ( ("!=" | "==") comparison )*
+func (p *parser) equality() (expr, error) {
+	return p.parseLeftAssocBinOps(
+		p.comparison,
+		tokenTypeBangEq, tokenTypeEqualEqual,
+	)
+}
+
+// comparison -> term ( ( ">" | ">=" | "<" | "<=" ) term )*
+func (p *parser) comparison() (expr, error) {
+	return p.parseLeftAssocBinOps(
+		p.term,
+		tokenTypeGreater, tokenTypeGreaterEq,
+		tokenTypeLess, tokenTypeLessEq,
+	)
+}
+
+// term -> factor ( ( "-" | "+" ) factor )*
+func (p *parser) term() (expr, error) {
+	return p.parseLeftAssocBinOps(
+		p.factor,
+		tokenTypeMinus, tokenTypePlus,
+	)
+}
+
+// factor -> unary ( ( "*" | "/" ) unary )*
+func (p *parser) factor() (expr, error) {
+	return p.parseLeftAssocBinOps(
+		p.unary,
+		tokenTypeSlash, tokenTypeStar,
+	)
+}
+
+// unary -> ( "!" | "-" ) unary | primary;
+func (p *parser) unary() (expr, error) {
+	if p.match(tokenTypeBang, tokenTypeMinus) {
+		op := p.previous()
+		r, err := p.unary()
+		return &unaryExpr{op, r}, err
+	}
+
+	return p.primary()
+}
+
+// primary -> STRING | NUMBER | "true" | "false" | "nil" | "(" expression ")"
+func (p *parser) primary() (expr, error) {
+	if p.match(tokenTypeTrue) {
+		return &literalExpr{true}, nil
+	}
+	if p.match(tokenTypeFalse) {
+		return &literalExpr{false}, nil
+	}
+	if p.match(tokenTypeNil) {
+		return &literalExpr{nil}, nil
+	}
+
+	if p.match(tokenTypeString, tokenTypeNumber) {
+		return &literalExpr{p.previous().Literal}, nil
+	}
+
+	if p.match(tokenTypeLeftParen) {
+		e, err := p.expression()
+		if err != nil {
+			return nil, err
+		}
+		_, err = p.consume(tokenTypeRightParen, "expected ')' after expression")
+		return &groupingExpr{e}, err
+	}
+
+	return nil, newSyntaxError(p.peek(), "expected expression")
+}
+
+func (p *parser) consume(tokenType tokenType, msg string) (*token, error) {
+	if p.check(tokenType) {
+		return p.advance(), nil
+	}
+
+	return nil, newSyntaxError(p.peek(), msg)
+}
+
+func (p *parser) synchronize() {
+	p.advance()
+
+	for !p.isAtEnd() {
+		if p.previous().Type == tokenTypeSemicolon {
+			return
+		}
+
+		if isKeyword(p.peek()) {
+			return
+		}
+		p.advance()
+	}
+}
+
+func (p *parser) match(tokenTypes ...tokenType) bool {
+	for _, t := range tokenTypes {
+		if p.check(t) {
+			p.advance()
+			return true
+		}
+	}
+
+	return false
+}
+
+func (p *parser) check(t tokenType) bool {
+	if p.isAtEnd() {
+		return false
+	}
+	return p.peek().Type == t
+}
+
+func (p *parser) advance() *token {
+	if !p.isAtEnd() {
+		p.current += 1
+	}
+	return p.previous()
+}
+
+func (p *parser) isAtEnd() bool {
+	return p.peek().Type == tokenTypeEOF
+}
+
+func (p *parser) peek() *token {
+	return p.tokens[p.current]
+}
+
+func (p *parser) previous() *token {
+	return p.tokens[p.current-1]
+}

scanner.go

@@ -222,7 +222,7 @@ func (s *scanner) string() bool {
 
 	// todo: escape sequences
 	value := s.source[s.start+1 : s.current-1]
-	s.addToken(tokenTypeString, value)
+	s.addToken(tokenTypeString, string(value))
 
 	return false
 }

tokentype.go

@@ -59,6 +59,17 @@ const (
 	tokenTypeEOF
 )
 
+var keywordTokenTypes = []tokenType{
+	tokenTypeClass,
+	tokenTypeFun,
+	tokenTypeVar,
+	tokenTypeFor,
+	tokenTypeIf,
+	tokenTypeReturn,
+	tokenTypeWhile,
+	tokenTypePrint,
+}
+
 type token struct {
 	Type    tokenType
 	Lexeme  string
@@ -69,3 +80,12 @@ type token struct {
 func (t token) String() string {
 	return fmt.Sprintf("%s %s %+v", t.Type, t.Lexeme, t.Literal)
 }
+
+func isKeyword(token *token) bool {
+	for _, kt := range keywordTokenTypes {
+		if token.Type == kt {
+			return true
+		}
+	}
+	return false
+}