enhance errors

parser/parser.go

@@ -5,6 +5,19 @@ import (
 	"strconv"
 )
 
+// ParseError represents a parsing error with location information
+type ParseError struct {
+	Message string
+	Line    int
+	Column  int
+	Token   Token
+}
+
+func (pe ParseError) Error() string {
+	return fmt.Sprintf("Parse error at line %d, column %d: %s (near '%s')",
+		pe.Line, pe.Column, pe.Message, pe.Token.Literal)
+}
+
 // Parser implements a recursive descent Pratt parser
 type Parser struct {
 	lexer *Lexer
@@ -15,14 +28,14 @@ type Parser struct {
 	prefixParseFns map[TokenType]func() Expression
 	infixParseFns  map[TokenType]func(Expression) Expression
 
-	errors []string
+	errors []ParseError
 }
 
 // NewParser creates a new parser instance
 func NewParser(lexer *Lexer) *Parser {
 	p := &Parser{
 		lexer:  lexer,
-		errors: []string{},
+		errors: []ParseError{},
 	}
 
 	p.prefixParseFns = make(map[TokenType]func() Expression)
@@ -81,12 +94,25 @@ func (p *Parser) ParseProgram() *Program {
 
 // parseStatement parses a statement
 func (p *Parser) parseStatement() Statement {
-	if p.curTokenIs(IDENT) && p.peekTokenIs(ASSIGN) {
+	switch p.curToken.Type {
+	case IDENT:
+		if p.peekTokenIs(ASSIGN) {
 			return p.parseAssignStatement()
 		}
-
+		p.addError("unexpected identifier, expected assignment or declaration")
-	// Skip unknown statements for now
 		return nil
+	case ASSIGN:
+		p.addError("assignment operator '=' without left-hand side identifier")
+		return nil
+	case ILLEGAL:
+		p.addError(fmt.Sprintf("unexpected token '%s'", p.curToken.Literal))
+		return nil
+	case EOF:
+		return nil
+	default:
+		p.addError(fmt.Sprintf("unexpected token '%s', expected statement", p.curToken.Literal))
+		return nil
+	}
 }
 
 // parseAssignStatement parses variable assignment
@@ -94,6 +120,7 @@ func (p *Parser) parseAssignStatement() *AssignStatement {
 	stmt := &AssignStatement{}
 
 	if !p.curTokenIs(IDENT) {
+		p.addError("expected identifier for assignment")
 		return nil
 	}
 
@@ -106,6 +133,10 @@ func (p *Parser) parseAssignStatement() *AssignStatement {
 	p.nextToken()
 
 	stmt.Value = p.parseExpression(LOWEST)
+	if stmt.Value == nil {
+		p.addError("expected expression after assignment operator")
+		return nil
+	}
 
 	return stmt
 }
@@ -119,6 +150,9 @@ func (p *Parser) parseExpression(precedence Precedence) Expression {
 	}
 
 	leftExp := prefix()
+	if leftExp == nil {
+		return nil
+	}
 
 	for !p.peekTokenIs(EOF) && precedence < p.peekPrecedence() {
 		infix := p.infixParseFns[p.peekToken.Type]
@@ -128,6 +162,9 @@ func (p *Parser) parseExpression(precedence Precedence) Expression {
 
 		p.nextToken()
 		leftExp = infix(leftExp)
+		if leftExp == nil {
+			return nil
+		}
 	}
 
 	return leftExp
@@ -143,8 +180,7 @@ func (p *Parser) parseNumberLiteral() Expression {
 
 	value, err := strconv.ParseFloat(p.curToken.Literal, 64)
 	if err != nil {
-		msg := fmt.Sprintf("could not parse %q as float", p.curToken.Literal)
+		p.addError(fmt.Sprintf("could not parse '%s' as number", p.curToken.Literal))
-		p.errors = append(p.errors, msg)
 		return nil
 	}
 
@@ -168,6 +204,9 @@ func (p *Parser) parseGroupedExpression() Expression {
 	p.nextToken()
 
 	exp := p.parseExpression(LOWEST)
+	if exp == nil {
+		return nil
+	}
 
 	if !p.expectPeek(RPAREN) {
 		return nil
@@ -186,6 +225,11 @@ func (p *Parser) parseInfixExpression(left Expression) Expression {
 	p.nextToken()
 	expression.Right = p.parseExpression(precedence)
 
+	if expression.Right == nil {
+		p.addError(fmt.Sprintf("expected expression after operator '%s'", expression.Operator))
+		return nil
+	}
+
 	return expression
 }
 
@@ -202,21 +246,48 @@ func (p *Parser) expectPeek(t TokenType) bool {
 	if p.peekTokenIs(t) {
 		p.nextToken()
 		return true
-	} else {
+	}
 	p.peekError(t)
 	return false
-	}
+}
+
+// Error handling methods
+func (p *Parser) addError(message string) {
+	p.errors = append(p.errors, ParseError{
+		Message: message,
+		Line:    p.curToken.Line,
+		Column:  p.curToken.Column,
+		Token:   p.curToken,
+	})
 }
 
 func (p *Parser) peekError(t TokenType) {
-	msg := fmt.Sprintf("expected next token to be %v, got %v instead",
+	message := fmt.Sprintf("expected next token to be %s, got %s instead",
-		t, p.peekToken.Type)
+		tokenTypeString(t), tokenTypeString(p.peekToken.Type))
-	p.errors = append(p.errors, msg)
+	p.errors = append(p.errors, ParseError{
+		Message: message,
+		Line:    p.peekToken.Line,
+		Column:  p.peekToken.Column,
+		Token:   p.peekToken,
+	})
 }
 
 func (p *Parser) noPrefixParseFnError(t TokenType) {
-	msg := fmt.Sprintf("no prefix parse function for %v found", t)
+	var message string
-	p.errors = append(p.errors, msg)
+	switch t {
+	case ASSIGN:
+		message = "unexpected assignment operator, missing left-hand side identifier"
+	case PLUS, MINUS, STAR, SLASH:
+		message = fmt.Sprintf("unexpected operator '%s', missing left operand", tokenTypeString(t))
+	case RPAREN:
+		message = "unexpected closing parenthesis"
+	case EOF:
+		message = "unexpected end of input"
+	default:
+		message = fmt.Sprintf("unexpected token '%s'", tokenTypeString(t))
+	}
+
+	p.addError(message)
 }
 
 func (p *Parser) peekPrecedence() Precedence {
@@ -234,6 +305,58 @@ func (p *Parser) curPrecedence() Precedence {
 }
 
 // Errors returns all parsing errors
-func (p *Parser) Errors() []string {
+func (p *Parser) Errors() []ParseError {
 	return p.errors
 }
+
+// HasErrors returns true if there are any parsing errors
+func (p *Parser) HasErrors() bool {
+	return len(p.errors) > 0
+}
+
+// ErrorStrings returns error messages as strings for backward compatibility
+func (p *Parser) ErrorStrings() []string {
+	result := make([]string, len(p.errors))
+	for i, err := range p.errors {
+		result[i] = err.Error()
+	}
+	return result
+}
+
+// tokenTypeString returns a human-readable string for token types
+func tokenTypeString(t TokenType) string {
+	switch t {
+	case IDENT:
+		return "identifier"
+	case NUMBER:
+		return "number"
+	case STRING:
+		return "string"
+	case TRUE, FALSE:
+		return "boolean"
+	case NIL:
+		return "nil"
+	case ASSIGN:
+		return "="
+	case PLUS:
+		return "+"
+	case MINUS:
+		return "-"
+	case STAR:
+		return "*"
+	case SLASH:
+		return "/"
+	case LPAREN:
+		return "("
+	case RPAREN:
+		return ")"
+	case VAR:
+		return "var"
+	case EOF:
+		return "end of file"
+	case ILLEGAL:
+		return "illegal token"
+	default:
+		return "unknown"
+	}
+}

parser/parser_test.go

@@ -1,6 +1,7 @@
 package parser
 
 import (
+	"strings"
 	"testing"
 )
 
@@ -19,19 +20,11 @@ func TestLiterals(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.input, func(t *testing.T) {
+			// Parse as expression directly - literals are not valid statements
 			l := NewLexer(tt.input)
 			p := NewParser(l)
-			program := p.ParseProgram()
-			checkParserErrors(t, p)
-
-			if len(program.Statements) != 0 {
-				t.Fatalf("expected 0 statements for literal, got %d", len(program.Statements))
-			}
-
-			// Parse as expression
-			l = NewLexer(tt.input)
-			p = NewParser(l)
 			expr := p.parseExpression(LOWEST)
+			checkParserErrors(t, p)
 
 			switch expected := tt.expected.(type) {
 			case float64:
@@ -159,10 +152,16 @@ func TestParsingErrors(t *testing.T) {
 	tests := []struct {
 		input         string
 		expectedError string
+		line          int
+		column        int
 	}{
-		{"x =", "no prefix parse function"},
+		{"= 5", "assignment operator '=' without left-hand side identifier", 1, 1},
-		{"= 5", "no prefix parse function"},
+		{"x =", "expected expression after assignment operator", 1, 3},
-		{"(1 + 2", "expected next token to be"},
+		{"(1 + 2", "expected next token to be )", 1, 7},
+		{"+ 5", "unexpected operator '+'", 1, 1},
+		{"1 +", "expected expression after operator '+'", 1, 3},
+		{"@", "unexpected token '@'", 1, 1},
+		{"invalid@", "unexpected identifier", 1, 1},
 	}
 
 	for _, tt := range tests {
@@ -170,11 +169,14 @@ func TestParsingErrors(t *testing.T) {
 			l := NewLexer(tt.input)
 			p := NewParser(l)
 
-			if tt.input == "x =" {
+			// Decide parsing strategy based on the type of error we're testing
-				p.ParseProgram()
+			switch tt.input {
-			} else {
+			case "(1 + 2", "+ 5", "1 +":
-				// Parse as expression to catch syntax errors
+				// These are expression-level errors
 				p.parseExpression(LOWEST)
+			default:
+				// These are statement-level errors
+				p.ParseProgram()
 			}
 
 			errors := p.Errors()
@@ -184,19 +186,69 @@ func TestParsingErrors(t *testing.T) {
 
 			found := false
 			for _, err := range errors {
-				if containsSubstring(err, tt.expectedError) {
+				if strings.Contains(err.Message, tt.expectedError) {
 					found = true
+					if err.Line != tt.line {
+						t.Errorf("expected error at line %d, got line %d", tt.line, err.Line)
+					}
 					break
 				}
 			}
 
 			if !found {
-				t.Errorf("expected error containing %q, got %v", tt.expectedError, errors)
+				errorMsgs := make([]string, len(errors))
+				for i, err := range errors {
+					errorMsgs[i] = err.Message
+				}
+				t.Errorf("expected error containing %q, got %v", tt.expectedError, errorMsgs)
 			}
 		})
 	}
 }
 
+func TestErrorRecovery(t *testing.T) {
+	input := `x = 42
+= 5
+y = "hello"`
+
+	l := NewLexer(input)
+	p := NewParser(l)
+	program := p.ParseProgram()
+
+	// Should have errors but still parse valid statements
+	if !p.HasErrors() {
+		t.Fatal("expected parsing errors")
+	}
+
+	errors := p.Errors()
+	found := false
+	for _, err := range errors {
+		if strings.Contains(err.Message, "assignment operator '=' without left-hand side identifier") {
+			found = true
+			if err.Line != 2 {
+				t.Errorf("expected error at line 2, got line %d", err.Line)
+			}
+			break
+		}
+	}
+
+	if !found {
+		t.Error("expected specific assignment error")
+	}
+
+	// Should still have parsed the valid statements
+	validStatements := 0
+	for _, stmt := range program.Statements {
+		if stmt != nil {
+			validStatements++
+		}
+	}
+
+	if validStatements < 2 {
+		t.Errorf("expected at least 2 valid statements, got %d", validStatements)
+	}
+}
+
 func TestProgram(t *testing.T) {
 	input := `x = 42
 y = "hello"
@@ -224,6 +276,39 @@ z = true + false`
 	}
 }
 
+func TestErrorMessages(t *testing.T) {
+	tests := []struct {
+		input           string
+		expectedMessage string
+	}{
+		{"= 5", "Parse error at line 1, column 1: assignment operator '=' without left-hand side identifier (near '=')"},
+		{"x =", "Parse error at line 1, column 3: expected expression after assignment operator (near '')"},
+		{"(", "Parse error at line 1, column 1: unexpected end of input (near '')"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			l := NewLexer(tt.input)
+			p := NewParser(l)
+			p.ParseProgram()
+
+			if !p.HasErrors() {
+				t.Fatal("expected parsing errors")
+			}
+
+			errors := p.Errors()
+			if len(errors) == 0 {
+				t.Fatal("expected at least one error")
+			}
+
+			errorStr := errors[0].Error()
+			if !strings.Contains(errorStr, "Parse error at line") {
+				t.Errorf("expected formatted error message, got: %s", errorStr)
+			}
+		})
+	}
+}
+
 // Helper functions for testing specific node types
 func testNumberLiteral(t *testing.T, expr Expression, expected float64) {
 	t.Helper()
@@ -315,17 +400,8 @@ func checkParserErrors(t *testing.T, p *Parser) {
 	}
 
 	t.Errorf("parser has %d errors", len(errors))
-	for _, msg := range errors {
+	for _, err := range errors {
-		t.Errorf("parser error: %q", msg)
+		t.Errorf("parser error: %s", err.Error())
 	}
 	t.FailNow()
 }
-
-func containsSubstring(s, substr string) bool {
-	for i := 0; i <= len(s)-len(substr); i++ {
-		if s[i:i+len(substr)] == substr {
-			return true
-		}
-	}
-	return false
-}