2.4 Parser (error handling)

Author: cedrickchee

parser/parser.go

@@ -4,6 +4,8 @@ package parser
 // lexer and produces as output an AST (Abstract Syntax Tree).
 
 import (
+	"fmt"
+
 	"github.com/cedrickchee/hou/ast"
 	"github.com/cedrickchee/hou/lexer"
 	"github.com/cedrickchee/hou/token"
@@ -13,13 +15,18 @@ import (
 type Parser struct {
 	l *lexer.Lexer
 
+	errors []string
+
 	curToken  token.Token
 	peekToken token.Token
 }
 
 // New constructs a new Parser with a Lexer as input.
 func New(l *lexer.Lexer) *Parser {
-	p := &Parser{l: l}
+	p := &Parser{
+		l:      l,
+		errors: []string{},
+	}
 
 	// Read two tokens, so curToken and peekToken are both set.
 	p.nextToken()
@@ -28,6 +35,19 @@ func New(l *lexer.Lexer) *Parser {
 	return p
 }
 
+// Errors check if the parser encountered any errors.
+func (p *Parser) Errors() []string {
+	return p.errors
+}
+
+// Add an error to errors when the type of peekToken doesn’t match the
+// expectation.
+func (p *Parser) peekError(t token.TokenType) {
+	msg := fmt.Sprintf("expected next token to be %s, got %s instead",
+		t, p.peekToken.Type)
+	p.errors = append(p.errors, msg)
+}
+
 // Helper method that advances both curToken and peekToken.
 func (p *Parser) nextToken() {
 	p.curToken = p.peekToken
@@ -98,9 +118,9 @@ func (p *Parser) expectPeek(t token.TokenType) bool {
 	if p.peekTokenIs(t) {
 		p.nextToken()
 		return true
-	} else {
-		return false
 	}
+	p.peekError(t)
+	return false
 }
 
 func (p *Parser) peekTokenIs(t token.TokenType) bool {

parser/parser_test.go

@@ -17,6 +17,8 @@ let foobar = 838383;
 	p := New(l)
 
 	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
 	if program == nil {
 		t.Fatalf("ParseProgram() returned nil")
 	}
@@ -65,3 +67,18 @@ func testLetStatement(t *testing.T, s ast.Statement, name string) bool {
 
 	return true
 }
+
+// Check the parser for errors and if it has any it prints them as test errors
+// and stops the execution of the current test.
+func checkParserErrors(t *testing.T, p *Parser) {
+	errors := p.Errors()
+	if len(errors) == 0 {
+		return
+	}
+
+	t.Errorf("parser has %d errors", len(errors))
+	for _, msg := range errors {
+		t.Errorf("parser error: %q", msg)
+	}
+	t.FailNow()
+}