parse let expressions, handle syntax errors

Readme.md

@@ -1,4 +1,4 @@
-These are my source files for working on Thorsten Ball's [Writing an interpreter in go](https://interpreterbook.com/) book. The ultimate goal of this source code is to interpret, and execute code written in a language called monkey.
+These are my source files for working on Thorsten Ball's [Writing an interpreter in go](https://interpreterbook.com/) book. The ultimate goal of this source code is to interpret, and execute code written in a language called monkey. I'm still going through the book so this repo maybe incomplete.
 
 An example of monkey is below. Monkey is a c-like language with closures, first class functions, and variable bindings.
 

parser/parser.go

@@ -1,6 +1,7 @@
 package parser
 
 import (
+	"fmt"
 	"monkey/ast"
 	"monkey/lexer"
 	"monkey/token"
@@ -11,10 +12,14 @@ type Parser struct {
 
 	curToken  token.Token
 	peekToken token.Token
+	errors    []string
 }
 
 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()
@@ -86,6 +91,16 @@ func (p *Parser) expectPeek(t token.TokenType) bool {
 		p.nextToken()
 		return true
 	} else {
+		p.peekError(t)
 		return false
 	}
 }
+func (p *Parser) Errors() []string {
+	return p.errors
+}
+
+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)
+}

parser/parser_test.go

@@ -16,6 +16,8 @@ let foobar = 838383;
 	p := New(l)
 
 	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
 	if program == nil {
 		t.Fatalf("ParseProgram() returned nil")
 	}
@@ -40,6 +42,19 @@ let foobar = 838383;
 	}
 }
 
+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()
+}
+
 func testLetStatement(t *testing.T, s ast.Statement, name string) bool {
 	if s.TokenLiteral() != "let" {
 		t.Errorf("s.TokenLiteral not 'let'. got=%q", s.TokenLiteral())