1.4: Add '==' and '!='

lexer/lexer.go

@@ -41,6 +41,14 @@ func (l *Lexer) readChar() {
 	l.readPosition += 1
 }
 
+func (l *Lexer) peekChar() byte {
+	if l.readPosition >= len(l.input) {
+		return 0 // NULL
+	} else {
+		return l.input[l.readPosition]
+	}
+}
+
 func (l *Lexer) readIdentifier() string {
 	position := l.position
 	for isLetter(l.ch) {
@@ -64,13 +72,27 @@ func (l *Lexer) NextToken() token.Token {
 
 	switch l.ch {
 	case '=':
-		tok = newToken(token.ASSIGN, l.ch)
+		if l.peekChar() == '=' {
+			ch := l.ch
+			l.readChar()
+			literal := string(ch) + string(l.ch)
+			tok = token.Token{Type: token.EQ, Literal: literal}
+		} else {
+			tok = newToken(token.ASSIGN, l.ch)
+		}
 	case '+':
 		tok = newToken(token.PLUS, l.ch)
 	case '-':
 		tok = newToken(token.MINUS, l.ch)
 	case '!':
-		tok = newToken(token.BANG, l.ch)
+		if l.peekChar() == '=' {
+			ch := l.ch
+			l.readChar()
+			literal := string(ch) + string(l.ch)
+			tok = token.Token{Type: token.NOT_EQ, Literal: literal}
+		} else {
+			tok = newToken(token.BANG, l.ch)
+		}
 	case '*':
 		tok = newToken(token.ASTERISK, l.ch)
 	case '/':

lexer/lexer_test.go

@@ -21,6 +21,9 @@ if (5 < 10) {
 } else {
 	return false;
 }
+
+10 == 10;
+10 != 9;
 `
 
 	tests := []struct {
@@ -92,6 +95,14 @@ if (5 < 10) {
 		{token.FALSE, "false"},
 		{token.SEMICOLON, ";"},
 		{token.RBRACE, "}"},
+		{token.INT, "10"},
+		{token.EQ, "=="},
+		{token.INT, "10"},
+		{token.SEMICOLON, ";"},
+		{token.INT, "10"},
+		{token.NOT_EQ, "!="},
+		{token.INT, "9"},
+		{token.SEMICOLON, ";"},
 		{token.EOF, ""},
 	}
 

token/token.go

@@ -19,6 +19,9 @@ const (
 	LT = "<"
 	GT = ">"
 
+	EQ     = "=="
+	NOT_EQ = "!="
+
 	// Delimiters
 	COMMA     = ","
 	SEMICOLON = ";"