add additional keywords and evaluators to lexer

lexer/lexer.go

@@ -25,6 +25,14 @@ func (l *Lexer) readChar() {
 	l.readPosition += 1
 }
 
+func (l *Lexer) peekChar() byte {
+	if l.readPosition >= len(l.input) {
+		return 0
+	} else {
+		return l.input[l.readPosition]
+	}
+}
+
 func (l *Lexer) NextToken() token.Token {
 	var tok token.Token
 
@@ -32,17 +40,50 @@ 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 '!':
+		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.SLASH, l.ch)
+	case '*':
+		tok = newToken(token.ASTERISK, l.ch)
+	case '<':
+		tok = newToken(token.LT, l.ch)
+	case '>':
+		tok = newToken(token.GT, l.ch)
 	case ';':
 		tok = newToken(token.SEMICOLON, l.ch)
+	case ',':
+		tok = newToken(token.COMMA, l.ch)
 	case '(':
 		tok = newToken(token.LPAREN, l.ch)
 	case ')':
 		tok = newToken(token.RPAREN, l.ch)
-	case ',':
-		tok = newToken(token.COMMA, l.ch)
-	case '+':
-		tok = newToken(token.PLUS, l.ch)
 	case '{':
 		tok = newToken(token.LBRACE, l.ch)
 	case '}':