Add string literal support to the lexer.

examples/print.g

@@ -0,0 +1,16 @@
+@data msg "hello world"
+
+.run
+SETX msg 	; set X register to the address of msg
+JUMP print
+
+.done
+HALT
+
+.print
+MOVX A		; move the value of address in X to A
+JAEZ done	; jump to label 'done' if A = 0
+OUTA		; print A
+INCX		; increment address stored in X
+JUMP print	; jump back to the start of .print to print the next character
+

lexer/lexer.go

@@ -40,6 +40,9 @@ func (l *Lexer) NextToken() token.Token {
 		case l.ch == '\'':
 			char := l.readCharacter()
 			return l.newToken(token.CHAR, char)
+		case l.ch == '"':
+			str := l.readString()
+			return l.newToken(token.STRING, str)
 		case l.ch == 0:
 			return l.newToken(token.EOF, "")
 		case unicode.IsDigit(l.ch):
@@ -75,6 +78,19 @@ func (l *Lexer) readUntil(r rune) string {
 	return l.input[start:l.position]
 }
 
+func (l *Lexer) readString() string {
+	position := l.position + 1
+	for {
+		l.readChar()
+		if l.ch == '"' || l.ch == 0 {
+			break
+		}
+	}
+	// consume closing quote
+	l.readChar()
+	return l.input[position : l.position-1]
+}
+
 func (l *Lexer) readCharacter() string {
 	start := l.position
 	l.readChar()

lexer/lexer_test.go

@@ -43,8 +43,11 @@ DECA
 PSHA
 POPA
 MOVA X
+MOVA Y
 OUTA
-HALT`
+HALT
+"test"
+""`
 	tests := []struct {
 		expectedType    token.TokenType
 		expectedLiteral string
@@ -69,9 +72,13 @@ HALT`
 		{token.OPCODE, "POPA", 20},
 		{token.OPCODE, "MOVA", 21},
 		{token.REGISTER, "X", 21},
-		{token.OPCODE, "OUTA", 22},
-		{token.OPCODE, "HALT", 23},
-		{token.EOF, "", 23},
+		{token.OPCODE, "MOVA", 22},
+		{token.REGISTER, "Y", 22},
+		{token.OPCODE, "OUTA", 23},
+		{token.OPCODE, "HALT", 24},
+		{token.STRING, "test", 25},
+		{token.STRING, "", 26},
+		{token.EOF, "", 26},
 	}
 
 	l := newLexerFromString(input)

token/token.go

@@ -11,6 +11,7 @@ const (
 	IDENT               = "IDENT"
 	INT                 = "INT"
 	CHAR                = "CHAR"
+	STRING              = "STRING"
 )
 
 var registers = map[string]TokenType{