add string comparison

evaluator/evaluator.go

@@ -242,14 +242,23 @@ func evalInfixExpression(operator string, left, right object.Object) object.Obje
 }
 
 func evalStringInfixExpression(operator string, left object.Object, right object.Object) object.Object {
-	if operator != "+" {
-		return newError("unknown operator: %s %s %s", left.Type(), operator, right.Type())
-	}
-
 	leftVal := left.(*object.String).Value
 	rightVal := right.(*object.String).Value
 
-	return &object.String{Value: leftVal + rightVal}
+	switch operator {
+	case "+":
+		return &object.String{Value: leftVal + rightVal}
+	case "<":
+		return nativeBoolToBooleanObject(leftVal < rightVal)
+	case ">":
+		return nativeBoolToBooleanObject(leftVal > rightVal)
+	case "==":
+		return nativeBoolToBooleanObject(leftVal == rightVal)
+	case "!=":
+		return nativeBoolToBooleanObject(leftVal != rightVal)
+	default:
+		return newError("unknown operator: %s %s %s", left.Type(), operator, right.Type())
+	}
 }
 
 func evalIntegerInfixExpression(operator string, left, right object.Object) object.Object {

evaluator/evaluator_test.go

@@ -62,6 +62,9 @@ func TestEvalBooleanExpression(t *testing.T) {
 		{"(1 < 2) == false", false},
 		{"(1 > 2) == true", false},
 		{"(1 > 2) == false", true},
+		{`"a" == "a"`, true},
+		{`"a" < "b"`, true},
+		{`"abc" == "abc"`, true},
 	}
 
 	for _, tt := range tests {