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
 
+	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 {

vm/vm.go

@@ -455,6 +455,10 @@ func (vm *VM) executeComparison(op code.Opcode) error {
 		return vm.executeIntegerComparison(op, left, right)
 	}
 
+	if left.Type() == object.STRING_OBJ && right.Type() == object.STRING_OBJ {
+		return vm.executeStringComparison(op, left, right)
+	}
+
 	switch op {
 	case code.OpEqual:
 		return vm.push(nativeBoolToBooleanObject(right == left))
@@ -581,6 +585,22 @@ func (vm *VM) executeStringIndex(str, index object.Object) error {
 	return vm.push(&object.String{Value: string(stringObject.Value[i])})
 }
 
+func (vm *VM) executeStringComparison(op code.Opcode, left, right object.Object) error {
+	leftValue := left.(*object.String).Value
+	rightValue := right.(*object.String).Value
+
+	switch op {
+	case code.OpEqual:
+		return vm.push(nativeBoolToBooleanObject(rightValue == leftValue))
+	case code.OpNotEqual:
+		return vm.push(nativeBoolToBooleanObject(rightValue != leftValue))
+	case code.OpGreaterThan:
+		return vm.push(nativeBoolToBooleanObject(leftValue > rightValue))
+	default:
+		return fmt.Errorf("unknown operator: %d", op)
+	}
+}
+
 func nativeBoolToBooleanObject(input bool) *object.Boolean {
 	if input {
 		return True

vm/vm_test.go

@@ -229,6 +229,9 @@ func TestBooleanExpressions(t *testing.T) {
 		{"!!false", false},
 		{"!!5", true},
 		{"!(if (false) { 5; })", true},
+		{`"a" == "a"`, true},
+		{`"a" < "b"`, true},
+		{`"abc" == "abc"`, true},
 	}
 
 	runVmTests(t, tests)