3.5 Evaluation (infix expressions)

Implement support for:
* group of operators that don't produces booleans as their result
* integer operands on either side of the operator
* boolean operands for the equality operators `==` and `!=`

evaluator/evaluator.go

@@ -53,6 +53,11 @@ func Eval(node ast.Node) object.Object {
 		// this evaluation with the operator.
 		right := Eval(node.Right)
 		return evalPrefixExpression(node.Operator, right)
+
+	case *ast.InfixExpression:
+		left := Eval(node.Left)
+		right := Eval(node.Right)
+		return evalInfixExpression(node.Operator, left, right)
 	}
 
 	return nil
@@ -123,3 +128,56 @@ func evalMinusPrefixOperatorExpression(right object.Object) object.Object {
 	// Allocate a new object to wrap a negated version of this value.
 	return &object.Integer{Value: -value}
 }
+
+func evalInfixExpression(
+	operator string,
+	left, right object.Object,
+) object.Object {
+	// Monkey's object system doesn't allow pointer comparison for integer
+	// objects. It has to unwrap the value before a comparison can be made.
+	// Thus the comparison between booleans is faster.
+
+	switch {
+	case left.Type() == object.INTEGER_OBJ && right.Type() == object.INTEGER_OBJ:
+		// The check for integer operands has to be higher up in the switch
+		// statement.
+		return evalIntegerInfixExpression(operator, left, right)
+	case operator == "==":
+		// Using pointer comparison to check for equality between booleans.
+		return nativeBoolToBooleanObject(left == right)
+	case operator == "!=":
+		// Using pointer comparison to check for equality between booleans.
+		return nativeBoolToBooleanObject(left != right)
+	default:
+		return NULL
+	}
+}
+
+func evalIntegerInfixExpression(
+	operator string,
+	left, right object.Object,
+) object.Object {
+	leftVal := left.(*object.Integer).Value
+	rightVal := right.(*object.Integer).Value
+
+	switch operator {
+	case "+":
+		return &object.Integer{Value: leftVal + rightVal}
+	case "-":
+		return &object.Integer{Value: leftVal - rightVal}
+	case "*":
+		return &object.Integer{Value: leftVal * rightVal}
+	case "/":
+		return &object.Integer{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 NULL
+	}
+}

evaluator/evaluator_test.go

@@ -14,6 +14,7 @@ func TestEvalIntegerExpression(t *testing.T) {
 	// position. Second, because this test function should grow to encompass
 	// all integer arithmetic in order to have one place that shows the desired
 	// behaviour in a clear and neat way.
+	// This test is also extended for the infix expressions.
 
 	tests := []struct {
 		input    string
@@ -23,6 +24,17 @@ func TestEvalIntegerExpression(t *testing.T) {
 		{"10", 10},
 		{"-5", -5},
 		{"-10", -10},
+		{"5 + 5 + 5 + 5 - 10", 10},
+		{"2 * 2 * 2 * 2 * 2", 32},
+		{"-50 + 100 + -50", 0},
+		{"5 * 2 + 10", 20},
+		{"5 + 2 * 10", 25},
+		{"20 + 2 * -10", 0},
+		{"50 / 2 * 2 + 10", 60},
+		{"2 * (5 + 10)", 30},
+		{"3 * 3 * 3 + 10", 37},
+		{"3 * (3 * 3) + 10", 37},
+		{"(5 + 10 * 2 + 15 / 3) * 2 + -10", 50},
 	}
 
 	for _, tt := range tests {
@@ -38,6 +50,23 @@ func TestEvalBooleanExpression(t *testing.T) {
 	}{
 		{"true", true},
 		{"false", false},
+		{"1 < 2", true},
+		{"1 > 2", false},
+		{"1 < 1", false},
+		{"1 > 1", false},
+		{"1 == 1", true},
+		{"1 != 1", false},
+		{"1 == 2", false},
+		{"1 != 2", true},
+		{"true == true", true},
+		{"false == false", true},
+		{"true == false", false},
+		{"true != false", true},
+		{"false != true", true},
+		{"(1 < 2) == true", true},
+		{"(1 < 2) == false", false},
+		{"(1 > 2) == true", false},
+		{"(1 > 2) == false", true},
 	}
 
 	for _, tt := range tests {