Eval Prefix, Boolean, and Infix Expressions

evaluator/evaluator.go

@@ -5,6 +5,12 @@ import (
 	"monkey/object"
 )
 
+var (
+	NULL  = &object.Null{}
+	TRUE  = &object.Boolean{Value: true}
+	FALSE = &object.Boolean{Value: false}
+)
+
 func Eval(node ast.Node) object.Object {
 	switch node := node.(type) {
 
@@ -18,6 +24,15 @@ func Eval(node ast.Node) object.Object {
 	// Expressions
 	case *ast.IntegerLiteral:
 		return &object.Integer{Value: node.Value}
+	case *ast.Boolean:
+		return nativeBoolToBooleanObject(node.Value)
+	case *ast.PrefixExpression:
+		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
@@ -32,3 +47,82 @@ func evalStatements(stmts []ast.Statement) object.Object {
 
 	return result
 }
+
+func nativeBoolToBooleanObject(input bool) object.Object {
+	if input {
+		return TRUE
+	}
+	return FALSE
+}
+
+func evalPrefixExpression(operator string, right object.Object) object.Object {
+	switch operator {
+	case "!":
+		return evalBangOperatorExpression(right)
+	case "-":
+		return evalMinusPrefixOperatorExpression(right)
+	default:
+		return NULL
+	}
+}
+
+func evalBangOperatorExpression(right object.Object) object.Object {
+	switch right {
+	case TRUE:
+		return FALSE
+	case FALSE:
+		return TRUE
+	case NULL:
+		return TRUE
+	default:
+		return FALSE
+	}
+}
+
+func evalMinusPrefixOperatorExpression(right object.Object) object.Object {
+	if right.Type() != object.INTEGER_OBJ {
+		return NULL
+	}
+
+	value := right.(*object.Integer).Value
+	return &object.Integer{Value: -value}
+}
+
+func evalInfixExpression(operator string, left, right object.Object) object.Object {
+	switch {
+	case left.Type() == object.INTEGER_OBJ && right.Type() == object.INTEGER_OBJ:
+		return evalIntegerInfixExpression(operator, left, right)
+	case operator == "==":
+		return nativeBoolToBooleanObject(left == right)
+	case operator == "!=":
+		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
+	}
+}