Errors

evaluator/evaluator.go

@@ -1,6 +1,7 @@
 package evaluator
 
 import (
+	"fmt"
 	"monkey/ast"
 	"monkey/object"
 )
@@ -11,6 +12,13 @@ var (
 	FALSE = &object.Boolean{Value: false}
 )
 
+func isError(obj object.Object) bool {
+	if obj != nil {
+		return obj.Type() == object.ERROR_OBJ
+	}
+	return false
+}
+
 func Eval(node ast.Node) object.Object {
 	switch node := node.(type) {
 
@@ -29,6 +37,9 @@ func Eval(node ast.Node) object.Object {
 
 	case *ast.ReturnStatement:
 		val := Eval(node.ReturnValue)
+		if isError(val) {
+			return val
+		}
 		return &object.ReturnValue{Value: val}
 
 	// Expressions
@@ -40,11 +51,20 @@ func Eval(node ast.Node) object.Object {
 
 	case *ast.PrefixExpression:
 		right := Eval(node.Right)
+		if isError(right) {
+			return right
+		}
 		return evalPrefixExpression(node.Operator, right)
 
 	case *ast.InfixExpression:
 		left := Eval(node.Left)
+		if isError(left) {
+			return left
+		}
 		right := Eval(node.Right)
+		if isError(right) {
+			return right
+		}
 		return evalInfixExpression(node.Operator, left, right)
 
 	}
@@ -58,9 +78,13 @@ func evalProgram(program *ast.Program) object.Object {
 	for _, statement := range program.Statements {
 		result = Eval(statement)
 
-		if returnValue, ok := result.(*object.ReturnValue); ok {
-			return returnValue.Value
+		switch result := result.(type) {
+		case *object.ReturnValue:
+			return result.Value
+		case *object.Error:
+			return result
 		}
+
 	}
 
 	return result
@@ -72,8 +96,11 @@ func evalBlockStatements(block *ast.BlockStatement) object.Object {
 	for _, statement := range block.Statements {
 		result = Eval(statement)
 
-		if result != nil && result.Type() == object.RETURN_VALUE_OBJ {
-			return result
+		if result != nil {
+			rt := result.Type()
+			if rt == object.RETURN_VALUE_OBJ || rt == object.ERROR_OBJ {
+				return result
+			}
 		}
 	}
 
@@ -94,7 +121,7 @@ func evalPrefixExpression(operator string, right object.Object) object.Object {
 	case "-":
 		return evalMinusPrefixOperatorExpression(right)
 	default:
-		return NULL
+		return newError("unknown operator: %s%s", operator, right.Type())
 	}
 }
 
@@ -113,7 +140,7 @@ func evalBangOperatorExpression(right object.Object) object.Object {
 
 func evalMinusPrefixOperatorExpression(right object.Object) object.Object {
 	if right.Type() != object.INTEGER_OBJ {
-		return NULL
+		return newError("unknown operator: -%s", right.Type())
 	}
 
 	value := right.(*object.Integer).Value
@@ -128,8 +155,10 @@ func evalInfixExpression(operator string, left, right object.Object) object.Obje
 		return nativeBoolToBooleanObject(left == right)
 	case operator == "!=":
 		return nativeBoolToBooleanObject(left != right)
+	case left.Type() != right.Type():
+		return newError("type mismatch: %s %s %s", left.Type(), operator, right.Type())
 	default:
-		return NULL
+		return newError("unknown operator: %s %s %s", left.Type(), operator, right.Type())
 	}
 }
 
@@ -155,12 +184,15 @@ func evalIntegerInfixExpression(operator string, left, right object.Object) obje
 	case "!=":
 		return nativeBoolToBooleanObject(leftVal != rightVal)
 	default:
-		return NULL
+		return newError("unknown operator: %s %s %s", left.Type(), operator, right.Type())
 	}
 }
 
 func evalIfExpression(ie *ast.IfExpression) object.Object {
 	condition := Eval(ie.Condition)
+	if isError(condition) {
+		return condition
+	}
 
 	if isTruthy(condition) {
 		return Eval(ie.Consequence)
@@ -183,3 +215,7 @@ func isTruthy(obj object.Object) bool {
 		return true
 	}
 }
+
+func newError(format string, a ...interface{}) *object.Error {
+	return &object.Error{Message: fmt.Sprintf(format, a...)}
+}