Tech Per

AS3 Inconsistency - Unable To Use …(rest) Keyword in Setter

public class Task {
    private var _tags : ArrayCollection = new ArrayCollection();

    ...

    public function set tags(...tags) : void {
        for each (var tag : Object in tags) {
            if (tag is String) {
                CollectionUtil.addAll(_tags, createTagsFromString(tag as String));
            } else if (tag is Array) {
                CollectionUtil.addAll(_tags, tag as Array);
            } else {
                throw new ArgumentError("value must be either strings or array");
            }
        }

    }

    private static function createTagsFromString(tags : String) : Array {
       ...
    }
}

var myTags : String = "dude, car";
var task : Task = new Task();
task.tags = myTags;

Implicit coercion of a value of type String to an unrelated type Array.        task.tags = tags;

public class Task {
    ...

    public function setTags(...newtags) : void {
        this.tags = newtags;
    }

    ...
}

var myTags : String = "dude, car";
var task : Task = new Task();
task.setTags(myTags);

9 Responses to “AS3 Inconsistency - Unable To Use …(rest) Keyword in Setter”

1. 1
   Alex Says:
   

   when using …rest on a method call you wouldn’t put “” around the parameters. In the examples above I see “”, which suggest just one argument. Would you expected a syntax like this to work?

   task.tags = “foo”, 1, ["foo","bar"];

   July 22nd, 2008 at 23:54
2. 2
   Ash Says:
   

   I’m not sure what you’re trying to achieve here.. A setter takes one argument by definition, rather than a list of arguments. How would you intend to pass a list of arguments to your setter?
   myProperty = 1, 2, 3?

   In that case you would just pass an array:
   myProperty = [1, 2, 3];

   and the argument for your setter would be of type Array.

   Otherwise you would just use a regular method, as in your last example

   July 23rd, 2008 at 00:38
3. 3
   senocular Says:
   

   This is as designed. You can’t use = to assign more than one value to any given target. Setters are defined in a way that let you create variables that convert = into a method call. So what may have been:

   target.setTags(value);

   can instead be

   target.tags = value;

   However, where you can pass any number of arguments to a function call, you can only use one value in an assignment. There’s no way to do something like

   target.tags = value, value1, valueN;

   The closest thing you have to that is assigning an array at which point you’re then back to dealing with one object (where the setter would be defined with one parameter of the type Array).

   July 23rd, 2008 at 00:47
4. 4
   Alex Bustin Says:
   

   Can’t you just do this… ?????

   public function set tags(value:String) : void {
   var tags:Array = value.split(”,”);
   …….
   }

   …(rest) is of the Array type, so ActionScript is right to assume that you want to set an Array.

   Getters and setters have to be matching in type. If you’re passing in a String to the setter, the getter would return a String. I know you haven’t implemented a matching getter and thats OK, but if you had, it would of had to of returned an Array to match the setter.

   Your second block of code works because you’re using
   …(rest) correctly. But your implementation is buggy, your myTags string will never be automatically split at the comma in ActionScript. So in the setTags method, your newtags Array will always contain only one String element. Passing the newtags Array to this.tags works because this.tags is expecting an Array. Before you were trying to pass a String.

   To use setTags correctly, you would say …

   setTags(”foo”, “bar”, “bob”);

   July 23rd, 2008 at 01:22
5. 5
   zwetan Says:
   

   nope it’s not an AS3 inconsistency and it’s certainly not a bug

   you try to do something that make no sens

   the rest parametter allow you to pass multi arguments to a function

   function foobar( …args )

   foobar( 0, 1, 2, 3 ); // args = [0,1,2,3]

   put this make no sens for a setter

   function set foobar( …args )

   obj.foobar = “you can pass only one argument here”;

   wen you try to do that
   —
   var myTags : String = “dude, car”;
   var task : Task = new Task();
   task.tags = myTags; //here your arg is not magically put into an array
   —

   so when you try to do that
   for each (var tag : Object in tags)

   you trying to do that on a String, not an Array
   hence the implicit coercion error

   I guess what you want is

   public function set tags(tags:Array) : void

   and

   var myTags : String = “dude,car”;
   var task : Task = new Task();
   task.tags = myTags.split(”,”);

   or

   var myTags : Array = ["dude","car"];
   var task : Task = new Task();
   task.tags = myTags;

   July 23rd, 2008 at 01:29
6. 6
   Jake Says:
   

   I think the inconsistency lies in the fact that the compiler allows using ‘…’ in a set method. varargs don’t make much sense in the context of properties (a property is singular by nature).

   July 23rd, 2008 at 04:06
7. 7
   polesen Says:
   

   Thanks for all the comments.

   @Alex: The “dude, car” string is not a try from me, to pass to strings into the setter. This is just the example here. And no, I would not expect task.tags = “foo”, 1, ["foo","bar"] to work. I would expect it to convert my single string var into a one element array.

   @Ash: At times, I am not sure myself, what I want to achive

   @Senocular:

   However, where you can pass any number of arguments to a function call, you can only use one value in an assignment. There’s no way to do something like

   target.tags = value, value1, valueN;

   If there is no way to pass any number of arguments to a setter, why does the setter with …rest then compile?

   I would not expect target.tags = value, value1, valueN; to work, but, given that the setter compiles, I would at least expect target.tags = value; to work.

   @Jake: EXACTLY!

   July 23rd, 2008 at 08:15
8. 8
   Darren Says:
   

   I think the Flex compiler is right here. It’s not necessary for the Flex compiler to put in a special check to see that “…rest” is not used as a parameter in a setter because the default behaviour is just to treat it as a normal array - without the normal “…rest” qualities because of the unique nature of setter functions - and catch your error if you try to present it with something else. You seem to be complaining because in this particular case, the compiler won’t do an automatic type conversion for you but it shouldn’t because setters don’t do this - just as it shouldn’t if you had written “var arr:Array = “some_string”. Is it really that surprising that a setter function handles a special type of parameter - which you shouldn’t have given it in the first place because it makes no sense in this context - in a way that you didn’t expect? For your use case, the setTags method is the appropriate solution, not a setter function. In an ideal world, the compiler would issue a warning when you use the “…rest” parameter in a setter but I’m personally comfortable that it’s not slowed down looking for particular use cases like this when an error will be thrown if you use it incorrectly anyway.

   July 24th, 2008 at 00:45
9. 9
   polesen Says:
   

   @Darren: I seems you are right. I just had an understanding of the set/get properties as just being syntactic sugar, that made the compiler generate actual methods for set and getters, and then convert calls like

   
task.tags = foo


   into

   
task.setter_tags(foo)


   But hey, I guess they are more special than that then.

   July 24th, 2008 at 08:20

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