Overloading Dart Operators for Great Evil

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One of the cool things about working through a brand new language like Dart is happening across weird little nooks and crannies of the language specification. One such example is the call operator which seems to be a way to get objects to behave like functions. Or I could be misreading the spec, I have a hard time with that kind of thing.

My best guess is that I can define an operator named call() to set this up, so I will try this out with my Hipster MVC framework. For now, I will make it an alias for save() on the HipsterModel class.

I have already defined other operator methods in Dart. In the HipsterModel, for instance, I have the square brackets operator that allows developers to lookup attributes in models as if the model were a Map (e.g. comic_book['title']). This is defined using the operator keyword:

class HipsterModel implements Hashable {
  // ...
  operator [](attr) => attributes[attr];
}

So I try doing something similar with call():

class HipsterModel implements Hashable {
  // ...
  operator call([callback]) {
    save(callback: callback);
  }
}

But, when I try to load the application that is build with Hipster MVC, I get a compile time error:

Internal error: 'http://localhost:3000/scripts/HipsterModel.dart': Error: line 23 pos 12: invalid operator overloading
  operator call([callback]) {
           ^

After fiddling a bit, I concede that perhaps I mis-read the spec. Perhaps call is just meant to represent the parens operator:

class HipsterModel implements Hashable {
  // ...
  operator ([callback]) {
    save(callback: callback);
  }
}

With that, I no longer receive a compile time warning. When I try to create a new model with something like:

new_model(callback:(event) {
  print("added a new model!");
});

I am greeted with a run time error:

2FIXME:1Exception: Object is not closure
Stack Trace:  0. Function: 'HipsterCollection.create' url: 'http://localhost:3000/scripts/HipsterCollection.dart' line:48 col:14
 1. Function: 'AddComicForm._submit_create_form@3b72b637' url: 'http://localhost:3000/scripts/Views.AddComicForm.dart' line:89 col:22
 2. Function: 'AddComicForm.function' url: 'http://localhost:3000/scripts/Views.AddComicForm.dart' line:24 col:26
 3. Function: 'HipsterView.function' url: 'http://localhost:3000/scripts/HipsterView.dart' line:41 col:15
 4. Function: 'EventListenerListImplementation.function' url: 'dart:htmlimpl' line:23128 col:35

Despite receiving my very favorite exception warning ("Object is not closure" seems to be little more than "something went wrong"), this is progress.

Maybe now is when I need call():

new_model.call(callback:(event) {
  print("added a new model!");
});

This changes the error, but I am still not quite right:

Exception: NoSuchMethodException : method not found: 'call'
Receiver: Instance of 'ComicBook'
Arguments: [Closure]
Stack Trace:  0. Function: 'Object.noSuchMethod' url: 'bootstrap' line:669 col:3
 1. Function: 'HipsterCollection.create' url: 'http://localhost:3000/scripts/HipsterCollection.dart' line:48 col:19
 2. Function: 'AddComicForm._submit_create_form@3b72b637' url: 'http://localhost:3000/scripts/Views.AddComicForm.dart' line:89 col:22
 3. Function: 'AddComicForm.function' url: 'http://localhost:3000/scripts/Views.AddComicForm.dart' line:24 col:26
 4. Function: 'HipsterView.function' url: 'http://localhost:3000/scripts/HipsterView.dart' line:41 col:15
 5. Function: 'EventListenerListImplementation.function' url: 'dart:htmlimpl' line:23128 col:35

I try:

new_model.(callback:(event) {
  print("added a new model!");
});

But get:

Internal error: 'http://localhost:3000/scripts/HipsterCollection.dart': Error: line 48 pos 15: identifier expected
    new_model.(callback:(event) {
              ^

Sigh. Re-reading the spec again, it really seems as though it ought to be call(). Perhaps it is not yet supported? I try a dummy operator to see if the message is the same, and indeed, "invalid operator overloading" seems to mean that the token is not an operator:

Internal error: 'http://localhost:3000/scripts/HipsterModel.dart': Error: line 23 pos 12: invalid operator overloading
  operator asdf([callback]) {
           ^

I try the latest version of Dartium only to find the same thing. Grr...

It is at this point that I finally stumble across a description of how call() will work in Dart. At the very top, it states that this is brand new and not implemented yet. Darn it. On the bright-side, it does seem that my understanding of how it will work is correct.

Regardless, I refuse to give up until I have done something tonight, no matter how ridiculous. So I overload the pipe operator for HispterModel so that it saves itself and pipes the response into the supplied callback:

class HipsterModel implements Hashable {
  // ...
  operator |(callback) => save(callback: callback);
}

I can then invoke this save/pipe like so:

new_model | (event) {
  print("added a new model!");
};

And that works.

It seems only fitting to end this post thusly: Mwah haha!


Day #310