Method Combinators in CoffeeScript

https://github.com/raganwald/homoiconic/blob/master/2012/08/method-decorators-and-combinators-in-coffeescript.md

by grifaton • 14 years ago

64 25 14 years ago

asolove3• 14 years ago

While the syntax here is CoffeeScript-specific, this is a perfectly fine pattern to apply in any JavaScript.

The key is to remember that, in JavaScript, you aren't really "defining instance methods" in the way you are in almost any other language. You're just passing anonymous functions as the value tied to some key in a dictionary, which happens to be the prototype of some set of objects. Once you get that into your mind and let it play around for a bit, you realize that you could be getting the anonymous function from anywhere!

From a function-factory, that takes some arguments and returns a function:

    render: renderWithTemplate("some_template")

From an anonymous function wrapped in something that controls its execution:

    onScroll: _.debounce(100, function(){ /* do something */ })

Or even by delegating to another object:

    viewInstance.onScroll = _.bind(somethingElse.render, somethingElse);

Thanks for the great post to help bend our minds around these possibilities.

Cushman2• 14 years ago

> Or even by delegating to another object:

> viewInstance.onScroll = somethingElse.render;

This one is a bit dangerous, because as you mention the "methods" are just anonymous functions assigned to objects. If `render` is expecting to be called on a `somethingElse`, this might not work right without being bound.

Xcelerate2• 14 years ago

That's why I never use "this" in Javascript. For me, dropping any pretense of OOP allows me to write more elegant programs.

asolove3• 14 years ago

Yeah, I apologize. I have changed it to read correctly.

raganwald4• 14 years ago

Jeremy Ashkenas provides the "tl;dr:"

Python decorators are a hack around the lack of proper lambda ;) Just pass the function: decorate -> … Where "decorate" is a fn

https://twitter.com/jashkenas/status/235012485009248256

jerf• 14 years ago

That makes no sense at all. The lambda complaint is mostly a syntax complaint, it is not a complaint about not having anonymous functions. It even has the ability to invoke __call__ on instances, allowing you to create your own "functions" that are actually objects, if you like, which is somewhat unusual.

There's no point in having an anonymous function decorator. You'd simply inline it into the body itself right on the spot, anything else would simply be obfuscation for the sake of obfuscation. This only makes sense if you're using a decorator applied elsewhere, at which point all languages require you to have named it, so there's no point complaining about lack of anonymous functions here.

Python has first-class functions. It even has first-class methods, with automatic instance binding. It just doesn't quite work how very functionally-oriented people want it to work. Part of the reason I don't like the kvetching that such people do is that it does seem to convince people that Python is lacking function references. Nope. It just doesn't spell them to everybody's taste (for instance, "it doesn't have Ruby blocks" translates to "it doesn't have block syntax", not "it can't do function references", and this is a taste issue not a fundamental capability issue), and the can rarely have moderately inconvenient scope rules if you want to write to outer scopes.

Decorators are syntax sugar, not a new feature.

asolove3• 14 years ago

Just FYI, he's not talking about the decorator being anonymous, he's talking about the function the decorator is wrapping being anonymous. What he means is that, since python lambda's are one-line, you can't do something like:

    ifAuthorized("admin", lambda x:
      // more code here

raganwald4• 14 years ago

I don't know about those other complaints, I don't personally have any complaints about Python. It doesn't have multi-line lambdas, that's a design trade-off like an iPhone not having a slide-out keyboard. But it does have first-class functions.

https://en.wikipedia.org/wiki/Python_syntax_and_semantics#Fi...

What I was speaking to is the desire to write:

  class SomeExampleModel:
  
    def setHeavyweightProperty:
      triggers('cache:dirty')(
        lambda self, property, value:
          ...something...

My understanding is that Python doesn't like two different things about this. First, the multi-line anonymous lambda being used as the target of the decorator. Second, a function being called with another function as its argument as an expression within an instance method definition.

I'm open to reëducation.

goblin892• 14 years ago

A little nitpick, which may be a misconception deserving clarification:

> Our decorators work just like Python method decorators, only we don't need any magic syntax for them because CoffeeScript, like JavaScript, already has this idea that functions can return functions and there's nothing particularly magic about defining a method, it's just an expression that evaluates to a function.

Python also has this idea (called ‘higher-order functions’). The difference is in syntax—function calls require parens in Python, and anonymous functions aren't that well supported. Therefore the need for special syntax construct to make decorator use convenient.

raganwald4• 14 years ago

You're speaking to the first part of my claim, but not the second, namely that JavaScript and CoffeeScript don't separate the idea of a function and a method, which is what allows you to use first-class functional combinators as decorators.

masklinn4• 14 years ago

> JavaScript and CoffeeScript don't separate the idea of a function and a method

Neither does Python, at definition time. All the difference is in the processing performed by the class constructor (`type`) when the class object is created. Before that, it's a bog-standard function.

> which is what allows you to use first-class functional combinators as decorators.

Which is exactly like Python, the original decorators[0][1] predate the syntactic extension by several versions, the original syntax for applying them was:

    def some_method(cls):
        pass
    some_method = classmethod(some_method)

And you could use the exact same syntax to define non-method functions (though not this decorator, of course, as it doesn't make sense for functions)

[0] http://docs.python.org/library/functions.html?highlight=clas...

[1] http://docs.python.org/library/functions.html?highlight=clas...

goblin892• 14 years ago

I only intended to comment on the first part. Decided against cutting the quote mid-sentence. (Which led to me learning something new from masklinn's comment. =))

quarterto• 14 years ago

In LiveScript: https://github.com/quarterto/homoiconic/blob/d6b78812c00eb4d...

ludicast• 14 years ago

Interesting, do like the "implicit currying". And the decorator motivations read better when they are expressed with the <| rather than when they are just "escaped line breaks".

Method Combinators in CoffeeScript

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