Argument Enhancements (Defaults and Splats)

[jashkenas]

Think about adding argument defaults to CoffeeScript function definitions. Something along the lines of:

start_reading: book, page: 1 => ...

Ideally, the arguments with default values could occur anywhere in the list.

Also, splats to siphon up the rest of the arguments:

demolition: *condominiums => wreck(condo) for condo in condominiums..

Neither of these should be too hard to implement -- both can be accomplished by inserting an expression that does the work at the top of the Code body.

[weepy]

Like it alot.

Could also have ruby 2.0 style named arguments? E.g

start_reading(book: "The Jungle Book", page: 0)

Might be more complexity than you're willing to add at this point.

[henrikh]

I like both ideas. Weepy's can be implemented using objects (The "normal" way in plain Javascript).

[jashkenas]

I like weepy's idea too, -- it's a nice shorthand in Ruby, and it would be nice to have it here. The only twist is that it's already valid CoffeeScript syntax.

start_reading(book: 'The Jungle Book', page: 0)

Compiles into:

var book, page;
start_reading(book = 'The Jungle Book', page = 0);

So we might need to keep the brackets wrapper just to be specific about what you mean.

[weepy]

how does ruby manage it ?

[jashkenas]

Well, in Ruby, assignment uses =, not :, so it's not ambiguous. I don't really mind the brackets though -- it's nice to know that you're passing an arguments object and not named arguments -- they don't work the same way within the method.

[jashkenas]

Alright -- splats in function definitions are now on master. Kick the tires if you'd like. This:

party: host, *guests =>
  guests.each(guest => host.danceWith(guest))

Compiles into:

var party;
party = function party(host) {
  var guests;
  guests = Array.prototype.slice.call(arguments, 1);
  return guests.each(function(guest) {
    return host.danceWith(guest);
  });
};

[jashkenas]

And splats are now available as arguments to function calls. Calling the function defined above:

party(me, *friends, *relatives)

Compiles into:

party.apply(this, [me].concat(friends).concat(relatives));

[jashkenas]

Going to leave default parameters out for the time being, because they would significantly complicate the grammar and have the potential to look really confusing within function assignments. If anyone has a proposal for what default parameters should look like, feel free to open a new ticket. Closing this one.

(If you'd like to have default arguments now, you can use the ||= operator.)

[josher19]

New ticket: http://github.com/jashkenas/coffee-script/issues#issue/92
discussing default parameters.

Problem with using ||= is that it is bug-prone.

Horse: name, speed => 
    this.name: name
    this.speed: speed || 45

deadHorse = new Horse("Gluey", 0);
"This dead horse has a speed of " + deadHorse.speed  + "!"
 # ... 45!