Currying, or partial function application, is explained here by Resig. He gives the following implementation:
Function.prototype.partial = function(){
var fn = this, args = Array.prototype.slice.call(arguments);
return function(){
var arg = 0;
for ( var i = 0; i < args.length && arg < arguments.length; i++ )
if ( args[i] === undefined )
args[i] = arguments[arg++];
return fn.apply(this, args);
};
};
This method saves all arguments, including the final arguments, which is usually not the desired behavior. Consider:
add = (a,b) ->
a + b
addOne = add.partial 1, undefined
> addOne(2)
3
> addOne(10)
3
The 2 is saved as the second argument, so the 10 is ignored in the second invocation of addOne. The solution is to use a copy of args in the inner function (slice() makes a copy of an array):
Function.prototype.partial = ->
fn = @
args_original = Array.prototype.slice.call arguments
->
args = args_original.slice()
arg = 0
i = 0
while i < args.length && arg < arguments.length
if args[i] is undefined
args[i] = arguments[arg++]
i++
fn.apply @, args
Now the second call to addOne will return 11 as intended.