JavaScript curry: what are the practical applications?

JavaScript curry: what are the practical applications?

I don’t think I’ve grokked currying yet. I understand what it does, and how to do it. I just can’t think of a situation I would use it.
Where are you using currying in JavaScript (or where are the main libraries using it)? DOM manipulation or general application development examples welcome.
One of the answers mentions animation. Functions like slideUp, fadeIn take an element as an arguments and are normally a curried function returning the high order function with the default “animation function” built-in. Why is that better than just applying the higher-up function with some defaults?
Are there any drawbacks to using it?
As requested here are some good resources on JavaScript currying:

http://www.dustindiaz.com/javascript-curry/
Crockford, Douglas (2008) JavaScript: The Good Parts
http://www.svendtofte.com/code/curried_javascript/
(Takes a detour into ML so skip the whole section from “A crash course in ML” and start again at “How to write curried JavaScript”)
http://web.archive.org/web/20111217011630/http://blog.morrisjohns.com:80/javascript_closures_for_dummies
How do JavaScript closures work?
http://ejohn.org/blog/partial-functions-in-javascript (Mr. Resig on the money as per usual)
http://benalman.com/news/2010/09/partial-application-in-javascript/

I’ll add more as they crop up in the comments.

So, according to the answers, currying and partial application in general are convenience techniques.
If you are frequently “refining” a high-level function by calling it with same configuration, you can curry (or use Resig’s partial) the higher-level function to create simple, concise helper methods.

Solutions/Answers:

Solution 1:

@Hank Gay

In response to EmbiggensTheMind’s comment:

I can’t think of an instance where currying—by itself—is useful in JavaScript; it is a technique for converting function calls with multiple arguments into chains of function calls with a single argument for each call, but JavaScript supports multiple arguments in a single function call.

Related:  Real time line graph with nvd3.js

In JavaScript—and I assume most other actual languages (not lambda calculus)—it is commonly associated with partial application, though. John Resig explains it better, but the gist is that have some logic that will be applied to two or more arguments, and you only know the value(s) for some of those arguments.

You can use partial application/currying to fix those known values and return a function that only accepts the unknowns, to be invoked later when you actually have the values you wish to pass. This provides a nifty way to avoid repeating yourself when you would have been calling the same JavaScript built-ins over and over with all the same values but one. To steal John’s example:

String.prototype.csv = String.prototype.split.partial(/,\s*/);
var results = "John, Resig, Boston".csv();
alert( (results[1] == "Resig") + " The text values were split properly" );

Solution 2:

Here’s an interesting AND practical use of currying in JavaScript that uses closures:

function converter(toUnit, factor, offset, input) {
    offset = offset || 0;
    return [((offset + input) * factor).toFixed(2), toUnit].join(" ");
}

var milesToKm = converter.curry('km', 1.60936, undefined);
var poundsToKg = converter.curry('kg', 0.45460, undefined);
var farenheitToCelsius = converter.curry('degrees C', 0.5556, -32);

milesToKm(10);            // returns "16.09 km"
poundsToKg(2.5);          // returns "1.14 kg"
farenheitToCelsius(98);   // returns "36.67 degrees C"

This relies on a curry extension of Function, although as you can see it only uses apply (nothing too fancy):

Function.prototype.curry = function() {
    if (arguments.length < 1) {
        return this; //nothing to curry with - return function
    }
    var __method = this;
    var args = toArray(arguments);
    return function() {
        return __method.apply(this, args.concat([].slice.apply(null, arguments)));
    }
}

Solution 3:

I found functions that resemble python’s functools.partial more useful in JavaScript:

function partial(fn) {
  return partialWithScope.apply(this,
    Array.prototype.concat.apply([fn, this],
      Array.prototype.slice.call(arguments, 1)));
}

function partialWithScope(fn, scope) {
  var args = Array.prototype.slice.call(arguments, 2);
  return function() {
    return fn.apply(scope, Array.prototype.concat.apply(args, arguments));
  };
}

Why would you want to use it? A common situation where you want to use this is when you want to bind this in a function to a value:

var callback = partialWithScope(Object.function, obj);

Now when callback is called, this points to obj. This is useful in event situations or to save some space because it usually makes code shorter.

Related:  HTML - how can I show tooltip ONLY when ellipsis is activated

Currying is similar to partial with the difference that the function the currying returns just accepts one argument (as far as I understand that).

Solution 4:

Agreeing with Hank Gay – It’s extremely useful in certain true functional programming languages – because it’s a necessary part. For example, in Haskell you simply cannot take multiple parameters to a function – you cannot do that in pure functional programming. You take one param at a time and build up your function. In JavaScript it’s simply unnecessary, despite contrived examples like “converter”. Here’s that same converter code, without the need for currying:

var converter = function(ratio, symbol, input) {
    return (input*ratio).toFixed(2) + " " + symbol;
}

var kilosToPoundsRatio = 2.2;
var litersToUKPintsRatio = 1.75;
var litersToUSPintsRatio = 1.98;
var milesToKilometersRatio = 1.62;

converter(kilosToPoundsRatio, "lbs", 4); //8.80 lbs
converter(litersToUKPintsRatio, "imperial pints", 2.4); //4.20 imperial pints
converter(litersToUSPintsRatio, "US pints", 2.4); //4.75 US pints
converter(milesToKilometersRatio, "km", 34); //55.08 km

I badly wish Douglas Crockford, in “JavaScript: The Good Parts”, had given some mention of the history and actual use of currying rather than his offhanded remarks. For the longest time after reading that, I was boggled, until I was studying Functional programming and realized that’s where it came from.

Related:  Scaling Images Proportionally in CSS with Max-width

After some more thinking, I posit there is one valid use case for currying in JavaScript: if you are trying to write using pure functional programming techniques using JavaScript. Seems like a rare use case though.

Solution 5:

Here’s an example.

I’m instrumenting a bunch of fields with JQuery so I can see what users are up to. The code looks like this:

$('#foo').focus(trackActivity);
$('#foo').blur(trackActivity);
$('#bar').focus(trackActivity);
$('#bar').blur(trackActivity);

(For non-JQuery users, I’m saying that any time a couple of fields get or lose focus, I want the trackActivity() function to be called. I could also use an anonymous function, but I’d have to duplicate it 4 times, so I pulled it out and named it.)

Now it turns out that one of those fields needs to be handled differently. I’d like to be able to pass a parameter in on one of those calls to be passed along to our tracking infrastructure. With currying, I can.

Solution 6:

It’s no magic or anything… just a pleasant shorthand for anonymous functions.

partial(alert, "FOO!") is equivalent to function(){alert("FOO!");}

partial(Math.max, 0) corresponds to function(x){return Math.max(0, x);}

The calls to partial (MochiKit terminology. I think some other libraries give functions a .curry method which does the same thing) look slightly nicer and less noisy than the anonymous functions.