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let double = mul.bind(null, 2);

alert( double(3) ); // = mul(2, 3) = 6
alert( double(4) ); // = mul(2, 4) = 8
alert( double(5) ); // = mul(2, 5) = 10

The call to mul.bind(null, 2) creates a new function double that passes calls to mul, fixing null as the context and 2 as the first argument. Further arguments are passed “as is”.

That’s called partial function application – we create a new function by fixing some parameters of the existing one.

Please note that here we actually don’t use this here. But bind requires it, so we must put in something like null.

The function triple in the code below triples the value:

let triple = mul.bind(null, 3);

alert( triple(3) ); // = mul(3, 3) = 9
alert( triple(4) ); // = mul(3, 4) = 12
alert( triple(5) ); // = mul(3, 5) = 15

Why do we usually make a partial function?

Here our benefit is that we created an independent function with a readable name (double, triple). We can use it and don’t write the first argument of every time, cause it’s fixed with bind.

In other cases, partial application is useful when we have a very generic function, and want a less universal variant of it for convenience.

For instance, we have a function send(from, to, text). Then, inside a user object we may want to use a partial variant of it: sendTo(to, text) that sends from the current user.

Going partial without context

What if we’d like to fix some arguments, but not bind this?

The native bind does not allow that. We can’t just omit the context and jump to arguments.

Fortunately, a partial function for binding only arguments can be easily implemented.

Like this:

function partial(func, ...argsBound) {
  return function(...args) { // (*)
    return func.call(this, ...argsBound, ...args);
  }
}

// Usage:
let user = {
  firstName: "John",
  say(time, phrase) {
    alert(`[${time}] ${this.firstName}: ${phrase}!`);
  }
};

// add a partial method that says something now by fixing the first argument
user.sayNow = partial(user.say, new Date().getHours() + ':' + new Date().getMinutes());

user.sayNow("Hello");
// Something like:
// [10:00] John: Hello!

The result of partial(func[, arg1, arg2...]) call is a wrapper (*) that calls func with:

So easy to do it with the spread operator, right?

Also there’s a ready _.partial implementation from lodash library.

Currying

Sometimes people mix up partial function application mentioned above with another thing named “currying”. That’s another interesting technique of working with functions that we just have to mention here.

Currying is translating a function from callable as f(a, b, c) into callable as f(a)(b)(c).

Let’s make curry function that performs currying for binary functions. In other words, it translates f(a, b) into f(a)(b):

function curry(func) {
  return function(a) {
    return function(b) {
      return func(a, b);
    };
  };
}

// usage
function sum(a, b) {
  return a + b;
}

let carriedSum = curry(sum);

alert( carriedSum(1)(2) ); // 3

As you can see, the implementation is a series of wrappers.

More advanced implementations of currying like _.curry from lodash library do something more sophisticated. They return a wrapper that allows a function to be called normally when all arguments are supplied or returns a partial otherwise.

function curry(f) {
  return function(...args) {
    // if args.length == f.length (as many arguments as f has),
    //   then pass the call to f
    // otherwise return a partial function that fixes args as first arguments
  };
}

Currying? What for?

Advanced currying allows both to keep the function callable normally and to get partials easily. To understand the benefits we definitely need a worthy real-life example.

For instance, we have the logging function log(date, importance, message) that formats and outputs the information. In real projects such functions also have many other useful features like: sending it over the network or filtering:

function log(date, importance, message) {
  alert(`[${date.getHours()}:${date.getMinutes()}] [${importance}] ${message}`);
}

Let’s curry it!

log = _.curry(log);

After that log still works the normal way:

log(new Date(), "DEBUG", "some debug");

…But also can be called in the curried form:

log(new Date())("DEBUG")("some debug"); // log(a)(b)(c)

Let’s get a convenience function for today’s logs:

// todayLog will be the partial of log with fixed first argument
let todayLog = log(new Date());

// use it
todayLog("INFO", "message"); // [HH:mm] INFO message

And now a convenience function for today’s debug messages:

let todayDebug = todayLog("DEBUG");

todayDebug("message"); // [HH:mm] DEBUG message

So:

  1. We didn’t lose anything after currying: log is still callable normally.
  2. We were able to generate partial functions that are convenient in many cases.

Advanced curry implementation

In case you’re interested, here’s the “advanced” curry implementation that we could use above.

function curry(func) {

  return function curried(...args) {
    if (args.length >= func.length) {
      return func.apply(this, args);
    } else {
      return function(...args2) {
        return curried.apply(this, args.concat(args2));
      }
    }
  };

}

function sum(a, b, c) {
  return a + b + c;
}

let curriedSum = curry(sum);

// still callable normally
alert( curriedSum(1, 2, 3) ); // 6

// get the partial with curried(1) and call it with 2 other arguments
alert( curriedSum(1)(2,3) ); // 6

// full curried form
alert( curriedSum(1)(2)(3) ); // 6

The new curry may look complicated, but it’s actually pretty easy to understand.

The result of curry(func) is the wrapper curried that looks like this:

// func is the function to transform
function curried(...args) {
  if (args.length >= func.length) { // (1)
    return func.apply(this, args);
  } else {
    return function pass(...args2) { // (2)
      return curried.apply(this, args.concat(args2));
    }
  }
};

When we run it, there are two branches:

  1. Call now: if passed args count is the same as the original function has in its definition (func.length) or longer, then just pass the call to it.
  2. Get a partial: otherwise, func is not called yet. Instead, another wrapper pass is returned, that will re-apply curried providing previous arguments together with the new ones. Then on a new call, again, we’ll get either a new partial (if not enough arguments) or, finally, the result.

For instance, let’s see what happens in the case of sum(a, b, c). Three arguments, so sum.length = 3.

For the call curried(1)(2)(3):

  1. The first call curried(1) remembers 1 in its Lexical Environment, and returns a wrapper pass.

  2. The wrapper pass is called with (2): it takes previous args (1), concatenates them with what it got (2) and calls curried(1, 2) with them together.

    As the argument count is still less than 3, curry returns pass.

  3. The wrapper pass is called again with (3), for the next call pass(3) takes previous args (1, 2) and adds 3 to them, making the call curried(1, 2, 3) – there are 3 arguments at last, they are given to the original function.

If that’s still not obvious, just trace the calls sequence in your mind or on the paper.

Fixed-length functions only

The currying requires the function to have a known fixed number of arguments.

A little more than currying

By definition, currying should convert sum(a, b, c) into sum(a)(b)(c).

But most implementations of currying in JavaScript are advanced, as described: they also keep the function callable in the multi-argument variant.

Summary

Tasks

importance: 5

The task is a little more complex variant of Ask losing this.

The user object was modified. Now instead of two functions loginOk/loginFail, it has a single function user.login(true/false).

What to pass askPassword in the code below, so that it calls user.login(true) as ok and user.login(false) as fail?

function askPassword(ok, fail) {
  let password = prompt("Password?", '');
  if (password == "rockstar") ok();
  else fail();
}

let user = {
  name: 'John',

  login(result) {
    alert( this.name + (result ? ' logged in' : ' failed to log in') );
  }
};

askPassword(?, ?); // ?

Your changes should only modify the highlighted fragment.

  1. Either use a wrapper function, an arrow to be concise:

    askPassword(() => user.login(true), () => user.login(false));

    Now it gets user from outer variables and runs it the normal way.

  2. Or create a partial function from user.login that uses user as the context and has the correct first argument:

    askPassword(user.login.bind(user, true), user.login.bind(user, false));
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