Discussion on: JavaScript Bites: Closure

[zaferberkun]

Strictly speaking, I don't believe your example of partial application is any different than the preceeding "counter" example--its just a higher order function that creates a closure when it returns the inner function. Partial application would be creating a logging function, and then creating another logging function that returns this existing logging function with some or the args already applied and the missing args supplied by the new function, thus the partial application or an already existing logging function. Correct me if im wrong or misunderstood your example.

[Brett Martin]

Hi zaferberkun - thanks for the comment!

I can see what you are saying - I think I need to come up with a better example in this case to show the partial application, as the underlying console.log only takes the one argument.

How about a simple add example in its place, where the example of the closure is in the partial function definition:

const partial = (fn, arg) => {
  return (...args) => {
    return fn(arg, ...args);
  };
};

const add = (x, y) => x + y;

const add10 = partial(add, 10);

console.log(add10(5));

[zaferberkun]

Hi Brett,

I like that! Also, after some consideration, I think I was actually wrong and your original example (and the one by peerreynders below) is a perfectly valid example of partial application.

What threw me was the use of an "intermediary" make function to create the partial. What I was expecting was that the function itself, logger in this case, could be applied partially "in place". You would have to add "undefined" arg checks, but that way you could do full or partial application without "intermediary" maker functions for each arg permutation. Of course, if you're planning using composition, you'll need to generate a partial that only takes one arg (which your logger make example is doing). Anyway, sorry about that, you can disregard my original post if you want.

Btw, great writing, hope you'll tackle some more functional programming concepts! I'm going to have to backtrack and read the ones you've already written.

[peerreynders]

I think I need to come up with a better example in this case to show the partial application, as the underlying console.log only takes the one argument.

const docLogger = mkLogger('DOCS', true);

logger('LIBS', 'Direct log message', false);
docLogger('This is my log message');
docLogger('Another log message');

function logger(route, message, showDate) {
  const header = showDate ? `${new Date().toISOString()} | ${route}` : route;
  console.log(`${header} | ${message}`);
}

function mkLogger(route, showDate = false) {
  // Implement "partial application" with the values
  // in the closure
  return (message) => logger(route, message, showDate);
}

[Brett Martin]

Thanks for writing this up - ill add it to the main article and shout you out!