Bind arguments in different order

Question

Say I have

function bake(egg, milk, batter) { ... }

And I already have the egg. Then I might do this

var f = bake.bind(null, theEgg);

to create a function that does the rest of the work.

But: what if I have the milk or the batter and I want to bind it, and it's the first argument that will be passed in later? Is there a functional way to twist the arguments without just creating an anonymous intermediary function and manually twisting the arguments so I can have fluid, functional-looking code with the argument bound?

In functional languages, functions are typically curried, so you can't immediately access the arguments out of order anyway. That's why Haskell has functionals like `flip`. I'd write out some signatures, but it kind of bothers me that you have "batter" as an argument, when usually "egg" and "milk" go into the batter so I'd rather not . — Marcel Besixdouze, Apr 15 '21 at 16:38

elclanrs · Answer 1 · 2014-08-14T02:33:57.650

I think you're looking for partial application, with placeholder. Here's a helper:

var _ = {} // placeholder

var partial = function(f) {
  var as = [].slice.call(arguments, 1)
  return function() {
    var args = as.concat([].slice.call(arguments))
    for (var i=args.length; i--;)
      if (args[i] === _)
        args[i] = args.splice(-1)[0]
    return f.apply(null, args)
  }
}

Then you can use it like so:

partial(bake, _, theMilk, _)(theEgg, theBatter)

score 0 · Answer 2 · answered Aug 14 '14 at 02:24

Using an object is usually the way to do this

var ingredients = {
    egg: theEgg,
    milk: theMilk,
    batter: theBatter
}

bake(ingredients);

Then you can vary the order in which egg, milk or batter are added to ingredients and you can handle null or undefined values within your bake method.

There are also libraries (e.g. jQuery extend) that allow you to merge 2 objects together, so for example, you could have a default set of ingredients that are extended with the values provided in the arguments

Bind arguments in different order

2 Answers2