partial.md

Partial Application {#simple-partial}

In Building Blocks, we discussed partial application, but we didn't write a generalized recipe for it. This is such a common tool that many libraries provide some form of partial application tool. You'll find examples in Lemonad from Michael Fogus, Functional JavaScript from Oliver Steele and the terse but handy node-ap from James Halliday.

These two recipes are for quickly and simply applying a single argument, either the leftmost or rightmost.¹ If you want to bind more than one argument, or you want to leave a "hole" in the argument list, you will need to either use a generalized partial recipe, or you will need to repeatedly apply arguments. It is context-agnostic.

var __slice = Array.prototype.slice;

function callFirst (fn, larg) {
  return function () {
    var args = __slice.call(arguments, 0);
    
    return fn.apply(this, [larg].concat(args))
  }
}

function callLast (fn, rarg) {
  return function () {
    var args = __slice.call(arguments, 0);
    
    return fn.apply(this, args.concat([rarg]))
  }
}

function greet (me, you) {
  return "Hello, " + you + ", my name is " + me
}

var heliosSaysHello = callFirst(greet, 'Helios');

heliosSaysHello('Eartha')
  //=> 'Hello, Eartha, my name is Helios'
  
var sayHelloToCeline = callLast(greet, 'Celine');

sayHelloToCeline('Eartha')
  //=> 'Hello, Celine, my name is Eartha'

As noted above, our partial recipe allows us to create functions that are partial applications of functions that are context aware. We'd need a different recipe if we wished to create partial applications of object methods.

Footnotes

1. callFirst and callLast were inspired by Michael Fogus' Lemonad. Thanks! ↩