Question

Reading through the Wikipedia article on First-Class functions, there is a nice table of language support for various aspects of functional programming: http://en.wikipedia.org/wiki/First-class_function#Language_support

JavaScript is listed as not having partial function application. However, there are techniques for creating a function that returns a function that with some of the parameters stored in a closure, ie:

var add = function(a, b){
    return a + b;
},
apply = function(fn, a){
    return function(b){ 
        return fn(a, b); 
    }
},
addFive = apply(add, 5);

console.log(addFive(2)); // prints 7

Is this not partial function application? If not, could someone please provide an example of partial function application in another language and explain how it is different?

Thanks!

Answer 1

What you show is an example of higher order functions, functions that take functions as arguments and/or return functions.

Partial application is something different. Here a Haskell example:

add     :: Int -> Int -> Int
add x y = x + y

addOne = add 1

add is a function that takes two Int and returns an Int, denoted as Int -> Int -> Int. If you’re unfamiliar with the syntax, in Javascript this would roughly look like this:

/**
 * @param int x
 * @param int y
 * @return int
 */
function add(x, y) {
    return x + y;
}

add 1 calls this function with only one parameter, which returns a new function that takes one Int and returns an Int (Int -> Int). The add function was not explicitly designed to be a higher order function, it was simply partially applied.