partial function application

partial.hs

-- in haskell, this is all abstracted away by syntactic sugar.
-- although the function type syntax probably looks familiar.
add :: Integer -> Integer -> Integer
add x y =  x + y

-- can curry as well
inc = add 1

-- haskell functions all only have one arg in reality, 
-- further arguments are always handled this way

partial.js

// partial function application

function add(a) {
  return function(b) {
    return a + b;
  }
}

// call it like this
add(1)(2);

// es6 gives you syntactic sugar for this
const add = (a) => (b) => {
  return a + b
}


// you can also "curry" a partial function, by pre-calling it with not enough args.
const inc = add(1);

inc(2);
// returns 3

// useful in js for doing stuff like parameterized buttons
const buttonPressed = (name) => () => {
  alert(name + " pressed!");
}
// <button onclick="buttonPressed('A')">A</button>
// <button onclick="buttonPressed('B')">B</button>

// in js, the number of args that each nested function is
// not fixed, could easily do something like this:
// foo(a, b)(c, d)