Questions tagged [partial-application]

Partial application is a programming technique for passing less than the full number of arguments to a function, in order to yield a new function that can be used later. It is particularly common in functional-programming languages that support currying.

Example (OCaml)

(* `add` is a function with arity 2 *)
let add a b = a + b

(* `add` is partially applied with the arguemnt `2`,
 * yielding a function of arity 1 *)
let add2 = add 2

(* `4` is applied to `add2`, making it fully applied
 * and yielding the result of evaluating `add`: `6` *)
let x = add2 4

Example (Python)

Consider the following function:

def add_together(a, b):
    return a + b

If we want to hold a constant, we can manually create the same function with a constant, for example, 5:

def add_together(b):
    return 5 + b

In programming, we typically want a programmatic way of setting our constant and generating our partial function. In Python, we can do this with a closure:

def set_a_on_add_together(a): # the outer function takes a parameter, a
    def add_a_to(b):          # outer function def's new function w/ parameter, 5
        return a + b          # the new function returns a, held constant, plus b
    return add_a_to           # outer function returns the newly created function

and would be used like this:

add_to_five = set_a_on_add_together(5)
add_to_five(4)

would return 9, and

add_to_five(10)

would return 15.