title: Functions
type: concept
summary: |
  Functions describe a process of turning *inputs* into *outputs*.

content: |
  In a purely mathematical setting, functions typically have one input and one
  output, but in functional programming, we can usually get around this either
  by using [tuples][1] or by [currying][2].

  [1]: page://fp-tuples
  [2]: page://fp-currying

exercises:

  - name: doubleIt
    style: gradedProgram
    description: |
      Write a function called `doubleIt` that takes an integer and doubles it.

    satisfiesConcept:
      - fp-function

    graders:
      ocaml:
        style: studentModule
        props:
          interface: |
            val doubleIt : int -> int
          driver: |
            open List
            let () = List.iter
              (fun x -> assert ((doubleIt x) = (x * 2)))
              (List.init 100 (fun x -> x + 1));

  - name: whichIsFunction
    style: multipleChoice
    description: |
      Which of the following can be described as a _function_?

    concepts:
      - fp-function

    graders:
      ocaml:
        style: multipleChoice
        props:
          foo: bar