#+title: currying

Currying, in programming, is a [[file:20200604222704-functional_programming.org][functional programming]] technique whereby a
[[file:20210521211721-function_programming.org][function]] of =n= arity is reduced to =n= unary functions. The term comes from
[[file:20210407163530-λ_calculus.org][lambda calculus]].

#+begin_src js :eval never
const add = (x, y, z) => x + y + z;
const curriedAdd = curry(add);
curriedAdd(1)(2)(3) === curriedAdd(1, 2)(3) === curriedAdd(1)(2, 3) === curriedAdd(1, 2, 3);
#+end_src

Currying could be implemented in [[file:20200720132835-javascript.org][JavaScript]] as such:

#+begin_src js :results output
function curry(func) {
  const arity = func.length;

  return function wrapper(...args) {
    if (args.length >= arity) {
      return func(...args);
    } else {
      return function(...otherArgs) {
        return wrapper(...args.concat(otherArgs));
      };
    }
  };
}

const add = (x, y, z) => x + y + z;
const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)(3) === 6);
console.log(curriedAdd(1, 2)(3) === 6);
console.log(curriedAdd(1)(2, 3) === 6);
console.log(curriedAdd(1, 2, 3) === 6);
#+end_src

#+RESULTS:
: true
: true
: true
: true

In [[file:20200708123930-haskell.org][Haskell]], all functions are automatically curried:

#+begin_src haskell
add x y z = x + y + z

add10 = add 10

add10 100 2
#+end_src