SquareThreeWays.re

/* Here's the same function written three different ways */

/* map */
let squareMap = ns => List.map(n => n * n, ns);

// squareMap([1,2,3]) => [1, 4, 9]

/* tail recursive */
let rec squareRec' = (out, ns) =>
  switch (ns) {
  | [] => out
  | [h, ...t] => squareRec'([h * h, ...out], t)
  };

let squareRec = ns => squareRec'([], ns);

/* stepping fn */
type stepIn =
  | Start(list(int))
  | DoThese(list(int), list(int));

type stepOut =
  | DidThese(list(int), list(int))
  | Done(list(int));

let squareStep = input =>
  switch (input) {
  | Start(ns) => DidThese([], ns)
  | DoThese(out, []) => Done(out)
  | DoThese(out, [h, ...t]) => DidThese([h * h, ...out], t)
  };

let rec square' = output =>
  switch (output) {
  | DidThese(a, b) => square'(squareStep(DoThese(a, b)))
  | Done(ns) => ns
  };

let square = ns => square'(squareStep(Start(ns)));