Web Development with ReasonML

Practicing More Recursion

As you saw in the chapter about collections, the Belt.Array and Belt.List modules provide a keep function that produces a new collection of only those items which return true when given to a filtering function.

What if we want to keep only the indices of the matching items? For example, when filtering this array for words less than 6 characters long: [| "cow", "aardvark", "squirrel", "fish", "snake", "capybara"|], the result would be [|0, 3, 4|].

Here’s the pseudocode, given an empty array of integers as the accumulated result, a position starting at index number zero, and a predicate function f:

If the position is at the end of the array:
  return the accumulated result (base case)
otherwise:
 If f(item at ...