Problem 5: Array.mem

For is_valid_guess.

Checking membership in an array

Array.mem x a answers one question directly: is x anywhere in a? It checks every element with =, the same structural equality a literal pattern uses.

let () =
  assert (Array.mem "house" [| "house"; "plant"; "chair" |]);
  assert (not (Array.mem "horse" [| "house"; "plant"; "chair" |]))

is_valid_guess is exactly this question, no more: "is this guess one of the words in words?"

The list equivalent

List.mem does the identical job for a list instead of an array ('a -> 'a list -> bool). Which one to reach for depends only on which data structure you already have, never on the question being asked.

It's not magic

Array.mem is an ordinary loop under a name, checking each element in turn and stopping the moment it finds a match:

let mem_manual x a =
  let found = ref false in
  Array.iter (fun y -> if y = x then found := true) a;
  !found

A real implementation stops scanning as soon as it finds a match rather than checking every remaining element regardless -- mem_manual above doesn't, but the behavior (true/false answer) is the same either way, which is the only thing a caller ever depends on.