Problem 2: Guards (when)

For line_winner.

Matching several values at once

When the answer depends on a combination of values, match a tuple of them in one go instead of nesting several matches.

let combine a b =
  match a, b with
  | None,   None   -> "both empty"
  | Some _, None   -> "first only"
  | None,   Some _ -> "second only"
  | Some _, Some _ -> "both present"

match a, b with is shorthand for match (a, b) with. line_winner needs the same idea for three values: the cells at positions i, j, and k.

The when guard

A pattern checks shape: "this is Taken of something." A guard adds a boolean check on top of a pattern, evaluated only if the pattern already matched.

let sign = function
  | n when n > 0 -> "positive"
  | n when n < 0 -> "negative"
  | _ -> "zero"