Problem 3: Matching a tuple, and or-patterns

For next_cell_state.

Matching several values at once

When a decision depends on a combination of values, match a tuple of them in one go instead of nesting separate matches.

let describe_response (method_, code) =
  match method_, code with
  | "GET", 200  -> "ok"
  | "GET", 404  -> "not found"
  | "POST", 201 -> "created"
  | _           -> "other"

match method_, code with is shorthand for match (method_, code) with. Each clause checks both components at once. A clause only matches if the method and the code agree with that row.

Or-patterns: one arm, several literals

Sometimes several different literal values should take the same branch. Writing one clause per value works, but repeats itself.

let is_weekend day =
  match day with
  | "Saturday" -> true
  | "Sunday"   -> true
  | _          -> false

An or-pattern collapses those into a single clause, with a | between the alternatives inside the pattern itself.

let is_weekend day =
  match day with
  | "Saturday" | "Sunday" -> true
  | _                     -> false

"Saturday" | "Sunday" matches if the value is either one. The whole pattern succeeds the moment any alternative does, and all of them share the one right-hand side. This is lighter than a when guard for this situation, "is this value one of a few specific literals?" A guard needs an explicit comparison, when day = "Saturday" || day = "Sunday". An or-pattern says the same thing without writing out the comparison.

Combining both

The two combine directly. An or-pattern can sit inside one component of a tuple pattern, matching several values there while the other component still matches normally.

let shipping_cost (country, weight) =
  match country, weight with
  | "domestic", (1 | 2 | 3) -> 5
  | "domestic", _           -> 8
  | _, _                    -> 20

(1 | 2 | 3) inside the tuple pattern matches a weight of 1, 2, or 3, the same as the standalone version above, just nested one level in.