OCaml 入門。http://try.ocamlpro.com/ の Lesson 4 辺り。

4.ml

(* 試す *)
let f = function
  | [| x; (_, _) |] -> x
  | _ -> 0, 0;;

f [| 1, 2; 3, 4 |];; (* => (1, 2) *)

let f = function
  | [| x; _ |]::_ -> x
  | _ -> 0, 0;;

f [[| 1, 2; 3, 4 |]];; (* => (1, 2) *)

(* http://try.ocamlpro.com/ の Lesson 4 Step 5 の下 *)
let f = function
  | []                 -> failwith "empty list"
  | [| _; (_, x) |]::_ -> x
  | _                  -> failwith "the first array should be of size two"

f [[| 1, 2; 3, 4 |]];; (* => 4 *)