And you can mix all the kind of values: let f = function | [] -> failwith "empty list" | [| ; (, x) |]::_ -> x | _ -> failwith "the first array should be of size two"

val f : ('a * 'b) array list -> 'b =

Instead of asking “how to do X in Elixir”, ask “how to solve Y in Elixir”. In other words, don’t ask how to implement a particular solution, instead describe the problem at hand. Stating the problem gives more context and less bias for a correct answer.