(*
    Exercise 4-9 from Functional Programming Using F#
    Copyright (c) 2016 Răzvan Mocanu

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*)

let rec zip = function
    | (x :: xs, y :: ys) ->
        let zs = zip (xs, ys)
        (x, y) :: zs
    | (x :: _, []) ->
        failwith "Expected pair lists to be of same length."
    | ([], y :: _) ->
        failwith "Expected pair lists to be of same length."
    | ([], []) -> []