Re: [Ocaml-lib-devel] List.map_pairs

[Nicolas C. <war...@fr...>]

> On the subject of new list functions, how about:
>
> let rec list_map_pairs f = function
>   | [] | [_] -> invalid_arg "list_map_pairs"
>   | [x; y] -> [f x y]
>   | x :: (y :: _ as xs) ->
>       f x y :: list_map_pairs f xs
>
> We use this sort of pattern to disaggregate data which has been
> collected using an incrementing counter - something like this:
>
> # let xs = [ 0; 1; 4; 5; 5; 5; 8; 9; 9; 10 ];;
> val xs : int list = [0; 1; 4; 5; 5; 5; 8; 9; 9; 10]
> # list_map_pairs (fun a b -> b - a) xs;;
> - : int list = [1; 3; 1; 0; 0; 3; 1; 0; 1]
>
> Rich.

I think that it's not very the Ocaml spirit of handling pairs, in general
you will first transform your "int list" into an "(int * int) list" with
error handling and then treat pairs uniformly in a typesafe manner.

Nicolas