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

[Alan P. <ap...@re...>]

In article <200...@fu...>, Richard Jones wrote:
> On Wed, Aug 10, 2005 at 06:34:19PM +0000, Alan Post wrote:
>> In article <200...@fu...>, Richard W. M. Jones wrote:
>> > 
>> > Well OK, but we don't even have a function to do the
>> > int list -> (int * int) list transformation.
>> 
>> A fold using a ref would work, yes?
> 
> Something like that might, but the whole point of HOFs is to avoid
> having to write ugly hacks all the time :-)

OK, here's a tail-recursive function using a fold without a ref:

let take_pairs =
  let f (i,l) j = (j, (i,j)::l)
  in function
  | [] -> []
  | h::tl -> List.rev (snd (List.fold_left f (h, []) tl))