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

[Richard J. <ri...@an...>]

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 :-)

Here are my two functions for taking pairs of elements from lists.
FWIW, I have used both of these patterns in real code on more than one
occasion, although the first one is used a lot more frequently than
the second (for disaggregating our counted data):

# let rec take_pairs = function
  | [] | [_] -> invalid_arg "take_pairs"
  | [x; y] -> [x, y]
  | x :: (y :: _ as xs) ->
    (x, y) :: take_pairs xs;;
val take_pairs : 'a list -> ('a * 'a) list = <fun>
# take_pairs [1; 2; 3; 4; 5; 6; 7; 8];;
- : (int * int) list =
[(1, 2); (2, 3); (3, 4); (4, 5); (5, 6); (6, 7); (7, 8)]

# let rec take_in_twos = function
  | [] -> []
  | [_] -> invalid_arg "take_in_twos"
  | x :: y :: xs -> (x, y) :: take_in_twos xs;;
val take_in_twos : 'a list -> ('a * 'a) list = <fun>
# take_in_twos [1; 2; 3; 4; 5; 6; 7; 8];;
- : (int * int) list = [(1, 2); (3, 4); (5, 6); (7, 8)]

Rich.

-- 
Richard Jones, CTO Merjis Ltd.
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