RE: [Caml-list] still puzzled on generic types

[David Allsopp <dra-news@metastack.com>]

Walter Cazzola wrote:
> Dear all,
> waiting for some hints on my other issue for functors and generic I was
> playing with an enumerate function (with tail recursion):
> 
>    let rec enumerate ?(l':((int * 'a) list)=[]) ?(n=0) l =
>      match l with
>           h::l1 -> enumerate (l'@[(n,h)]) (n+1) l1
>      |  [] -> l'
> 
> this should just take a list and return a list of couple whose first entry
> is the position in the list of the element; to be clearer:
> 
>    enumerate ['a'; 'b'; 'c'] -> [(0,'a'); (1,'b'); (2,'c')];
> 
> but this doesn't work as by type mismatch:
> 
>   Error: This expression has type (int * 'a) list
>          but an expression was expected of type 'a list

I think it's probably something to do with optional arguments not behaving as you expect. But this is a bad use for optional arguments - you should instead use a nested function to pass the accumulator values. This works fine:

let enumerate l =
  let rec enumerate acc n = function
    h::ls -> enumerate ((n, h)::acc) (n + 1) ls
  | [] -> List.rev acc
  in
    enumerate [] 0 l

Concatenating lists is also expensive in terms of the left list so your function is about as slow as possible! Much better when aiming for tail recursion, accumulate a reversed list and then reverse it in the basis case.


David