Re: [Caml-list] List.fold_left vs. Hashtbl.fold

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2012/11/29, at 1:41, Virgile Prevosto <virgile.prevosto@m4x.org> wrote:

> 2012/11/28 Adrien <camaradetux@gmail.com>:
>> Well, MoreLabels.Hashtbl has
>>  val  fold  : f:(key:'a -> data:'b -> 'c -> 'c) -> ('a, 'b) t -> init:'c -> 'c
>> 
> 
> Fair enough, but there's no label (say acc) to the last argument of f
> (nor for the table itself), so you can't really permute the current
> element and the accumulator in the argument list.

Not true for fold itself: no label is equivalent to the empty label,
so if you write "fold t" you are partially applying to the table.
On the other hand, changing parameter order is not allowed
in functions passed as argument. So reordering of arguments to
f is not automatic. But at least it ensures that you do not get it wrong.

Also a good reason to use labels in fold is when both remaining arguments
have the same type:

  List.fold_left (fun l x -> if List.mem x l then l else x::l)

since the accumulator and the input list have the same type, I'm always
afraid of getting them wrong (ending up doing nothing).

	Jacques Garrigue