Re: [Ocaml-lib-devel] Re: some proposals

[Brian H. <bri...@ql...>]

On Thu, 22 May 2003, Alan Post wrote:

> The perl map function allows the number of outgoing items to be
> other than 1, for instance:
> 
>   my @l2 = map { if ( filter( $_ ) { $_ } else {()}} @l1;
> 
> or:
> 
>   my %h = map { $_, f( $_ )} @l;
> 
> I imagine it would be a bit tricky to implement an Enum function like
> that ("merge_map"?), but it would generalize the partial map function
> above.
> 
> 
> A more general question:  how many map/iterate functions will be a
> part of the data structures, as opposed to having users stick to the
> Enum versions?  For instance,
> 
>   List.map f l 
> 
>     vs
> 
>   List.of_enum (Enum.map f (List.enum l))
> 
> Will each datastructure have its own map function, to make things
> convenient for users?
> 

I considered implementing this as an enum.  But here's the problem: count 
requires you to know in advance how many elements you have.  The only way 
I can see doing this would be to create a list of pre-converted elements 
that didn't return None.  A stream might be a better choice.  

let stream_partial_map f src =
    let rec next curr_ref x =
        match !curr_ref with
          [] -> None
          | h :: t -> 
              curr_ref := t;
              match f h with
                  None -> next curr_ref x
                  | Some t -> Some t
    in
    Stream.from (next (ref src))
;;


But I hate having two different almost-identical-but-subtly-different
ways of doing things.  I think that instead of Enum module, we should 
consider an ExtStream module, with Enum functionality.

Brian