Re: [Ocaml-lib-devel] some proposals

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

On Fri, 23 May 2003, Nicolas Cannasse wrote:

> > Which means we're probably stuck with something like:
> >
> > let Int.compare x y = if (x < y) then -1 else if (x > y) then 1 else 0
> 
> Uh !
> This time Brian you're using up to TWO times polymorphic comparison !

You're right.  Need a typecast:

let Int.compare (x: int) (y: int) = if (x < y) then -1 else if (x > y) 
then 1 else 0



> I think the most easy solution is really to use the substraction, but that
> will cause the problems you showed with max_int / min_int , and doing some
> previous boundary tests before return x - y will increase the cost of it. So
> my conclusion is this is an user-specific problem (some might need max_int
> while some might not ) which should be addressed by not adding the module
> Int to the ExtLib, although it was a good idea.

Actually, handling the corner cases is a good argument, IMHO, *FOR* adding 
class Int.  This strikes me as being a common error.

> 
> About the partial_map, this can be done using a fold_left, which is more
> appropriate since you don't allocate None/Some blocks . But I think perhaps
> fold_left/right are not so easy to use in the first place, so maybe having a
> partial_map would be nice , but I would rename it to filter_map since this
> can be done using one map + one filter.

Allocating the option blocks doesn't worry me much.  I assumed that we 
wanted to a) avoid allocating an unnecessary interim list, and b) wanted 
to construct the list forwards, as opposed to doing a List.rev.  If either 
of these are relaxed, then yes, the function can be fairly easily 
implemented given existing functions.

> 
> BTW, I would prefer a more simple version of the Brian one, which does not
> need two loops :
> 
> let filter_map f l =
>     let rec loop dst = function
>         | [] -> ()
>         | h :: t ->
>             match f t with
>             | None -> loop dst t
>             | Some x ->
>                     let r = [ x ] in
>                     Obj.set_field (Obj.repr dst) 1 (Obj.repr r);
>                     loop r t
>     in
>     let dummy = [ Obj.magic () ] in
>     loop dummy l;
>     List.tl dummy
> 

I forgot about this trick.  This is a better implementation.

Brian