Re: [Caml-list] Fwd: Polymorphic optional label argument, with default

[Gerd Stolpmann <info@gerd-stolpmann.de>]

On Sam, 2004-04-10 at 23:36, Richard Jones wrote:
> Hi:
> 
> I posted the message below originally on ocaml_beginners.  There were
> many helpful responses, and the general consensus was that this was
> not possible, but no one could give a definitive answer.  I just
> wanted to confirm on the 'grown-ups' list (:-) if this is really true
> before I finalise the API I am designing.
> 
> Thanks,
> 
> Rich.
> 
> To: ocaml_beginners@yahoogroups.com
> From: Richard Jones <rich@annexia.org>
> Subject: Polymorphic optional label argument, with default
> 
> I have a function defined:
> 
>   let plot ?(labels = string_of_int) graph =
>     (* ... *)
> 
> The idea of the optional 'labels' argument is to specify a polymorphic
> function 'a -> string which is used to print labels stored in the
> 'graph' argument.
> 
> The problem is that because string_of_int is obviously int -> string,
> my function is typed as:
> 
>   plot : ?labels : (int -> string) -> (* ... int ... *) -> unit
> 
> but I want it to be typed as:
> 
>   plot : ?labels : ('a -> string) -> (* ... 'a ... *) -> unit
> 
> [The 'a types are the same type.  If I leave out the initializer, then
> it works.]
> 
> How to?

This does not work. If "plot" were not specialised to int labels, you
could call

plot string_graph

(i.e. without ~labels), and where string_graph would be another
specialisation to string labels. This is unsound, because you would
apply string_of_int to strings.

One way out is the two function solution already discussed in
ocaml-beginners (i.e. having "plot" for the polymorphic case, and
"plot_int" for the int case).

Another idea is to store the default for "labels" in the graph. (Or to
implement the graphs with classes, which is a similar approach, but more
effective when you have several such defaults.)

I suppose there is also a good solution with extensional polymorphism,
but this isn't yet in mainstream O'Caml.

Gerd
-- 
------------------------------------------------------------
Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany 
gerd@gerd-stolpmann.de          http://www.gerd-stolpmann.de
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