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

[Brian Hurt <bhurt@spnz.org>]

On Sat, 10 Apr 2004, 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.

Hmm.  Somehow I've gotten unsubscribed from that list.  I should subscribe 
again.

> 
> 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?
> 

I don't think it's possible.  Consider the following situation: I pass in 
a graph of floats for example, and then forget to specify a labels 
argument.  Now you're passing a float to string_of_int.

-- 
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                                - Gene Spafford 
Brian

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