[m-dev.] type classes and the color blue

[Fergus Henderson]

On 14-Jan-2000, Michael Day <mcda at cat.cs.mu.OZ.AU> wrote:
> 
> So really I do want:
> 
> :- typeclass thing(T) where
> 	[
> 		pred is_blue(T),
> 		mode is_blue(in) is semidet
> 	].
> 
> even if is_blue never actually examines its argument explicitly, it
> requires the argument to get the typeclass_info to know which is_blue to
> call. Or something along those lines, albeit better phrased.

If is_blue never actually examines its argument, then you can
declare the argument to have mode `unused'.  Then you can
pass an underscore variable, qualified with the appropriate type

	is_blue(_ : int)	% XXX the `term : type' syntax is
				% not yet supported


-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
--------------------------------------------------------------------------
mercury-developers mailing list
Post messages to:       mercury-developers at cs.mu.oz.au
Administrative Queries: owner-mercury-developers at cs.mu.oz.au
Subscriptions:          mercury-developers-request at cs.mu.oz.au
--------------------------------------------------------------------------