[mercury-users] "some" and existential types

[Fergus Henderson]

On 02-Oct-1999, Ralph Becket <rbeck at microsoft.com> wrote:
> 
> :- pred give_me_a_value(list(anything)).
...
> :- type anything ---> some [T] thing(T) => printable(T).
> 
> means you can do things like
> 
> foo :-
> 	give_me_a_value(Xs),
> 	print_anythings(Xs).
> 
> print_anythings([]) --> [].
> print_anythings([thing(X) | Xs]) --> print(X), print_anythings(Xs).
> 
> The things in the list can all be different, but because of the typeclass
> constraint give_me_a_value/1 promises that they will all have a print//1
> method defined for them.
> 
> QUESTION FOR THE MERCURY SQUAD: can I write something like the following,
> 
> print_anythings(Xs) --> list__foldl(print, Xs).
> 
> I suspect not, because the print//1 method may be different in each case.
> However, to disallow this sort of code would be a great shame, IMHO.  Not
> sure what the solution is, assuming this is the case.

There's an easy solution here: just declare `anything' to be an
instance of `printable':

	:- instance printable(anything) where [
		pred(print/3) is print_anything
	].

	:- pred print_anything(anything::in, io__state::di, io__state::uo)
		is det.
	print_anything(thing(X)) --> print(X).

Then you can write `print_anythings(Xs) --> list__foldl(print, Xs)'
and it should work just fine.

-- 
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.
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