Hello,I recently encountered a situation where I had (effectively) the following polymorphic type:
type 'a record = { id : int; data : 'a }
and the following compare function
let compare {id = id1} {id = id2} = Pervasives.compare id1 id2
and wanted to put such records into a set. However, I could not
figure out how to make the polymorphic 'a in the type definition
"disappear" in the module argument to the Set.Make functor. For
example, the obvious
Set.Make(struct type t = 'a record let compare = compare end)fails because the 'a in the type definition for t is unbound. Is there no way to do this? I'm thinking of some sort of "forall" designation, which universally quantifies the type parameter, like
Set.Make(struct type t = forall 'a : 'a record let compare = compare end)(I'm sure that there is better terminology for this---please pardon my ignorance about types and type theory.)
I ended up solving my problem by placing the record type into a functor, whose argument specified the concrete type for data, but I'm curious if other solutions exist.
Thanks, Will
PGP.sig
Description: This is a digitally signed message part
_______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
