From: Philippe Veber <philippe.ve...@googlemail.com> >> Constrained types have their uses, > > which are, in brief ? Now I can't see a typical use for closed polymorphic > variant types (I mean types of the form 'a t constraint 'a = [< ... ])
A typical use would be for objects, particularly when representing virtual types (see the advanced part of the tutorial, in the reference manual). Another application is to use a record notation for type variables: type 'r t = .... constraint 'r = < env:'env; loc:'loc; typ:'typ; .. > This way you can share multiple type parameters at once, and even allow transparent addition of new parameters. This was not the original goal of constraints, but I find it handy. I have no immediate example with polymorphic variants, but your arguments about scalability are valid: in some cases, constraints allow more compact types. But at a cost. >> but I find them often confusing as >> the type variable you write is not really a type variable. >> > why isn't it the case ? Aren't they simply type variables restricted to a > finite number of types ? Yes they are constrained type variables. But the problem is that the constraint is left implicit. For instance, if somewhere you have defined type 'a t constraint 'a = [< `a | `b] and inside an interface you write val f : 'a t -> int the real meaning is val f : [< `a | `b] t -> int I.e., you are hiding something. This is not the same thing as type abbreviations, because constraints are applied from the outside, while abbreviations just have to be expanded. Jacques Garrigue _______________________________________________ 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
