There is no way to avoid coercions altogether: OCaml doesn't have inference for subtyping, using unification with row variables gives the intersection behavior. But the language could be changed, with a syntax like
match x return e with | ... -> ... | ... -> ... meaning that all branches should be coerced to e. On Thu, Aug 14, 2008 at 4:18 AM, Jacques Carette <[EMAIL PROTECTED]> wrote: > Here is a much simplified version from a (much) larger problem I have > recently encountered: > > type 'a a = [`A of 'a b] > and 'a b = [`B of 'a a] > and 'a c = [`C ] > > type 'a d = [ 'a a | 'a b | 'a c] > type e = e d > > # this code gives an error (details below) > let f1 (x:e) : e = match x with > | `A n -> n > | `B n -> n > | `C -> `C > > # this works > let f2 (x:e) : e = match x with > | `A n -> (n :> e) > | `B n -> (n :> e) > | `C -> `C > > f1 gives an error on the "| `B n -> n" line, pointing to the second 'n' > with > This expression has type e a but is used with type e b > These two variant types have no intersection > > Indeed, they have no intersection, but they have a union! That is what it > seems the coercion in f2 'forces' the type-checker to realize, and all works > fine. But of course, such coercions end up polluting my code all over the > place (since the actual example is made of 9 types with 20 tags in total, > and the 'recursive knot' requires 2 parameters to close properly). > > So, is this a bug? Is there a way to avoid these coercions? > > Jacques > > _______________________________________________ > 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 > _______________________________________________ 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
