]> On Sun, Dec 14, 2003 at 09:38:24PM +0000, Benjamin Geer wrote: > Jean-Baptiste Rouquier wrote: > > compare_things : thing -> thing -> ('a -> 'a -> bool) -> bool > >doesn't mean that comparison_fun has to be polymorphic, but it means > >that any function with type ('a -> 'a -> bool), where 'a is any type, is > >acceptable. For instance, the type checker would accept a function of > >the type (char -> char -> bool). > > OK, but it seems that the type checker has inferred the type of > comparison_fun just by looking at its first use in compare_things, and > has decided that it must be monomorphic; this is an incorrect inference. > My naive impression is that if the type checker was willing to accept > both uses of comparison_fun as valid (or perhaps if it also looked at > the places where compare_things is called), it would conclude that > comparison_fun does indeed have to be polymorphic. In that case, I > would get a compile error if I tried to pass a non-polymorphic function > to compare_things. Would this be this asking too much? (I suspect the > answer is yes; I'm just curious...) Polymorphism in ML is flat, that is all type variables are quantified before the function, not in the arguments. The type ('a -> foo) -> bar means forall 'a. ('a -> foo) -> bar, and not (forall 'a. 'a -> foo) -> bar. This is one of the costs of type inference. In OCaml you can simulate deep polymorphism with polymorphic records (but you have to write explicit type then). -- : Michal Moskal :: http://www.kernel.pl/~malekith : GCS {C,UL}++++$ a? !tv : When in doubt, use brute force. -- Ken Thompson : {E-,w}-- {b++,e}>+++ h ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners