]> Benjamin Geer [Sunday 14 December 2003] : > > 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...) You could use a record holding a field with a fully polymorphic type : type t = { c : 'a. 'a -> 'a -> bool } let compare_things t1 t2 comparison_fun = match (t1, t2) with (Int i, _) -> comparison_fun.c i (int_of_thing t2) | (String s, _) -> comparison_fun.c s (string_of_thing t2) ;; -- Olivier ------------------- 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