]> 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...) Ben > [...] > Benjamin Geer wrote: > >> Can anyone explain to me why the following doesn't compile: >> >> ---------- >> >> type thing = Int of int | String of string ;; >> >> let int_of_thing t = >> match t with >> Int i -> i >> | String s -> int_of_string s ;; >> >> let string_of_thing t = >> match t with >> Int i -> string_of_int i >> | String s -> s ;; >> >> let compare_things t1 t2 comparison_fun = >> match (t1, t2) with >> (Int i, _) -> comparison_fun i (int_of_thing t2) >> | (String s, _) -> comparison_fun s (string_of_thing t2) ;; >> >> let is_less_than t1 t2 = >> compare_things t1 t2 (<) ;; >> >> let is_greater_than t1 t2 = >> compare_things t1 t2 (>) ;; >> >> ---------- >> >> The compiler says: >> >> File "test.ml", line 16, characters 38-39: >> This expression has type string but is here used with type int >> >> It seems to have decided that in the function compare_things, >> comparison_fun is int -> int -> 'a because that's how it's used in the >> first pattern, not realising that I mean to pass a polymorphic >> function, 'a -> 'a -> bool. >> >> Is there a way to make this work? (Specifying the type of >> comparison_fun as 'a -> 'a -> bool makes no difference.) >> >> Ben ------------------- 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