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[Caml-list] polymorphic function not recognised as such
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Date: 2003-12-14 (22:06)
From: Olivier Andrieu <andrieu@i...>
Subject: Re: [Caml-list] polymorphic function not recognised as such
 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) ;;


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