Re: [Caml-list] issue with polymorphism

[Gabriel Scherer <gabriel.scherer@gmail.com>]

This is possibly, but you have to write it a bit differently. See the FAQ entry,
  "How to write a function with polymorphic arguments?"
  https://ocaml.org/learn/faq.html#Howtowriteafunctionwithpolymorphicarguments

On Mon, May 16, 2016 at 11:45 AM, Guillaume Hennequin
<g.hennequin@eng.cam.ac.uk> wrote:
> Dear caml-list,
>
> let id x = x
>
> is polymorphic, and indeed I can apply it to various types:
>
> let _ = print_float (id 1.)
> let _ = print_int (id 1)
>
> Now, say you want to write a function that takes a function of the same
> ['a->'a] type as [id] above, and applies it to two different types:
>
> let print_both f =
>  print_int (f 1);
>  print_float (f 1.0)
>
> That in fact won't compile:
>
> Error: This expression (1.0) has type float but an expression was expected
> of int
>
> Naively trying to enforce polymorphism doesn't work either:
>
> let print_both: 'a. ('a -> 'a) -> unit = fun f ->
>  print_int (f 1);
>  print_float (f 1.0)
>
> As a matter of fact, neither will this:
>
> let print1: 'a. ('a -> 'a) -> unit = fun f -> print_int (f 1)
>
> Error: This definition has type (int -> int) -> unit
> which is less general than 'a. ('a -> 'a) -> unit
>
> What am I missing? How would you go about writing such a function?
>
> Many thanks,
> Guillaume
>
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