Hi Guillaume,

You need to use either records or objects to pass a polymorphic argument
(note that the type you want for print_both is ('a. 'a -> 'a) -> unit, not
'a. ('a -> 'a) -> unit).

For example, with objects you could do:

  let print_both (o : <f: 'a. 'a -> a>) =
    print_int (o # f 1);
    print_float (o # f 1.0)

Best wishes,
Nicolas


On Mon, May 16, 2016 at 5:45 PM, 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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