Re: [Caml-list] Returning printf from a function

[Florian Angeletti <octa@polychoron.fr>]

This is the value restriction at work, you can avoid the issue by replacing

let printer = get_printer true in ...


where the type of printer is "Format.formatter -> ('_weak1, 
Format.formatter, unit) format -> '_weak1",  with

let printer x = get_printer true x in ...


− octachron.

On 23/08/2018 11:15, Richard W.M. Jones wrote:
> This is a problem we hit from time to time and I don't think I've ever
> really understood why it happens.  This made up example shows the
> essence:
>
> ----------------------------------------------------------------------
> let get_printer out =
>    if out then Printf.printf else Printf.fprintf stderr
>
> let () =
>    let printer = get_printer true in
>    printer "hello\n";
>    printer "goodbye %s\n" "everyone"      (* line 7 *)
> ----------------------------------------------------------------------
>
> $ ocamlopt test2.ml
> File "test2.ml", line 7, characters 2-9:
> Error: This function has type (unit, out_channel, unit) format -> unit
>         It is applied to too many arguments; maybe you forgot a `;'.
>
> Type inference in line 6 seems to overspecify the inferred type of
> printer, so that we get the error on line 7.
>
> Why?  And how to fix it?
>
> Rich.
>