Re: [Caml-list] Why type inference fails in this code

[Gabriel Kerneis <kerneis@pps.jussieu.fr>]

On Fri, Oct 09, 2009 at 07:25:41AM +0200, rouanvd@softwarerealisations.com wrote:
> ======================================================
> type t = MyInt of int | MyFloat of float | MyString of string ;;
> 
> let foo printerf = function
>   | MyInt i -> printerf string_of_int i
>   | MyFloat x -> printerf string_of_float x
>   | MyString s -> printerf (fun x -> x) s
> ;;
> ======================================================

Type-inference has nothing to do with it, it's the type system itself
which fails.

What is the type you would expect? I guess:
(forall 'a. ('a -> string) -> 'a -> 'b) -> t -> 'b

This is impossible with Ocaml: you cannot have a universal quantifier,
type variables are existantially quantified.

> I read that there are lots of workarounds for the above code snippet,
> but I would like to Know why this code fails and what is the best
> workaround for this type of code.

You have many ways to emulate universal types in Ocaml (objects,
records) and browsing the archives of this list should provide many good
examples.

In you very specific case, though, I guess refactoring it is possible:
let foo printerf = function
  | MyInt i -> printerf (string_of_int i)
  | MyFloat x -> printerf (string_of_float x)
  | MyString s -> printerf s
;;

(depending on what your printerf function does, but I can't find any
counter-example)

Regards,
-- 
Gabriel Kerneis