[MLton] Universal type wiki page wrong?

[Stephen Weeks]

> The page http://mlton.org/UniversalType seems to suggest that the following
> UNIVERSAL_TYPE implementation would be correct, just leaky:
> 
> structure U2:> UNIVERSAL_TYPE =
>   struct
>     type t = unit -> unit
>     fun embed () =
>       let
>         val r = ref NONE
>         fun inject a () = r := SOME a
>         fun project f = (f (); !r)
>       in (inject, project) end
>   end
> 
> I think this is wrong

You're right.  A correct, but leaky implementation would be

          fun project f = (r := NONE; f (); !r)