Re: [Caml-list] Locally abstract type with type parameters

["John F. Carr" <jfc@mit.edu>]

I hit the same limitation last year:

https://sympa.inria.fr/sympa/arc/caml-list/2013-04/msg00159.html

The inability to constrain parameterized types in packed module
types is a documented restriction.  You will have to ask the type
theory experts whether it is a necessary restriction.

A followup message offered a workaround for some cases.

https://sympa.inria.fr/sympa/arc/caml-list/2013-04/msg00160.html

 > To avoid type-escaping-its-scope errors, we need to define a locally abstract
 > type in the implementation of 'actually_process'.  Something like this:
 > 
 >   let actually_process (type u) (module Logger: LOGGER with type 'a Monad.t = 'a u) x =
 >       let open Logger in
 >       let (>>=) t f = Monad.bind t f in
 >       Logger.log () >>= fun () ->
 >       Monad.return x
 > 
 > 
 > Which does not actually compile.  Is it at all possible to use a locally
 > abstract type when that type has type parameters?  And is there a solution
 > to this problem that does not require a) moving the implementation of
 > 'actually_process' to the inside of a functor, or b) pass each function
 > of the first-class module as a separate parameter to 'actually_process'?
 > 
 > Thanks in advance for your time!
 > Best regards,
 > Dario Teixeira