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

[Leo White <lpw25@cam.ac.uk>]

Hi Dario,

> 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
>

This function relies on higher-kinded polymorphism (i.e. it is
polymorphic in type `u` which has type parameters). Since OCaml's core
language does not provide direct support for higher-kinded polymorphism
the usual solution is to make `actually_process` into a functor. A
possible alternative solution, which may suit your use case, is outlined
in the paper "Lightweight higher-kinded polymorphism" [1] using the
"higher" library available on OPAM.

Regards,

Leo

[1] Lightweight higher-kinded polymorphism
    Jeremy Yallop and Leo White   FLOPS 2014
    http://www.lpw25.net/flops2014.pdf