Re: [Caml-list] Type That's Concrete From Within A Library Abstract From Without

[Malcolm Matalka <mmatalka@gmail.com>]

Jun Inoue <jun.lambda@gmail.com> writes:

> Dear list,
>
> Is there a way to make a type concrete inside a library, yet opaque to
> library users, preferably in a way that works with -pack?  This is a
> nagging issue in our sundials package
> (http://inria-parkas.github.io/sundialsml/).
>
> Basically, we have a type declared in one module of the library that
> is pattern-matched upon in other modules, like:
>
> (* private.ml *)
> type opaque_type = Foo | Bar
>
> (* public.ml *)
> let f : opaque_type -> int = function
>   | Foo -> 0
>   | Bar -> 1
>

Doesn't using .mli files do exactly what you want?

(* public.mli *)
type opaque_type
val f : opaque_type -> int

(* public.ml *)
type opaque_type = Foo | Bar

let f t = ....

> There are a few constraints:
> - We don't want users to be able to pattern-match on opaque_type.
> - We need multiple modules in the library to pattern-match on
> opaque-type (so moving opaque_typ e to public.ml is not an option).
> - To avoid namespace pollution, we want to pack the whole library
> (with ocamlc -pack) as a single Sundials module, so the user sees a
> Sundials.Public module instead of just Public.
>
> Is this possible?  Right now, we just collect public.cmo and
> private.cmo into sundials.cma and throw away private.cmi.  But this
> doesn't work with packing:
>
> $ ocamlc -pack -o sundials.cmo private.cmo public.cmo
>
> demands that there be a private.cmi.
>
> -- 
> Jun Inoue