Re: [Caml-list] A limitation of "with type" declarations for first-class modules

[Jacques Le Normand <rathereasy@gmail.com>]

first post! let's see if anyone can do better...

module type Foo = sig type t end
module type Bar = sig type foo_t module Foo : Foo type t = foo_t   end

      (* compiles *)
let g (type a) (m : (module Foo with type t = a)) = ()

    (* fails to compile with a syntax error *)
let f (type a) (m : (module Bar with type foo_t = a)) = ()


On Tue, Sep 20, 2011 at 3:36 AM, Yaron Minsky <yminsky@gmail.com> wrote:
> For some reason, 1st-class modules have more restrictive "with" syntax,
> which turns out to be a practical problem.
>
> The main constraint is that with constraints do not seem to be able to refer
> to sub-modules.  Consider the following code snippet:
>
>>     module type Foo = sig type t end
>>     module type Bar = sig module Foo : Foo end
>>
>>     (* compiles *)
>>     let g (type a) (m : (module Foo with type t = a)) = ()
>>
>>     (* fails to compile with a syntax error *)
>>     let f (type a) (m : (module Bar with type Foo.t = a)) = ()
>
> Of course, ordinary modules have no such constraint.  Any thoughts as to
> what is going on here, and whether it can be fixed?  This has really
> restricted designs I've been using, forcing me to flatten out structures
> that are more naturally nested.
>
> y
>