Re: Problem with information hiding.

On Fri, Oct 02, 2015 at 11:57:17PM +0200, Gabriel Scherer [email protected] [ocaml_beginners] wrote:
> Signatures in OCaml have two purpose: they check compatibility, and
> they enforce type abstract. They check, and they hide.
> When you write
> 
>   module Chargram : GRAMMAR = struct
>      ...
>   end
> 
> you are enforcing the signature of Chargram to be GRAMMAR, and in
> particular to have an *abstract* token type (this is a sealing
> signature ascription). You could fix the type error by not enforcing a
> signature there:
> 
>   module Chargram = struct
>     ...
>   end
> 
> This removes hiding. You can still check by doing it separately, with
> for example:
> 
>   let () = let module C : GRAMMAR = Chargram in ()
> 
> You can also use the GRAMMAR signature, but relaxed to have the
> equality (type token = char), so that you hide a bit but keep what you
> need:
> 
> module Chargram : (GRAMMAR with type token = char) = struct
>    ...
> end

This looks like what I wanted.

In the complete program, Chargram is passed as a parameter to a functor 
that takes a GRAMMAR, so the first solution you propose won't work in 
context.

Other modules of type GRAMMAR will have a different type for token, and 
will also be passed to this functor.

I couldn't just leave out the typing of Chargram as GRAMMAR. 

I tried the second approach, and it works fine in the complete 
program.  I needed to know how to relax the signature.  It now 
type-checks and runs correctly.

Thank you.

-- hendrik

> 
> Either change makes your example code type-check on my machine.
> 
> On Fri, Oct 2, 2015 at 11:49 PM, Hendrik Boom [email protected]
> [ocaml_beginners] <[email protected]> wrote:
> > On Wed, Sep 30, 2015 at 09:41:20PM +0200, Gabriel Scherer [email protected] [ocaml_beginners] wrote:
> >> Could you provide a self-contained piece of code that fails to compile? It
> >> is easier to debug such problems when we see the error source, and when it
> >> is possible to experiment (check that a fix suggestion work) before sending
> >> a reply. Note that you don't need to make it minimal.
> >
> > I made it minimal, and it no longer looks like what I wanted to do -- I
> > managed to preserve the error while removing an entire module.
> >
> > But it still fails in the sme way:
> >
> > open Stream
> >
> > module type GRAMMAR =
> >   sig
> >     type token
> >   end;;
> >
> >
> > module Chargram : GRAMMAR =
> >   struct
> >     type token = char;; (* Should really be a module's type parameter *)
> >   end;;
> >
> > let testx : Chargram.token Stream.t = Stream.of_string "str";;
> >
> >
> > hendrik@notlookedfor:~/dv/parse/priority$ ocamlc ko1.ml
> > File "ko1.ml", line 15, characters 38-60:
> > Error: This expression has type char Stream.t
> >        but an expression was expected of type Chargram.token Stream.t
> >        Type char is not compatible with type Chargram.token
> > hendrik@notlookedfor:~/dv/parse/priority$
> >
> >
> >
> > I want to use chargram in a way that requires that token = char.
> >
> > I also want to have other modules of type GRAMMAR with different  types
> > for token.
> >
> >
> > -- hendrik