Hiding top level declaration with tilde

[Herbert Valerio Riedel]

On 2014-01-09 at 08:46:28 +0100, Ramin Honary wrote:

[...]

> So as we all know, when there is no exports list in a module declaration,
> everything in the top level is exported by default. 
>
> If you provide an exports list, everything is hidden except what you
> declare in the exports list.

Btw, that's almost symmetric with an import statement lacking an
'impspec' and one providing a non-"hiding" 'impspec'; 

Otoh I've been wondering for some time why the export declaration
doesn't support the "hiding" modifier, e.g.

  module M hiding (A, B(..), C(..), abc) where
  ...

to make the export declaration a bit more symmetric with 'impsec' on the
import side.

> I think a good way to hide things without an exports list might be to
> simply prefix the type signature with a tilde character. For example:
>
>     module M where
>
>     ~type A = ()
>     ~newtype B = B ()
>     ~data C = MkB1 () | MkB2 ()
>
>     ~abc :: ()
>     abc = ()
>
>     increment :: Int -> Int
>     increment = (+1)
>
> So in this example, the module M would only export the "increment"
> function, everything else would be hidden.

So in order to hide a function/value this way, you have to write
a type-signature for 'abc', in order to be able to declare 'abc'
non-exported?

[...]

Cheers,
  hvr