instance export decls

[Serge D. Mechveliani]

On Thu, May 01, 2008 at 03:21:11PM +0100, Simon Peyton-Jones wrote:
> Indeed! I think it'd be good to allow type signatures, including 
> instance signatures, in export lists
> 
> module Foo(
>         data T (f :: * -> *),
>         instance Functor f => Eq (T f),
>         g :: T f -> T f
>   )
> 
> The first step is to evolve a well-worked-out design. I think that'd be 
> a very valuable thing for someone to do.  Indeed, I'd really like to 
> see it in Haskell', but it doesn't meet the "tried and tested" criterion.
 
Maybe, the GHC developers will first try it in GHC ?

> I'm a bit reluctant to invest effort in half-way-house solutions, though.

My idea was that the GHC developers may like to convert my suggestion into 
a carefully designed one and to implement it.



> | Behalf Of Serge D. Mechveliani
> | Sent: 22 April 2008 13:33
> | To: glasgow-haskell-users at haskell.org
> | Subject: instance export decls
> |
> | Dear GHC developers, people,
> |
> | Do you agree that there exists such a problem for a programmer as
> | recalling exported instances?
> | Have Haskell and/or GHC some constructs and tools to help the programmer
> | to recall the exported instances for a module?
> | Could GHC support the instance export messages for each module?
> |
> | In Haskell-98, the exported instances cannot be named explicitly in the
> | export list.
> | Right?
> | But for easier understanding of a program, it is desirable to allow to
> | name (in a short form) instances in the export list.
> | For this reason, I add comments, and write the export like this:
> |
> |   module Poly
> |   (WithHead(..), WithTail(..), -- classes
> |    Mon(..), Polynomial(..),
> |    lc, polMons
> |    -- , instances
> |    -- for Mon:         WithHead, List;
> |    -- for Polynomial:  WithHead, Cast Polynomial Mon
> |   )
> | The comment of kind  "-- , instances ..."
> | helps to recall which instances are exported, without inspecting all
> | the (lengthy) module source.
> |
> | But when the programmer changes the source, one often forgets to
> | add/remove the needed comments about instances.
> | I often forget them (maybe, lazy to recall) when I move pieces of code
> | between modules.
> |
> | So, I suggest the following feature for GHC.
> |
> | 1. To allow the export declarations of kind   instance {<type>}
> |    (maybe to suggest this for Haskell ?)
> | 2. If the module under compilation has the word `instance' in its
> |    export list,
> |    then  ghc  finds the difference set  diff  for the exported instance
> |    kinds  eKinds  and the instance kinds  nKinds  named in the export
> |    list. If  not $ null diff,  it issues the message:
> |    "Warning:  the exported instance kinds and the instance kinds named
> |               in the export differ in the following items: ...".
> |
> | Seeing such a message, the user corrects the export list in the source
> | according to  diff.
> |
> | For the export list and for their messages, it is probably better to
> | use a short denotation: the instance kind rather than full instance
> | declaration. In the instance kind, the part of "(...) =>" is skipped.
> |
> | What people think of this suggestion?
> |
> | Thank you in advance for your notes and help,
> |
> | -----------------
> | Serge Mechveliani
> | mechvel at botik.ru
>