Relaxin the PVP with regards to adding instances

[Michael Sloan]

Interesting ideas!

One way to make explicit re-export pragmas less necessary would be to
ignore package-local re-exports.  This might seem funky initially,
there's precedent for involving packages with import semantics
(package-qualified imports), and this is a sensible granularity for
such judgments because it's reasonable to assume "global" knowledge of
instances inside a package.

To minimize need for explicit pragmas, we could have another variety
of module export that specifies which imports provide instances that
should be explicitly re-exported.  While the explicit pragma might
still be desirable (seems fiddly, though..), I think that this would
cover a great deal of the problems.  The syntax might look like:

module Foo(..., module instances Bar) where

import Bar

Could also think about "filtering" based on partial specifications -
"Only re-export Monad instances", "Only re-export things involving
Maybe", "Hide everything involving Maybe", etc etc.  This gets a bit
more complicated:

module Foo(..., module instances Bar(Monad, Maybe)) where

import Foo

I'm not at all sold on this more complicated one, but something along
this line might be good.

-mgsloan

On Thu, Sep 6, 2012 at 2:10 AM, Henning Thielemann
<schlepptop at henning-thielemann.de> wrote:
> Am 06.09.2012 02:43, schrieb Michael Sloan:
>
>> I agree that it's quite unfortunate that adding instances causes a
>> major bump.  I think that instead it would make sense to only bump
>> your major version whenever you add *orphan* instances.  This is
>> because it is known to be dangerous to create orphan instances, and
>> the errors are relatively transparent.  While I suppose it could break
>> proper dependency resolution, a big reason for upper bounds is
>> preventing a cascade of confusing errors.  With orphan instances the
>> errors are very finite.
>>
>> In general here are a few different things that would help with the
>> orphans problem:
>
>
> There have been some ideas about explicit import of instances.
>
> I also had some thoughts about how GHC can help managing orphan instances
> when they are necessary:
>   http://www.haskell.org/pipermail/haskell-cafe/2011-July/094014.html
>