Relaxin the PVP with regards to adding instances

[Ganesh Sittampalam]

On 11/09/2012 18:04, Michael Sloan wrote:
> On Tue, Sep 11, 2012 at 5:32 AM, Ben Millwood <haskell at benmachine.co.uk> wrote:
>> On Tue, Sep 11, 2012 at 6:14 AM, Michael Sloan <mgsloan at gmail.com> wrote:
>>> Glad to hear that you agree that it's consistent!  This would make the
>>> PVP simpler.
>>>
>>> I've been thinking, there's still a way to retain the property that
>>> you can specify version bounds which retain compilation, even with
>>> orphan instances.  If your package really needs to add an instance to
>>> a datatype / class exported by some dependency, then you can put an
>>> upper limit on the minor version.  Harsh, I know, but orphans muck
>>> things up any way you slice it.
>>>
>>> This means that any orphans you use, you'd need to explicitly depend
>>> on an A.B.C upper-bounded version of the packages that define the
>>> datatypes (and classes, if you're being entirely correct), and not
>>> consume them through something that re-exports them.  Ideally, this
>>> could be mechanically checked.
>>
>> This seems sensible. However, in case this wasn't already clear, it
>> means when you /add/ an orphan instance, you still need a major
>> version bump: suppose I depend on package A and package B and have an
>> orphan for a type in package B, then you suggest I depend on
>> package-B-minor and package-A-major – so if package A adds an orphan
>> instance, they need to make their version bump major to stop me from
>> breaking.
> 
> I actually didn't write down what I meant to - instead of "any orphans
> you use" --> "any orphans you define".  Using orphans does not cause
> finer grained upper bounds.
> 
> Hmm, I hadn't thought of what you're pointing out!  So, the way to get
> the properties I was going for, even with orphans, is that as soon as
> you define an orphan instance, you must specify minor bounds on every
> package that could realistically want to define their own orphans.
> Definitely not ideal!  I guess the alternative is the "instance
> manifest" solution, and have Cabal use them for resolution (also hairy
> and troublesome)..

What's the problem with forcing a major version bump in every release
that adds an orphan instance?

Ganesh