Relaxin the PVP with regards to adding instances

Michael Sloan mgsloan at
Tue Sep 11 07:14:56 CEST 2012

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.

The maintenance of this kind of stuff can be reduced by centralizing
stuff in orphans package  (does Hackage 2.0 provide a way to
email-subscribe to a package? might also be good for maintaining these
sorts of things).  If a central orphans package isn't acceptable, then
you're probably not writing a library for general consumption anyway,
so upper limits on minor bounds wouldn't be problematic.


On Thu, Sep 6, 2012 at 4:24 AM, Simon Marlow <marlowsd at> wrote:
> On 06/09/2012 01:16, Johan Tibell wrote:
>> Hi all,
>> The PVP says:
>> "A.B is known as the major version number, and C the minor version
>> number. When a package is updated, the following rules govern how the
>> version number must change relative to the previous version:
>> 1. If any entity was removed, or the types of any entities or the
>> definitions of datatypes or classes were changed, or instances were
>> added or removed, then the new A.B must be greater than the previous
>> A.B. Note that modifying imports or depending on a newer version of
>> another package may cause extra instances to be exported and thus
>> force a major version change."
>> The part about adding instances and the one about modifying imports
>> makes it hard to follow the PVP. Bumping the major version number is a
>> quite disruptive change for your users if they use upper bounds on
>> their dependencies. Minor version bumps are not nearly as disruptive
>> as you can you can depend on x.y.* as long as you only use qualified
>> imports and/or explicit import lists.
>> Assuming no one uses orphan instances, adding instances is always safe
>> because you can only add instances for types from packages you depend
>> on, which means that they can't be depending on your package in turn
>> and have defined a non-orphan instance for a type or class defined in
>> your package.
>> I suggest that the rule be changed to not require a major version bump
>> if instances are added.
> Yes, I think this is reasonable.  The client already has some obligations if
> they want to be independent of minor versions: they have to use explicit
> import lists.  So adding another obligation, no orphan instances, is
> consistent with this and shouldn't cause problems in the majority of cases.
> Cheers,
>         Simon
>> P.S. I believe the same reasoning can be applied to the import part of
>> the rule above.
>> -- Johan
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