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.
-Michael
On Thu, Sep 6, 2012 at 4:24 AM, Simon Marlow
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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