Relaxin the PVP with regards to adding instances

[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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