PVP proposal: don't require major version bump when adding non-orphan instances

Leon Smith leon.p.smith at gmail.com
Wed Feb 26 17:28:35 UTC 2014

I feel indifferent on this proposal.

I certainly think the PVP is wrong in the case when a library both adds a
newtype or data type declaration and then adds an instance for it,  as it's
impossible to break downstream dependencies in this way.   So I've been
ignoring the strict letter of the PVP in this particular case,  although
I'd argue that I'm still following the spirit of the PVP.    So whether or
not this change is adopted,  I do think this point should be clarified.

My opinion on orphaned instances is that they should (almost always) be
avoided in hackage-released libraries,  but that they are mostly harmless
in executables,  whether hackage-released or not.   So under these
assumptions, it's still possible to break something;  the question in my
mind is whether or not it would be more work to tighten up dependencies
after the fact (under the optimistic assumption that things usually won't
break) or loosen things up.  Given that I'm not, at the moment, a fan of
upper bounds at all,  I don't feel like this tradeoff impacts me much.
 (Although obviously I'm coming down on the side of the optimistic
assumption.)  But I do think it's important to have a good semantics for
the PVP.

(And incidentally,  once (if?) we start adjusting version ranges without
updating the version of the package,  then I will adopt upper bounds,  and
I also feel that this issue would become mostly moot anyway.)

I also second Christian's comment that we really do need a more coherent
story on orphaned instances,  as avoiding them really does put a damper on


On Wed, Feb 26, 2014 at 11:01 AM, Mario Blažević <mblazevic at stilo.com>wrote:

> On 14-02-26 07:37 AM, Johan Tibell wrote:
>> ...
>> If this is true, we could change the first two PVP rules to (differences
>> in /italics/):
>>   * If any entity was removed, or the types of any entities or the
>>     definitions of datatypes or classes were changed, or /orphan/
>>     instances were added or /any instances were/ 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.
>>   * Otherwise, if only new bindings, types, classes, /non-orphan
>>     instances/, or modules (but see below) were added to the interface,
>>     then A.B may remain the same but the new C must be greater than the
>>     old C.
>> and add the following clarifying sentence:
>> /If a package defines an orphan instance, it must depend on the minor
>> version of the packages that define the data type and the type class to
>> be backwards compatible. For example, build-depends: mypkg >= 2.1.1 && <
>> 2.1.2./
> +1, except I'd add the following ammendment:
> In the interim period (for a year or so after the PVP change), the library
> maintainers that wish to take advantage of the change (i.e., add a new
> instance while incrementing only the minor package version) should check if
> any of the packages depending on their libraries contain clashing orphan
> instances. If so, they need to try to contact the depending package
> maintainers and get them to tighten the dependency bounds before the minor
> release. After the interim period is over, the responsibility falls onto
> the maintainers of the depending packages.
>         Also, we should consider reserving the third component of the
> version for instance additions, and relegating function and type additions
> to lower components.
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