We need to add role annotations for 7.8

Edward Kmett ekmett at gmail.com
Thu Mar 13 18:53:06 UTC 2014

Note with vector, Data.Vector.Vector is has a representational arg, but all
the other vector types have nominal args.

On Thu, Mar 13, 2014 at 2:52 PM, Edward Kmett <ekmett at gmail.com> wrote:

> Yep.
> The role annotation should be CPP guarded.
> On Thu, Mar 13, 2014 at 2:51 PM, Carter Schonwald <
> carter.schonwald at gmail.com> wrote:
>> it'd just need some CPP to maintain compat right?
>> We need it for vector and friends too right?
>> On Thu, Mar 13, 2014 at 2:49 PM, Johan Tibell <johan.tibell at gmail.com>wrote:
>>> If you send a pull request for containers I'll merge it and make a
>>> release. It needs to be backwards compatible with the last 3 major GHC
>>> releases. I think we need to bump the minor version number so people can
>>> reliable write dependency bounds to depend on the role being there (as that
>>> might be the difference between code compiling or not).
>>> Here's a guess which params should be representational:
>>> Map k v -- k: nominal, v: represententional
>>> Set a -- k: nominal
>>> IntMap v -- v: represententional
>>> Sequence a -- a: represententional
>>> Tree a -- a: represententional
>>> On Thu, Mar 13, 2014 at 7:41 PM, Richard Eisenberg <eir at cis.upenn.edu>wrote:
>>>> Hi libraries, Johan, Edward,
>>>> tl;dr: We really should add role annotations to the libraries that ship
>>>> with GHC 7.8.
>>>> In November, I sent this out:
>>>> http://www.haskell.org/pipermail/libraries/2013-November/021707.html
>>>> It got several good responses. Some time later, Joachim and I noticed
>>>> that no action was taken, so we made a bug report, #8718, with a milestone
>>>> of 7.8.1. I believe we both assumed that would take care of it. However,
>>>> Simon PJ and I were surprised earlier this week to discover that Map and
>>>> Set still do not have role annotations. Perhaps the GHC Trac isn't the
>>>> right place for libraries issues, but posting here didn't quite work, so
>>>> that seemed like the logical next step.
>>>> In any case, now that we have the ability to prevent abuses of
>>>> datatypes like Map and Set through erroneous use of
>>>> GeneralizedNewtypeDeriving (and the new `coerce`), we should take advantage
>>>> of this fact for 7.8. See my earlier email for a full explanation.
>>>> Possibly folks' unfamiliarity with roles are in part to blame for not
>>>> adding these role annotations. I am happy to help sort out what goes where.
>>>> But, I need to work with someone with an intimate familiarity with the
>>>> libraries themselves to help determine what the correct annotations are.
>>>> For example, Map's first parameter should be nominal (its Ord instance is
>>>> very relevant) but its second should be representational (there are no
>>>> particular invariants related to the values of a map). Map is
>>>> straightforward in this respect; other types probably aren't. Furthermore,
>>>> I don't have anywhere near a complete list of what even needs attention.
>>>> (It should be any type abstractly exported from a library.)
>>>> Can someone help? I've included Johan as the most recent uploader of
>>>> `containers` and Edward as the head of the core libraries committee.
>>>> Thanks!
>>>> Richard
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