We need to add role annotations for 7.8

Edward Kmett ekmett at gmail.com
Thu Mar 13 18:50:17 UTC 2014

That looks correct to me. Analogously for HashMap in unordered-containers
of course.

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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