We need to add role annotations for 7.8

[Carter Schonwald]

I think something like that is on the table, at least for a future ghc
release.  I'm not sure if it made it into the patches for 7.8.1. But this
was actually suggested on the relevant ticket last week. (I'm Afk otherwise
id dig up the link)

On Wednesday, March 26, 2014, Casey McCann <cam at uptoisomorphism.net> wrote:

> Were any rules considered along the lines of "Representational by
> default if all the type's constructors are exported by a module not
> named 'Internal', nominal by default otherwise"? Better would probably
> include "exported by a module the package exposes" but that's
> disgustingly non-local if it's even possible at all. The module name
> thing is hacky to the extreme but at least it's a simple rule rather
> than some obscure and opaque heuristics.
>
> Anyway, the goal of something like that would be not so much "figure
> out what it should be", since that's impossible, but more "default to
> nominal if and only if there's clear indication the user is already
> thinking about restricting how the type is used".
>
> Not that I'm really even suggesting such a rule, just wondering if it
> was discussed.
>
> - C.
>
>
> On Tue, Mar 25, 2014 at 7:23 PM, Richard Eisenberg <eir at cis.upenn.edu>
> wrote:
> > Hi Mark,
> >
> > I appreciate your analysis in terms of classes of users -- I think that
> is
> > helpful for framing the discussion.
> >
> > About transitivity: I think we're in the clear here. Let's say package A
> > exports types missing role annotations. If package B imports package A
> and
> > wants to have the full safety afforded by roles, that is no problem
> > whatsoever. Package B has annotations on its types (which may use package
> > A's types) that may restrict certain parameters to be nominal, as
> > appropriate. If package A had role annotations, it's quite possible that
> > package B could omit some annotations (as role inference propagates
> nominal
> > roles), but there is no problem inherent in this. (Indeed, if package A
> adds
> > annotations in the future, package B would have redundant, but harmless,
> > annotations.) So, I disagree with Mark's "partially" below -- I think
> we're
> > fully OK in this regard.
> >
> > About heuristics: we briefly considered some, though there's no
> > documentation of this anywhere. Specifically, we thought about giving
> > nominal roles to parameters used in class constraints. The problem is, in
> > the actual datatype definition, the constraints tend not to appear?
> Should
> > we look around for other functions with constraints? That seems likely
> to be
> > more confusing than helpful. Furthermore, I strongly don't like the idea
> of
> > using heuristics to infer a feature such as this -- it can cause strange
> > behavior and is hard to specify.
> >
> > Richard
> >
> > On Mar 25, 2014, at 11:09 AM, Mark Lentczner wrote:
> >
> > Thank you to everyone who has been helping me understand this issue in
> > greater depth.
> >
> > tl;dr: As long as we don't expect any libraries beyond to core to
> annotate,
> > I'm cool. This presumes that the extra safety isn't, in practice,
> dependent
> > on transitive adoption by libraries. It also implies that
> representational
> > is the only possible default, and that there can be no migration from it.
> >
> > My approach to thinking about this is guided by thinking about
> supporting an
> > eco-system with 1000s of libraries (hackage), a few dozen of which are
> > heavily promoted (the platform), and a small set that are closely tied to
> > the compiler (the core). The availability, speed of release, motivation,
> and
> > even skill of the the developers varies widely over that range.
> >
> > I also think about the various "stances" of different developers:
> >
> > End developer: makes use of libraries, but just builds apps
> > Internal developer: makes libraries for internal use in a project
> > Casual library writer: makes libraries, primarily for their own needs,
> but
> > distributed on hackage
> > Popular library writer: actively maintains libraries which are widely
> used
> > Core library writer: maintainer of a core package that stays in lock step
> > with the compiler
> >
> > Then, I think about, for each of these, what is the effect on a new
> feature
> > on them, their existing code, and future code? Does it affect them only
> if
> > they are using the feature? If they aren't using the feature? For library
> > writers, how does the feature affect clients? If a client wants to use a
> > feature, under what conditions does the library need to do something?
> This
> > last issue of the "transitivity" the feature is often the biggest
> concern.
> >
> > Given that... onto type roles:
> >
> > The default of representational is the only option, because a default of
> > nominal would require far too many developers to have to update their
> code.
> > I don't believe that we can ever migrate to nominal as default.
> >
> > The feature implies that any abstract data type that uses a type
> parameter
> > in certain ways needs annotate to get the full safety afforded now
> afforded.
> > However, without annotation, the data type is still no worse off than it
> was
> > before (there is added safety, but not perhaps relevant to the stand
> point
> > of the library writer). Further, this (pre-existing) non-safety isn't
> likely
> > a huge concern. Making sure the docs take the tone that most developers
> need
> > to nothing, and when developers need to be concerned seems like an
> important
> > way to ensure the right outcome.
> >
> > A key question here is transitivity: Is it possible for module A to not
> > annotate a type, and then have module B b>
> _______________________________________________
> > Libraries mailing list
> > Libraries at haskell.org <javascript:;>
> > http://www.haskell.org/mailman/listinfo/libraries
> >
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org <javascript:;>
> http://www.haskell.org/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/libraries/attachments/20140326/8609519a/attachment-0001.html>