Changes to Typeable

[José Pedro Magalhães]

Hi,

On Mon, Oct 29, 2012 at 10:28 PM, Simon Peyton-Jones
<simonpj at microsoft.com>wrote:

>  Right. Re the library packages that have to change, I replied (4 Oct)
> thus****
>
> It’s tiresome, but for good reasons.  You have to talk to each of the
> maintainers of the other repos, and ask them to implement a patch, which
> you’ll supply.  The patch will probably have ****
>
>      #ifdef __GLASGOW_HASKELL__ >= 7.7****
>
> so that it’ll still work with older versions of the compiler. To give them
> the background you’ll probably want to update the wiki page describing the
> design and its benefits.****
>
> ** **
>
> However, you are right that if Typeable is always derived for all types,
> then maybe we can get away with no changes in those packages, which would
> be good.
>

Well, they will at least start throwing warnings, which means validate
won't pass.


> ****
>
> ** **
>
> 7.8: Any uses of "deriving Typeable" would give rise to a warning saying
> that it is no longer
> necessary. Any instances of Typeable would give rise to a warning saying
> that this code
> is being ignored, and replaced by an internal Typeable instance. Packages
> might break,
> or change runtime behaviour due to this change.****
>
> ** **
>
> Yes, that looks right.  Actually I think it’s extremely unlikely that
> anything would break unless there is something Extremely Naughty happening.
> ****
>
> ** **
>
> I say, go for it.  Any objections?  Would you be willing to do that
> Pedro?  Thanks!
>

Yes, but you'll have to guide me a bit on how to internalise the Typeable
instances...


Cheers,
Pedro


> ****
>
> ** **
>
> Simon****
>
> ** **
>
> *From:* josepedromagalhaes at gmail.com [mailto:josepedromagalhaes at gmail.com]
> *On Behalf Of *José Pedro Magalhães
> *Sent:* 29 October 2012 09:06
> *To:* Gábor Lehel; Simon Peyton-Jones; libraries at haskell.org
>
> *Subject:* Re: Changes to Typeable****
>
>  ** **
>
> Sorry, I got a bit lost in this discussion. Let me try to provide a
> summary.
>
>
> Current status: I have a local branch with the new poly-kinded Typeable
> working fine.
> It works as described in [1]. It actually allows deriving Typeable for
> things involving
> the Constraint kind, but this can be easily disabled. Either way, I think
> most of this
> is necessary for whatever might follow next. But I'm not sure of how to
> push the changes,
> because I had to make some changes to these repos: array, containers, dph,
> template-haskell, and vector. Worse, I also had to change time, which gets
> built from
> a tarball. It might not be worth contacting the authors of these packages
> for changes
> if we're still going to get rid of "deriving Typeable" altogether, so I've
> been holding this
> back.
>
> It's been proposed to remove the possibility to derive Typeable or write
> instances for it.
> I'm supposing the way that this would be implemented would be:
>
> 7.8: Any uses of "deriving Typeable" would give rise to a warning saying
> that it is no longer
> necessary. Any instances of Typeable would give rise to a warning saying
> that this code
> is being ignored, and replaced by an internal Typeable instance. Packages
> might break,
> or change runtime behaviour due to this change.
>
> 7.10: Explicit uses of "deriving Typeable" or instances are an error.
>
> Regarding split :: (a ~ f i) => Dict (Typeable f, Typeable i), I'm not
> sure I can judge how
> much work that would be. But let's first try to draft a plan for removing
> Typeable definitions
> from the user, and then consider more extensions.
>
>
> Cheers,
> Pedro
>
> [1] http://hackage.haskell.org/trac/ghc/wiki/GhcKinds/PolyTypeable****
>
>  On Tue, Oct 16, 2012 at 8:32 AM, Emil Axelsson <emax at chalmers.se> wrote:*
> ***
>
> 2012-10-15 23:50, Gábor Lehel skrev:****
>
> ** **
>
> On Mon, Oct 15, 2012 at 7:58 AM, Emil Axelsson <emax at chalmers.se> wrote:**
> **
>
>
> I have a use case:
>
>
>
> http://hackage.haskell.org/packages/archive/syntactic/1.3/doc/html/Data-DynamicAlt.html
>
> This is a reimplementation of Data.Dynamic to support casting type `a` to
> `Dynamic` given a constraint `Typeable (a -> b)`:
>
>    toDyn :: Typeable (a -> b) => P (a -> b) -> a -> Dynamic
>
> With your suggestion, it seems I should be able to use the ordinary
> Data.Dynamic instead.
>
> / Emil****
>
>
> Great! Do you like my plan? Or perhaps know of a better one?
>
> (Relatedly, *does* this have to go through a separate libraries
> process? Or are we considering Typeable as getting completely
> replaced, and everything pertaining to it gets discussed here?)****
>
> ** **
>
> Your plan certainly seems general enough! But I'm afraid I can't really
> speak about the implications on libraries etc.
>
> / Emil****
>
> ** **
>
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