Constrained Type Families?

[Edward Kmett]

I'd pretty much just assumed that a class associated type with a default
definition was modeling the same thing as a default signature, just without
the 'default' keyword

-Edward

On Wed, Mar 9, 2016 at 7:39 AM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> Notably if a class associated type has a more general kind, we currently
> can't give a default definition for it that has a tighter kind.
>
>
>
> This is the same situation as holds for default class methods.
>
>
>
> BUT for the latter we invented default method signatures
> –XdefaultSignatures (user manual Section 9.8.1.4
> <http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#class-declarations>),
> which can be more restrictive than the signature implied by the method
> signature of the class.
>
>
>
> Maybe we can do the same for default declarations for associated types?
>
>
>
> Would someone like to open a ticket and tell the story?
>
>
>
> Simon
>
>
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Richard
> Eisenberg
> *Sent:* 09 March 2016 01:32
> *To:* Edward Kmett <ekmett at gmail.com>
> *Cc:* ghc-devs <ghc-devs at haskell.org>
> *Subject:* Re: Constrained Type Families?
>
>
>
> I see no good reason for this restriction -- I think that we should just
> remove the restriction instead of cooking up a workaround. Have you brought
> this up before? Perhaps make a ticket.
>
>
>
> Richard
>
>
>
> On Mar 8, 2016, at 8:24 PM, Edward Kmett <ekmett at gmail.com> wrote:
>
>
>
> If and when that feature lands would it be possible to use it to bypass a
> current limitation in class associated types?
>
>
>
> Notably if a class associated type has a more general kind, we currently
> can't give a default definition for it that has a tighter kind.
>
>
>
> e.g. I have some classes which are technically polykinded but where 90% of
> the instances instantiate that kind as *. The status quo prevents me from
> putting in a type default that would only be valid when the kind argument
> is *.
>
>
>
> -Edward
>
>
>
> On Tue, Mar 8, 2016 at 8:21 PM, Richard Eisenberg <eir at cis.upenn.edu>
> wrote:
>
>
>
> On Mar 8, 2016, at 7:17 PM, Evan Austin <e.c.austin at gmail.com> wrote:
>
> The wiki page for Phase I of Dependent Haskell describes an approach to
> constrained type families:
>
>
> https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase1#Typefamilyequationscanbeconstrained
>
>
>
> Did that land in GHC 8.0 and, if so, is the updated syntax documented
> somewhere?
>
>
>
> No, it didn't make it. The motivating test case seemed contrived and so we
> punted on this one.
>
>
>
> Do you have a use case that really needs this feature? That would help to
> motivate it for 8.2 or beyond.
>
>
>
> Thanks!
>
> Richard
>
>
>
>
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