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 <[email protected]> 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:[email protected]] *On Behalf Of *Richard > Eisenberg > *Sent:* 09 March 2016 01:32 > *To:* Edward Kmett <[email protected]> > *Cc:* ghc-devs <[email protected]> > *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 <[email protected]> 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 <[email protected]> > wrote: > > > > On Mar 8, 2016, at 7:17 PM, Evan Austin <[email protected]> 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 > > > > > _______________________________________________ > ghc-devs mailing list > [email protected] > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > <https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc-devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c548849e42ffa4b50f01208d347ba9228%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=QKme1PQXRrwAkxAODUaPscdFk7ovwwwxHAFhDxpBXb8%3d> > > > > >
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