I'm sure I've used the feature that you're proposing to remove, but I'll adapt. To be clear, the change means no loss of expressiveness, just that I'll sometimes have to use a helper type family (closed or open), right?
If I'm right there, then no complaints from me. Richard On Jun 24, 2014, at 4:07 AM, Simon Peyton Jones <simo...@microsoft.com> wrote: > I’m not sure when this ”feature” was added, but I’m pretty sure that my > original implementation of associated types was exactly what you describe in > the solution. Or did I miss anything? > > I think you are right. I think I added the new stuff in a fit of enthusiasm > one day, a fit that I am now regretting! But I’m just checking that no one > has meanwhile become addicted to it. > > Simon > > From: Manuel Chakravarty [mailto:mchakrava...@me.com] > Sent: 24 June 2014 08:54 > To: Simon Peyton Jones > Cc: GHC List; ghc-d...@haskell.org > Subject: Re: Associated type instances > > Simon, > > I’m not sure when this ”feature” was added, but I’m pretty sure that my > original implementation of associated types was exactly what you describe in > the solution. Or did I miss anything? > > Manuel > > Simon Peyton Jones <simo...@microsoft.com>: > Friends > > I want to make withdraw (or, rather, simplify) a little-known feature in GHC, > but before I do so I want to check that no one is going to have a heart > attack. > > Relevant bits of the user manual: > http://www.haskell.org/ghc/docs/latest/html/users_guide/type-families.html#assoc-decl > > All of this arose when thinking about fixing Trac #9063. > > I believe that this change will affect essentially nobody, and I propose to > implement forthwith in HEAD (and hence 7.10). > > Does anyone object? > > Thanks > > Simon > > > > The issue > > Consider this: > > class C a where > type T a b :: * > > instance C [x] where > type T [x] b = x -> b > That is just what you’d expect. But currently this is allowed too: > > instance C [x] where > type T [x] Int = x -> Int > type T [x] Bool = Bool -> x > That is, GHC 7.8 allows many associated type instances, provided they don’t > overlap. But, oddly you can’t further instantiate the instance pattern. This > would make just as much sense, but isn’t allowed: > > instance C [x] where > type T [Int] b = b -> Int > type T [Bool] b = Bool -> b > Moreover, as the user manual says, for an open kind like *, none of this > really makes sense. It really only makes sense for a closed kind. Something > like > > class D a where > type S (b :: Bool) a :: * > Now this would make some kind of sense: > > instance D [x] where > type S True [x] = x -> x > type S False [x] = x > But for closed kinds, you really want a closed type family. So this would be > better: > > instance D [x] where > type S b [x] = SHelp x b > > type family SHelp x b where > SHelp x True = x -> x > SHelp x False = x > > So yes, you do have to declare a named helper type, but you get something > much more perspicuous and explicit in exchange. > > All of this also applies to the default declaration(s) which you can supply > for an associated type (see 7.7.3.2 in the link above), only it’s a bit more > complicated and indirect. > > My solution > > I propose to simplify substantially, as follows: > > · The “shared arguments” of an associated type are the argument > positions that mention a type variable from the class header. So in class C > above, the first argument position of T is “shared”; and in class D, the > second argument position of S is shared. > > · A instance for an associated type (in a class instance declaration) > cf 7.7.3.1 must have > > o type variables in the non-shared argument positions, and > > o an exact copy of the corresponding instance header type in the shared > positions > > · For each associated type you can have > > o at most one default declaration in the class declaration > > o at most one type instance declaration in the class instance declaration > > > > > > _______________________________________________ > ghc-devs mailing list > ghc-d...@haskell.org > http://www.haskell.org/mailman/listinfo/ghc-devs > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
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