I agree with David that using explicit `coerce`s can be quite verbose and may need ScopedTypeVariables and InstanceSigs. But visible type application should always work, because class methods always have a fixed type argument order. Regardless, requiring users to do all this for GND on Monad would be frustrating.
Actually, I just had an insight about this: there is no reason to use one deriving strategy for all methods in an instance. I can think of 4 ways to fill in the implementation of a class method in an instance: 1. Explicit, hand-written implementation 2. Defaulting to the implementation written in the class (or `error "undefined method"` in the absence of a default. This is essentially the default default.) 3. Stock implementation provided by GHC 4. Coerce Ways 2, 3, and 4 all have extra restrictions: Way 2 might have extra type constraints due to a `default` signature. Way 3 restricts the choice of class and type. Way 4 works only on newtypes and then imposes role restrictions on the method's type. GHC provides a `deriving` mechanism so that you can request Way 2 (`default`), 3 (`stock`), or 4 (`newtype`) to fill in every method in a class. But there's no need to provide this feature at such a course granularity. What about: > newtype N a = MkN (Foo a) > instance Blah a => C (N a) where > meth1 = ... > deriving default meth2 -- a bit silly really, as you can just leave meth2 > out > deriving stock meth3 -- also silly, as C isn't a stock class, but you > get the idea > deriving newtype meth4 We could also imagine > deriving newtype instance Blah a => Monad (N a) where > deriving default join -- not so silly anymore! This syntax allows a `where` clause on standalone deriving allowing you to override the overall `deriving` behavior on a per-method basis. I actually quite like this extension... Richard > On Jan 8, 2017, at 11:54 PM, David Feuer <david.fe...@gmail.com> wrote: > > You *can* do this, but it's often not so concise. When the type constructor > has parameters, you need to pin them down using ScopedTypeVariables. So you > end up needing to give a signature for the method type in order to bring into > scope variables you then use in the argument to coerce. If you have > > newtype Foo f a = Foo (Foo f a) > > then you may need > > instance Bar f => Bar (Foo f) where > bah = coerce (bah @ f @ a) > :: forall a . C a => ... > > to pin down the C instance. > > If you don't want to use explicit type application (e.g., you're using a > library that does not claim to have stable type argument order), things get > even more verbose. > > On Jan 8, 2017 11:32 PM, "Joachim Breitner" <m...@joachim-breitner.de > <mailto:m...@joachim-breitner.de>> wrote: > Hi, > > just responding to this one aspect: > > Am Sonntag, den 08.01.2017, 21:16 -0500 schrieb David Feuer: > > but using defaults for > > the others would give poor implementations. To cover this case, I > > think it would be nice to add per-method GND-deriving syntax. This > > could look something like > > > > instance C T where > > deriving f > > g = .... > > Assuming > newtype T = MkT S > > You can achieve this using > > instance C T where > f = coerce (f @F) > g = .... > > (which is precisely what GND does), so I don’t think any new syntax is > needed here. > > Greetings, > Joachim > > -- > Joachim “nomeata” Breitner > m...@joachim-breitner.de <mailto:m...@joachim-breitner.de> • > https://www.joachim-breitner.de/ <https://www.joachim-breitner.de/> > XMPP: nome...@joachim-breitner.de <mailto:nome...@joachim-breitner.de> • > OpenPGP-Key: 0xF0FBF51F > Debian Developer: nome...@debian.org <mailto:nome...@debian.org> > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org <mailto:Glasgow-haskell-users@haskell.org> > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > <http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users> > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
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