Narrower (per-method) GND

[David Feuer]

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" <mail at 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
>   mail at joachim-breitner.de • https://www.joachim-breitner.de/
>   XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
>   Debian Developer: nomeata at debian.org
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