Narrower (per-method) GND

[Richard Eisenberg]

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.feuer at 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" <mail at joachim-breitner.de <mailto: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 <mailto:mail at joachim-breitner.de> • https://www.joachim-breitner.de/ <https://www.joachim-breitner.de/>
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