Re: Narrower (per-method) GND

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

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