instantiating visible parameters in when deriving instances

[Andrew Martin]

I like number option number two. I don't really expect any of the
TypeInType stuff to work with the deriving machinery. I think that, at the
moment, for a normal deriving clause, GHC never adds in constraints (I
might be wrong on this). Whenever constraints, I feel like
StandaloneDeriving is the right choice. I don't know if StandaloneDeriving
works with DeriveFunctor or not, but if it does, then this is what I would
expect:

    REJECTED
    data Proxy k (a :: k) = P
      deriving Functor

    ACCEPTED
    data Proxy k (a :: k) = P
    deriving instance (k ~ *) => Functor Proxy k

But if the second code snippet involving StandaloneDeriving can't be made
to work, I would still prefer for the first snippet to be rejected as well.
Just my two cents.

-Andrew Thaddeus Martin

On Mon, Mar 28, 2016 at 8:55 AM, Richard Eisenberg <eir at cis.upenn.edu>
wrote:

> Hi devs,
>
> Consider the following:
>
> > data Proxy k (a :: k) = P
> >   deriving Functor
>
> What should happen when this is compiled?
>  1. Issue an error saying that `deriving` cannot instantiate visible
> parameters.
>  2. Type error: cannot match `k` with `*`.
>  3. Successfully produce `instance (Proxy *)`.
>
> Currently, GHC does #3. But this ends up choosing a value for a visible
> (i.e. explicit) parameter to Proxy. Is this a good idea? I myself have
> flip-flopped on this issue; see
> https://ghc.haskell.org/trac/ghc/ticket/11732, comments 4 and 9.
>
> I'd love to get feedback on this point.
>
> Thanks!
> Richard
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>



-- 
-Andrew Thaddeus Martin
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20160328/52d810a8/attachment-0001.html>