Yuck.
But, many, many thanks for discovering this now instead of later.
For various silly reasons, I didn't have a platform (i.e. a recent enough HEAD
in the right state, etc.) for me to explore this today (Monday). But, I should
have the time and ability to look closer on Tuesday.
If it's really as bad as you say, we'll need to think carefully about how to
proceed. I have various thoughts about how to fix the problem, but none would
be ready for 7.8 by a long shot. So, do we issue warnings (that a GND, or
perhaps a use of `coerce` might not be safe) in 7.8? How would a user disable
these warnings?
Anyway, more tomorrow.
Richard
On Oct 13, 2013, at 6:01 PM, Edward Kmett <ekm...@gmail.com> wrote:
> I didn't think I was using GND much at all, but apparently I was wrong.
>
> After Ben's initial foray into building linear, I went and looked at the
> other applications of GeneralizedNewtypeDeriving in my code and found that in
> parsers, all of the derived instances for the supplied parser transformers
> fail.
>
> This also affects any attempt to use GND to do deriving for any monad
> transformer stack that isn't fully instantiated to concrete terms. This is
> actually a very common idiom to avoid writing boilerplate on top of
> transformers:
>
> newtype T m a = T { runT : StateT MyState (ReaderT MyEnv) m a }
> deriving (Functor, Monad, Applicative, MonadState MyState, MonadReader
> MyEnv, ...)
>
> As I rummage through more of my code, I actually can't find any instances of
> GND that do still work that aren't of the form:
>
> newtype Foo a = Foo { runFoo :: a } deriving ...
>
> Literally every other example I have of GND in the code I maintain has
> something in it that causes it to run afoul of the new roles machinery.
>
> I'd say the problem is more widespread than we thought.
>
> -Edward
>
>
> On Sun, Oct 13, 2013 at 5:26 PM, Edward Kmett <ekm...@gmail.com> wrote:
> Ben Gamari was trying to update my linear package to work with GHC HEAD.
>
> Along the way he noted an example of the new GND+role inference machinery
> failing rather spectacularly.
>
> http://hackage.haskell.org/package/linear-1.3/docs/src/Linear-Affine.html#Point
>
> Note the number of classes that the current proposal would force us to hand
> implement.
>
> =(
>
> -Edward
>
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