Proposal: merge either into transformers

[Ross Paterson]

On Wed, Dec 12, 2012 at 05:20:06PM +0000, Henning Thielemann wrote:
> On Wed, 12 Dec 2012, Ross Paterson wrote:
> 
> > On Sun, Dec 09, 2012 at 01:24:36AM +0000, Ross Paterson wrote:
> >> A more economical variation on this idea would be to lift these classes
> >> to functors, e.g.
> >>
> >> class ShowF f where
> >>     showsPrecF :: Show a => Int -> f a -> ShowS
> >>
> >> instance (ShowF m, Show e, Show a) => Show (EitherT e m a) where
> >>     showsPrec d (EitherT m) = showParen (d > 10) $
> >>         showString "EitherT " . showsPrecF 11 m
> >>
> >> instance (ShowF m, Show e) => ShowF (EitherT e m) where
> >>     showsPrecF = showsPrec
> >
> > To also lift Prelude classes like Show to Compose (which seems
> > desirable), we'd need to use an explicit dictionary instead:
> >
> > class ShowF f where
> >    showsPrecF :: (Int -> a -> ShowS) -> Int -> f a -> ShowS
> >
> > instance (ShowF f, ShowF g) => ShowF (Compose f g) where
> >    showsPrecF sp d (Compose x) = showParen (d > 10) $
> >        showString "Compose " . showsPrecF (showsPrecF sp) 11 x
> >
> > instance (ShowF f, ShowF g, Show a) => Show (Compose f g a) where
> >    showsPrec = showsPrecF showsPrec
> 
> If you tolerate a (Functor f) constraint you may use a helper type, that 
> lifts (ShowF f, Show a) to (Show (f a)):
> 
> class ShowF f where
>      showsPrecF :: (Show a) => Int -> f a -> ShowS
> 
> newtype Inner g a = Inner (g a)
> 
> instance (ShowF g, Show a) => Show (Inner g a) where
>      showsPrec p (Inner g) = showsPrecF p g
> 
> instance (Functor f, ShowF f, ShowF g) => ShowF (Compose f g) where
>      showsPrecF d (Compose x) = showParen (d > 10) $
>          showString "Compose " . showsPrecF 11 (fmap Inner x)
> 
> instance (Functor f, ShowF f, ShowF g, Show a) => Show (Compose f g a) where
>      showsPrec = showsPrecF

Doing an fmap just to make an instance work is pretty unpleasant, but
having to hand-write all the instances, as the explicit dictionary
version would require, is probably worse.

> You may also make Functor a superclass of ShowF.

I'm not sure about that -- one might want to make Set an instance.

Assuming we have Eq1, Ord1, Show1 and Read1, what would be a good name
for the module defining them?