[Haskell-cafe] "SameConstraints?" type constraints

[Sean Leather]

On Fri, Jun 19, 2015 at 2:38 PM, Ivan Lazar Miljenovic wrote:

> On 19 June 2015 at 22:09, Leza Morais Lutonda wrote:
> > On 06/19/2015 03:02 AM, MigMit wrote:
> >>
> >> Typo.
> >>
> >> {-# LANGUAGE ConstraintKinds, GADTs #-}
> >> data SC c e where SC :: c d => d -> e -> SC c e
> >
> >
> > Yes, I have activated the GADTs extension too and the data definition
> itself
> > typechecks but the Show instance do not:
> >
> > instance Show e => Show (S c e) where
> >       show (SC x y) = show x ⧺ show y
> >
> > Because: Could not deduce (Show d) arising from a use of ‘show’
>
> This works for me if I also enable FlexibleInstances and restrict it to:
>
> instance (Show e) => Show (S Show e) where ...
>

Note this won't work for a Num instance mentioned earlier because the
existentially quantified d types in two SC values are not provably the same
type. In other words, you can't write

instance Num e => Show (S Num e) where
    SC x1 y1 + SC x2 y2 = SC (x1 + x2) (y1 + y2)

because x1 and x2 can have different types.

Regards,
Sean
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