[Haskell-cafe] MPTCs and extended defaulting

[Tom Murphy]

With GHC 8.0.1 I get a type error:

error:
    • Could not deduce (Foo x a0)
      from the context: Foo x a
        bound by the type signature for:
                   foo :: Foo x a => x -> AB


On Fri, Sep 23, 2016 at 8:56 AM, Christopher Allen <cma at bitemyapp.com>
wrote:

> {-# LANGUAGE DataKinds #-}
> {-# LANGUAGE ExtendedDefaultRules #-}
> {-# LANGUAGE FlexibleInstances #-}
> {-# LANGUAGE KindSignatures #-}
> {-# LANGUAGE MultiParamTypeClasses #-}
> {-# LANGUAGE TypeFamilies #-}
>
> import GHC.TypeLits
>
> data AB = A | B Double deriving (Show)
>
> class Foo x (a :: [Symbol]) where
>    foo :: x -> AB
>
> instance (b ~ Double) => Foo b a where
>    foo = B
>
> main = print $ foo 5
>
> On Thu, Sep 22, 2016 at 6:39 PM, Tom Murphy <amindfv at gmail.com> wrote:
> > Bump -- is this feasible? It would be extremely helpful for an EDSL of
> mine.
> >
> > Tom
> >
> > On Tue, Aug 30, 2016 at 5:23 AM, Tom Murphy <amindfv at gmail.com> wrote:
> >>
> >> I've got a case in a library I'm working on where having
> >> -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it
> >> possible?
> >>
> >> I.e., we can now write:
> >>
> >> ```
> >> {-# LANGUAGE ExtendedDefaultRules #-}
> >>
> >> data AB = A | B Double deriving (Show)
> >>
> >> class Foo x where
> >>    foo :: x -> AB
> >>
> >> instance Foo Double where
> >>    foo = B
> >>
> >> main = print $ foo 5
> >> ```
> >>
> >> And -XExtendedDefaultRules makes sure we don't need to write "5 ::
> Double"
> >>
> >>
> >> If, though, I want 'Foo' to take another parameter (here, a ::
> [Symbol]),
> >> it falls apart:
> >>
> >> ```
> >> {-# LANGUAGE DataKinds #-}
> >> {-# LANGUAGE ExtendedDefaultRules #-}
> >> {-# LANGUAGE KindSignatures #-}
> >> {-# LANGUAGE MultiParamTypeClasses #-}
> >>
> >> import GHC.TypeLits
> >>
> >> data AB = A | B Double deriving (Show)
> >>
> >> class Foo x (a :: [Symbol]) where
> >>    foo :: x -> AB
> >>
> >> instance Foo Double a where
> >>    foo = B
> >>
> >> main = print $ foo 5
> >> ```
> >>
> >> Is there a reason MPTCs can't support ExtendedDefaultRules?
> >>
> >> Thanks!
> >> Tom
> >>
> >>
> >>
> >>
> >
> >
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>
>
>
> --
> Chris Allen
> Currently working on http://haskellbook.com
>
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