[Haskell-cafe] Bug or feature?

[Miguel Mitrofanov]

Hi cafe!

I'm a bit confused by the DefaultSignatures extension. It's unclear whether to consider the following an example of clever use of this extension, or an example of abuse of it:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
module SubClass where
import GHC.Prim(Constraint)
data Void (c :: * -> Constraint) = Void
data Evidence c a where Evidence :: c a => Evidence c a
class c1 :<= c2 where
    isSubClass :: c1 a => Void c1 -> Evidence c2 a
    default isSubClass :: c2 a => Void c1 -> Evidence c2 a
    isSubClass Void = Evidence
instance Show :<= Show
instance Floating :<= Fractional
instance Real :<= Num
-- instance Fractional :<= Floating -- NO ROTTEN WAY

{- Examples -}
data Wrapper c where Wrapper :: c a => a -> Wrapper c
instance (c :<= Show) => Show (Wrapper c) where
    show (Wrapper (a :: t)) = case isSubClass (Void :: Void c) :: Evidence Show t of Evidence -> show a
absWrap :: (c :<= Num) => Wrapper c -> Wrapper c
absWrap (Wrapper (a :: t) :: Wrapper c) = case isSubClass (Void :: Void c) :: Evidence Num t of Evidence -> Wrapper (abs a)

What do you think?

Also, it's a bit strange that the first example (instance (c :<= Show) => Show (Wrapper c)) requires UndecidableInstances, while the second one (absWrap) requires FlexibleContexts - although being remarkably similar.