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.
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