{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-}
import Data.Typeable;
data EnumBox = forall s. (Enum s, Show s) => EB s
deriving Typeable
instance Show EnumBox where
show (EB s) = "EB " ++ show s
これは機能します。しかし、EnumBox のように Class Enum のインスタンスを追加したい場合:
instance Enum EnumBox where
succ (EB s) = succ s
次のメッセージで失敗します。
Could not deduce (s ~ EnumBox)
from the context (Enum s, Show s)
bound by a pattern with constructor
EB :: forall s. (Enum s, Show s) => s -> EnumBox,
in an equation for `succ'
at typeclass.hs:11:9-12
`s' is a rigid type variable bound by
a pattern with constructor
EB :: forall s. (Enum s, Show s) => s -> EnumBox,
in an equation for `succ'
at typeclass.hs:11:9
In the first argument of `succ', namely `s'
In the expression: succ s
In an equation for `succ': succ (EB s) = succ s
最初のショーは推測できるのに、2 番目の成功は推測できないのはなぜですか?