typeclass0.hs の例でのコンパイルエラーを前に少し悩んだので。

typeclass0.hs

class Producer p where
     get :: Resource r => p -> r

class Resource a where
     fire :: a -> Int

data A = A Int
data B = B Int

instance Producer A where
     get (A x) = B x

instance Resource B where
     fire (B x) = x


c = A 32
r = get c :: B
a = fire r

{-
typeclass0.hs|11 col 18 error| Could not deduce (r ~ B)
|| Could not deduce (r ~ B)
|| from the context (Resource r)
||   bound by the type signature for get :: Resource r => A -> r
||   at test.hs:11:6-20
-}

typeclass1.hs

{-# LANGUAGE MultiParamTypeClasses #-}

class Producer p r where
     get :: Resource r => p -> r

class Resource a where
     fire :: a -> Int

data A = A Int
data B = B Int

instance Producer A B where
     get (A x) = B x

instance Resource B where
     fire (B x) = x


c = A 32
r = get c :: B
a = fire r