haskell-jp / questions #39

newtype X a = X {unX = ReaderT A IO a}
 deriving
 (MonadReader A)

newtype Y a = Y {unY = ReaderT B X a}
 deriving
 (
  --MonadReader A -- 複数のMonadReaderにはできない
  --MonadReader B
 )

class MonadReader A => C1 where ...
class MonadReader B => C2 where ...
-- YをMonadReader A,Bどちらを要求するコンテキスト下でも使いたい。

Another solution to many of the same problems has been known for a while: free monads and extensible effects. As it happens, capability and free monads can be formally compared. In this paper, Mauro Jaskelioff and Russell O'Connor, prove that free monads are a special case of capabilities (it's not phrased in these terms, of course, but that's what the paper amounts to).

newtype YA a = YA (Y a) deriving (MonadReader A)
newtype YB a = YB (Y a) deriving (MonadReader B)

askA = coerce (ask @A @YA)
askB = coerce (ask @B @YB)

class 型クラス 型 where
　条件式

instance 型クラス 型 where
　条件式