On Thu, 2009-04-09 at 01:24 +0200, Ben Franksen wrote:
BTW, is this (ContT t) somehow related to the 'free monad' over t?
The free monad over t is just data FreeMonad t a = Return a | JoinLift (t (FreeMonad t a)) instance Functor t => Monad (FreeMonad t) where return = Return Return x >>= f = f x JoinLift a >>= f = JoinLift ((>>= f) <$> a) lift :: Functor t => t a -> FreeMonad t a lift a = JoinLift (return <$> a) So they're obviously different. Here's what free monads are for: picking a functor f so that FreeMonad f becomes a randomly chosen monad :), we could define data IOStmt a = GetChar (Char -> a) | PutChar Char a instance Functor IOStmt where fmap f (GetChar g) = GetChar (f . g) fmap f (PutChar ch x) = PutChar ch (f x) getCharStmt :: IOStmt Char getCharStmt = GetChar id putCharStmt :: Char -> IOStmt () putCharStmt ch = PutChar ch () type IO = FreeMonad IOStmt getChar :: IO Char getChar = lift getCharStmt putChar :: Char -> IO () putChar = lift . putCharStmt jcc