djp@arqux.com wrote:
class Monad m => MonadState s m | m -> s where
It is the vertical bar | that confuses me. What does that mean? How would one read this line in "natural language", say English?
The vertical bar signifies a functional dependency (cf. the ghc users' guide for details). It says here, that the monad determines the type of the state.
Does that mean it's ghc specific, i.e. not in Haskell 98 / 2010?
It's a type system extension, which is not in any standard. But associated types (a special kind of type families) serve essentially the same purpose in a cleaner way. class MonadState m where type StateOf m get :: m (StateOf m) put :: StateOf m -> m () instance MonadState (State s) where type StateOf (State s) = s ... This is equivalent to the definition with fundeps. You can find TF-based monad transformer libraries. Their package names usually end with "-tf" to indicate this. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/