[Haskell-beginners] class (Monad m) => MonadState s m | m -> s where - What does m | m -> s construct do?
daniel.is.fischer at googlemail.com
Mon Apr 11 17:44:46 CEST 2011
On Monday 11 April 2011 17:18:44, Amitava Shee wrote:
> I can across the following construct in ghc source
> class (Monad m) => MonadState s m | m -> s where
> get :: m s
> put :: s -> m ()
> I am not aware of the syntactic construct | m -> s just before the
> where? Can someone please explain it?
It introduces a "functional dependency" (look up FunctionalDependencies in
the GHC users' guide, section 7.??).
The '|' separates the instance head (MonadState s m) from the functional
dependenc(y|ies). Here there's one FunDep, m -> s, saying the class
parameter m (the monad) determines the class parameter s (the state).
So for each monad m, there can be at most one state type s such that m is a
'stateful monad' with state s.
If you're already familiar with type families, the remark that this
functional dependency is roughly equivalent to
class (Monad m) => MonadState m where
type MState m :: *
get :: m (MState m)
put :: MState m -> m ()
might be helpful.
> Thanks & Regards,
> Amitava Shee
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