Control.Monad.Levels.State

Monad environments for stateful computations.

Documentation

state :: forall m s a. HasState s m => (s -> (a, s)) -> m a

Embed a simple state action into the monad stack.

get :: HasState s m => m s

Obtain the state environment.

gets :: HasState s m => (s -> a) -> m a

Apply a function to the stateful environment. Equivalent to fmap f get.

put :: HasState s m => s -> m ()

Replace the stateful environment.

modify :: HasState s m => (s -> s) -> m ()

Map the old state to a new state, and discard the old one. Equivalent to gets f >>= put.

modify' :: HasState s m => (s -> s) -> m ()

A variant of modify in which the computation is strict in the new state.

type HasState s m = SatisfyConstraint (IsState s) m

A monad stack containing a stateful environment of type s.

class MonadTower m => IsState s m

The minimal definition needed for a monad providing a stateful environment.

Instances

MonadTower m => IsState s (StateT s m)

MonadTower m => IsState s (StateT s m)

(MonadTower m, Monoid w) => IsState s (RWST r w s m)

(MonadTower m, Monoid w) => IsState s (RWST r w s m)

ValidConstraint (IsState s)

MonadTower m => ConstraintPassThrough (IsState s) (ListT m) True

MonadTower m => ConstraintPassThrough (IsState s) (ContT r m) True

type ConstraintSatisfied (IsState s) m