| Copyright | (c) Ross Paterson 2003 |
|---|---|
| License | BSD-style (see the LICENSE file in the distribution) |
| Maintainer | [email protected] |
| Stability | experimental |
| Portability | non-portable (multi-parameter type classes) |
| Safe Haskell | Safe-Inferred |
| Language | Haskell98 |
Control.Arrow.Transformer.State
Description
An arrow transformer that adds a modifiable state, based of section 9 of Generalising Monads to Arrows, by John Hughes, Science of Computer Programming 37:67-111, May 2000.
Synopsis
- newtype StateArrow s (a :: Type -> Type -> Type) b c = StateArrow (a (b, s) (c, s))
- runState :: Arrow a => StateArrow s a e b -> a (e, s) (b, s)
- class (ArrowState s a, Arrow a') => ArrowAddState s (a :: Type -> Type -> Type) (a' :: Type -> Type -> Type) | a -> a' where
Documentation
newtype StateArrow s (a :: Type -> Type -> Type) b c Source #
An arrow type that augments an existing arrow with a modifiable
state. The ArrowState class contains the operations on this state.
Constructors
| StateArrow (a (b, s) (c, s)) |
Instances
runState :: Arrow a => StateArrow s a e b -> a (e, s) (b, s) Source #
Encapsulation of a state-using computation, exposing the initial and final states.
Typical usage in arrow notation:
proc p -> do
...
(result, final_state) <- (|runState cmd|) init_stateclass (ArrowState s a, Arrow a') => ArrowAddState s (a :: Type -> Type -> Type) (a' :: Type -> Type -> Type) | a -> a' where Source #
Adding a StateArrow to an
arrow type, but not necessarily as the outer arrow transformer.
Typically a composite arrow type is built by applying a series
of arrow transformer to a base arrow (usually either a function
arrow or a Kleisli arrow. One can add a transformer to the
top of this stack using the lift
method of the ArrowTransformer class,
or remove a state transformer from the top of the stack using the
runState encapsulation operator.
The methods of this class add and remove state transformers anywhere
in the stack. In the instance
instance Arrow a => ArrowAddState s (ArrowState s a) a
they are equivalent to lift and
runState respectively.
Instances are lifted through other transformers with
instance ArrowAddState s a a' =>
ArrowAddState s (FooArrow a) (FooArrow a')Methods
liftState :: a' e b -> a e b Source #
Lift a computation from an arrow to one with an added state.
Typical usage in arrow notation:
proc p -> ...
(|liftState cmd|)elimState :: a e b -> a' (e, s) (b, s) Source #
Elimination of a state transformer from a computation, exposing the initial and final states.
Typical usage in arrow notation:
proc p -> do
...
(result, final_state) <- (|elimState cmd|) init_state