| 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.Reader
Description
Arrow transformer that adds a read-only state (i.e. an environment).
Synopsis
- newtype ReaderArrow r (a :: Type -> Type -> Type) b c = ReaderArrow (a (b, r) c)
- runReader :: Arrow a => ReaderArrow r a e b -> a (e, r) b
- class (ArrowReader r a, Arrow a') => ArrowAddReader r (a :: Type -> Type -> Type) (a' :: Type -> Type -> Type) | a -> a' where
- liftReader :: a' e b -> a e b
- elimReader :: a e b -> a' (e, r) b
Documentation
newtype ReaderArrow r (a :: Type -> Type -> Type) b c Source #
An arrow type that augments an existing arrow with a read-only state
(or environment). The ArrowReader class contains the operations
on this state.
Constructors
| ReaderArrow (a (b, r) c) |
Instances
runReader :: Arrow a => ReaderArrow r a e b -> a (e, r) b Source #
Encapsulation of a state-reading computation, taking a value for the state.
Typical usage in arrow notation:
proc p -> ...
(|runReader cmd|) envclass (ArrowReader r a, Arrow a') => ArrowAddReader r (a :: Type -> Type -> Type) (a' :: Type -> Type -> Type) | a -> a' where Source #
Adding a ReaderArrow 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
runReader encapsulation operator.
The methods of this class add and remove state transformers anywhere
in the stack. In the instance
instance Arrow a => ArrowAddReader r (ArrowReader r a) a
they are equivalent to lift and
runReader respectively.
Instances are lifted through other transformers with
instance ArrowAddReader r a a' =>
ArrowAddReader r (FooArrow a) (FooArrow a')Methods
liftReader :: a' e b -> a e b Source #
Lift a computation from an arrow to one with an added environment.
Typical usage in arrow notation:
proc p -> ...
(|liftReader cmd|)elimReader :: a e b -> a' (e, r) b Source #
Elimination of a state reader from a computation, taking a value for the state.
Typical usage in arrow notation:
proc p -> ...
(|elimReader cmd|) env