Portability | non-portable (multi-parameter type classes) |
---|---|
Stability | experimental |
Maintainer | ross@soi.city.ac.uk |
Arrow transformer that adds accumulation of output.
- data WriterArrow w a b c
- runWriter :: (Arrow a, Monoid w) => WriterArrow w a e b -> a e (b, w)
- class (ArrowWriter w a, Arrow a') => ArrowAddWriter w a a' | a -> a' where
- liftWriter :: a' e b -> a e b
- elimWriter :: a e b -> a' e (b, w)
Documentation
data WriterArrow w a b c Source
An arrow type that augments an existing arrow with accumulating
output. The ArrowWriter
class contains the relevant operations.
runWriter :: (Arrow a, Monoid w) => WriterArrow w a e b -> a e (b, w)Source
Encapsulation of a writer computation, providing the accumulated output.
Typical usage in arrow notation:
proc p -> do ... (result, output) <- (|runWriter cmd|)
class (ArrowWriter w a, Arrow a') => ArrowAddWriter w a a' | a -> a' whereSource
Adding a Control.Arrow.Transformer.Writer.WriterArrow
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 Control.Arrow.Transformer.lift
method of the Control.Arrow.Transformer.ArrowTransformer
class,
or remove a state transformer from the top of the stack using the
Control.Arrow.Transformer.Writer.runWriter
encapsulation operator.
The methods of this class add and remove state transformers anywhere
in the stack. In the instance
instance Arrow a => ArrowAddWriter w (ArrowWriter w a) a
they are equivalent to Control.Arrow.Transformer.lift
and
Control.Arrow.Transformer.Writer.runWriter
respectively.
Instances are lifted through other transformers with
instance ArrowAddWriter w a a' => ArrowAddWriter w (FooArrow a) (FooArrow a')
liftWriter :: a' e b -> a e bSource
Lift a computation from an arrow to one with added output.
Typical usage in arrow notation:
proc p -> ... (|liftWriter cmd|)
elimWriter :: a e b -> a' e (b, w)Source
Elimination of an output writer from a computation, providing the accumulated output.
Typical usage in arrow notation:
proc p -> do ... (result, output) <- (|elimWriter cmd|)
ArrowAddWriter w a a' => ArrowAddWriter w (Automaton a) (Automaton a') | |
(Arrow a, Monoid w) => ArrowAddWriter w (WriterArrow w a) a | |
(ArrowAddWriter w a a', ArrowChoice a, ArrowChoice a') => ArrowAddWriter w (ErrorArrow ex a) (ErrorArrow ex a') | |
ArrowAddWriter w a a' => ArrowAddWriter w (StateArrow s a) (StateArrow s a') | |
(ArrowAddWriter w a a', Applicative f) => ArrowAddWriter w (StaticArrow f a) (StaticArrow f a') | |
ArrowAddWriter s a a' => ArrowAddWriter s (ReaderArrow r a) (ReaderArrow r a') |