| Copyright | (c) Ross Paterson 2003 |
|---|---|
| License | BSD-style (see the LICENSE file in the distribution) |
| Maintainer | R.Paterson@city.ac.uk |
| Stability | experimental |
| Portability | non-portable (multi-parameter type classes) |
| Safe Haskell | Safe-Inferred |
| Language | Haskell98 |
Control.Arrow.Transformer.Writer
Description
Arrow transformer that adds accumulation of output.
Synopsis
- newtype WriterArrow w a b c = WriterArrow (a b (c, w))
- 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
newtype WriterArrow w a b c Source #
An arrow type that augments an existing arrow with accumulating
output. The ArrowWriter class contains the relevant operations.
Constructors
| WriterArrow (a b (c, w)) |
Instances
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' where Source #
Adding a 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 lift
method of the ArrowTransformer class,
or remove a state transformer from the top of the stack using the
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 lift and
runWriter respectively.
Instances are lifted through other transformers with
instance ArrowAddWriter w a a' =>
ArrowAddWriter w (FooArrow a) (FooArrow a')Methods
liftWriter :: a' e b -> a e b Source #
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|)