| Copyright | (c) Ross Paterson 2003 |
|---|---|
| License | BSD-style (see the LICENSE file in the distribution) |
| Maintainer | ross@soi.city.ac.uk |
| Stability | experimental |
| Portability | non-portable (multi-parameter type classes) |
| Safe Haskell | Safe |
| Language | Haskell98 |
Control.Arrow.Operations
Description
Subclasses of Arrow providing additional operations.
The signatures are designed to be compatible with the proposed notation for arrows, cf. http://www.haskell.org/arrows/.
Synopsis
- class Arrow a => ArrowState s a | a -> s where
- class Arrow a => ArrowReader r a | a -> r where
- class (Monoid w, Arrow a) => ArrowWriter w a | a -> w where
- class Arrow a => ArrowError ex a | a -> ex where
- tryInUnlessDefault :: (ArrowError ex a, ArrowChoice a) => a e b -> a (e, b) c -> a (e, ex) c -> a e c
- class ArrowLoop a => ArrowCircuit a where
Conventions
The arrow classes defined in this module have names like ArrowFoo,
and contain operations specific to such arrows. Some of these include
a method newFoo, which maps computations to computations of the
same arrow type, but exposing some of the internals of the arrow.
Arrow transformers have names like BarArrow, and are
instances of appropriate arrow classes. For each arrow
transformer, there is typically an encapsulation operator
runBar that removes that transformer from the outside of an
arrow type. The lift method of the
ArrowTransformer class adds an arrow
transformer to the outside of an arrow type.
Typically a composite arrow type is built by applying a series of arrow
transformers to a base arrow (usually either a function arrow or a
Kleisli arrow. The lift method and the
runBar function operate only on the arrow transformer at the top
of this stack. For more sophisticated manipulation of this stack of
arrow transformers, many arrow transformers provide an ArrowAddBar
class, with methods methods liftBar and elimBar to add and remove
the transformer anywhere in the stack.
State transformers
class Arrow a => ArrowState s a | a -> s where #
An arrow type that provides a modifiable state, based of section 9 of Generalising Monads to Arrows, by John Hughes, Science of Computer Programming 37:67-111, May 2000.
Methods
Obtain the current value of the state.
Assign a new value to the state.
Instances
| ArrowState s a => ArrowState s (Automaton a) # | |
| ArrowState s a => ArrowState s (StreamArrow a) # | |
Defined in Control.Arrow.Transformer.Stream | |
| (ArrowState s a, ArrowChoice a) => ArrowState s (ErrorArrow ex a) # | |
Defined in Control.Arrow.Transformer.Error | |
| ArrowState s a => ArrowState s (ReaderArrow r a) # | |
Defined in Control.Arrow.Transformer.Reader | |
| Arrow a => ArrowState s (StateArrow s a) # | |
Defined in Control.Arrow.Transformer.State | |
| (ArrowState s a, Applicative f) => ArrowState s (StaticArrow f a) # | |
Defined in Control.Arrow.Transformer.Static | |
| (ArrowState s a, Monoid w) => ArrowState s (WriterArrow w a) # | |
Defined in Control.Arrow.Transformer.Writer | |
State readers
class Arrow a => ArrowReader r a | a -> r where #
An arrow type that provides a read-only state (an environment).
If you also need to modify the state, use ArrowState.
Methods
Obtain the current value of the state.
newReader :: a e b -> a (e, r) b #
Run a subcomputation in the same arrow, but with a different environment. The environment of the outer computation is unaffected.
Typical usage in arrow notation:
proc p -> ...
(|newReader cmd|) envInstances
Monoid writers
class (Monoid w, Arrow a) => ArrowWriter w a | a -> w where #
An arrow type that collects additional output (of some Monoid type).
Methods
Add a piece of additional output.
newWriter :: a e b -> a e (b, w) #
Run a subcomputation in the same arrow, making its additional output accessible.
Typical usage in arrow notation:
proc p -> do
...
(value, output) <- (|newWriter cmd|)Instances
Errors
class Arrow a => ArrowError ex a | a -> ex where #
An arrow type that includes errors (or exceptions).
Minimal definition: raise and tryInUnless.
TODO: the operations here are inconsistent with other arrow transformers.
Minimal complete definition
Methods
Raise an error.
Arguments
| :: a e b | computation that may raise errors |
| -> a (e, ex) b | computation to handle errors |
| -> a e b |
Traditional exception construct.
Typical usage in arrow notation:
proc p -> ...
body `handle` \ex -> handlerArguments
| :: a e b | computation that may raise errors |
| -> a (e, b) c | computation to receive successful results |
| -> a (e, ex) c | computation to handle errors |
| -> a e c |
Exception construct in the style of Exceptional Syntax, by Nick Benton and Andrew Kennedy, JFP 11(4):395-410, July 2001.
Typical usage in arrow notation:
proc p -> ...
(|tryInUnless
body
(\res -> success)
(\ex -> handler)
|)newError :: a e b -> a e (Either ex b) #
Handler that returns the error as a value.
Instances
Arguments
| :: (ArrowError ex a, ArrowChoice a) | |
| => a e b | computation that may raise errors |
| -> a (e, b) c | computation to receive successful results |
| -> a (e, ex) c | computation to handle errors |
| -> a e c |
A suitable value for tryInUnless when the arrow type belongs to
ArrowChoice. To use it, you must define either handle or newError.
Synchronous circuits
class ArrowLoop a => ArrowCircuit a where #
An arrow type that can be used to interpret synchronous circuits.
Minimal complete definition
Methods
Arguments
| :: b | the value to return initially. |
| -> a b b | an arrow that propagates its input with a one-tick delay. |
A delay component.
Instances
| ArrowLoop a => ArrowCircuit (Automaton a) # | |
Defined in Control.Arrow.Transformer.Automaton | |
| ArrowLoop a => ArrowCircuit (StreamArrow a) # | |
Defined in Control.Arrow.Transformer.Stream Methods delay :: b -> StreamArrow a b b # | |
| ArrowCircuit a => ArrowCircuit (ReaderArrow r a) # | |
Defined in Control.Arrow.Transformer.Reader Methods delay :: b -> ReaderArrow r a b b # | |
| ArrowCircuit a => ArrowCircuit (StateArrow s a) # | |
Defined in Control.Arrow.Transformer.State Methods delay :: b -> StateArrow s a b b # | |
| (ArrowCircuit a, Applicative f) => ArrowCircuit (StaticArrow f a) # | |
Defined in Control.Arrow.Transformer.Static Methods delay :: b -> StaticArrow f a b b # | |
| (ArrowCircuit a, Monoid w) => ArrowCircuit (WriterArrow w a) # | |
Defined in Control.Arrow.Transformer.Writer Methods delay :: b -> WriterArrow w a b b # | |