arrows-0.4.3.0: Arrow classes and transformers

Portability non-portable (multi-parameter type classes) experimental ross@soi.city.ac.uk

Control.Arrow.Transformer.Error

Description

An arrow transformer that adds error handling.

TODO: the operations here are inconsistent with other arrow transformers.

Synopsis

# Documentation

newtype ErrorArrow ex a b c Source

An arrow that augments an existing arrow with possible errors. The ArrowError class contains methods for raising and handling these errors.

Constructors

 ErrorArrow (a b (Either ex c))

Instances

Arguments

 :: ArrowChoice a => ErrorArrow ex a e b computation that may raise errors -> a (e, ex) b computation to handle errors -> a e b

Encapsulate an error-raising computation, by completely handling any errors.

Typical usage in arrow notation:

proc p -> ...
body `runError` \ex -> handler

class (ArrowError ex a, Arrow a') => ArrowAddError ex a a' | a -> a' whereSource

Adding a Control.Arrow.Transformer.Error.ErrorArrow 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.Error.runError encapsulation operator. The methods of this class add and remove state transformers anywhere in the stack. In the instance

instance Arrow a => ArrowAddError ex (ArrowError ex a) a

they are equivalent to Control.Arrow.Transformer.lift and Control.Arrow.Transformer.Error.runError respectively. Instances are lifted through other transformers with

instance ArrowAddError ex a a' =>
ArrowAddError ex (FooArrow a) (FooArrow a')

This could be combined with Control.Arrow.Transformer.Error.handle, since the resulting arrow is always the arrow of the handler. Separating them has the advantage of consistency with the other arrows, and might give more helpful type error messages.

Methods

liftError :: a' e b -> a e bSource

Lift a computation from an arrow to one with error handling.

Typical usage in arrow notation:

proc p -> ...
(|liftError cmd|)

elimError :: a e b -> a' (e, ex) b -> a' e bSource

Elimination of errors from a computation, by completely handling any errors.

Typical usage in arrow notation:

proc p -> ...
body `elimError` \ex -> handler

Instances