arrows-0.2: Arrow classes and transformersSource codeContentsIndex
Portabilitynon-portable (multi-parameter type classes)

An arrow transformer that adds error handling.

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

data ErrorArrow ex a b c
runError :: ArrowChoice a => ErrorArrow ex a e b -> a (e, ex) b -> a e b
class (ArrowError ex a, Arrow a') => ArrowAddError ex a a' | a -> a' where
liftError :: a' e b -> a e b
elimError :: a e b -> a' (e, ex) b -> a' e b
data 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.
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:: ArrowChoice a
=> ErrorArrow ex a e bcomputation that may raise errors
-> a (e, ex) bcomputation 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.

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