| Copyright | (c) Ross Paterson 2003 |
|---|---|
| License | BSD-style (see the LICENSE file in the distribution) |
| Maintainer | [email protected] |
| Stability | experimental |
| Portability | non-portable (multi-parameter type classes) |
| Safe Haskell | Safe-Inferred |
| Language | Haskell98 |
Control.Arrow.Transformer.Error
Description
An arrow transformer that adds error handling.
TODO: the operations here are inconsistent with other arrow transformers.
Synopsis
- newtype ErrorArrow ex (a :: Type -> Type -> Type) b c = ErrorArrow (a b (Either ex 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 :: Type -> Type -> Type) (a' :: Type -> Type -> Type) | a -> a' where
Documentation
newtype ErrorArrow ex (a :: Type -> Type -> Type) 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 -> handlerclass (ArrowError ex a, Arrow a') => ArrowAddError ex (a :: Type -> Type -> Type) (a' :: Type -> Type -> Type) | a -> a' where Source #
Adding a 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 lift
method of the ArrowTransformer class,
or remove a state transformer from the top of the stack using the
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 lift and
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 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 b Source #
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 b Source #
Elimination of errors from a computation, by completely handling any errors.
Typical usage in arrow notation:
proc p -> ...
body `elimError` \ex -> handler