Copyright | (C) 2014-2016 Edward Kmett |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | Edward Kmett <ekmett@gmail.com> |

Stability | provisional |

Portability | Control.Monad.Error |

Safe Haskell | Safe |

Language | Haskell98 |

## Synopsis

- catching :: MonadError e m => Getting (First a) e a -> m r -> (a -> m r) -> m r
- catching_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r
- handling :: MonadError e m => Getting (First a) e a -> (a -> m r) -> m r -> m r
- handling_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r
- trying :: MonadError e m => Getting (First a) e a -> m r -> m (Either a r)
- catches :: MonadError e m => m a -> [Handler e m a] -> m a
- data Handler e m r = Handler (e -> Maybe a) (a -> m r)
- class Handleable e (m :: * -> *) (h :: * -> *) | h -> e m where
- throwing :: MonadError e m => AReview e t -> t -> m x
- throwing_ :: MonadError e m => AReview e () -> m x

# Catching

catching :: MonadError e m => Getting (First a) e a -> m r -> (a -> m r) -> m r #

Catch exceptions that match a given `ReifiedPrism`

(or any `ReifiedGetter`

, really).

`catching`

::`MonadError`

e m =>`Prism'`

e a -> m r -> (a -> m r) -> m r`catching`

::`MonadError`

e m =>`Lens'`

e a -> m r -> (a -> m r) -> m r`catching`

::`MonadError`

e m =>`Traversal'`

e a -> m r -> (a -> m r) -> m r`catching`

::`MonadError`

e m =>`Iso'`

e a -> m r -> (a -> m r) -> m r`catching`

::`MonadError`

e m =>`ReifiedGetter`

e a -> m r -> (a -> m r) -> m r`catching`

::`MonadError`

e m =>`ReifiedFold`

e a -> m r -> (a -> m r) -> m r

catching_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r #

Catch exceptions that match a given `ReifiedPrism`

(or any `ReifiedGetter`

), discarding
the information about the match. This is particuarly useful when you have
a

where the result of the `Prism'`

e ()`ReifiedPrism`

or `ReifiedFold`

isn't
particularly valuable, just the fact that it matches.

`catching_`

::`MonadError`

e m =>`Prism'`

e a -> m r -> m r -> m r`catching_`

::`MonadError`

e m =>`Lens'`

e a -> m r -> m r -> m r`catching_`

::`MonadError`

e m =>`Traversal'`

e a -> m r -> m r -> m r`catching_`

::`MonadError`

e m =>`Iso'`

e a -> m r -> m r -> m r`catching_`

::`MonadError`

e m =>`ReifiedGetter`

e a -> m r -> m r -> m r`catching_`

::`MonadError`

e m =>`ReifiedFold`

e a -> m r -> m r -> m r

# Handling

handling :: MonadError e m => Getting (First a) e a -> (a -> m r) -> m r -> m r #

A version of `catching`

with the arguments swapped around; useful in
situations where the code for the handler is shorter.

`handling`

::`MonadError`

e m =>`Prism'`

e a -> (a -> m r) -> m r -> m r`handling`

::`MonadError`

e m =>`Lens'`

e a -> (a -> m r) -> m r -> m r`handling`

::`MonadError`

e m =>`Traversal'`

e a -> (a -> m r) -> m r -> m r`handling`

::`MonadError`

e m =>`Iso'`

e a -> (a -> m r) -> m r -> m r`handling`

::`MonadError`

e m =>`ReifiedFold`

e a -> (a -> m r) -> m r -> m r`handling`

::`MonadError`

e m =>`ReifiedGetter`

e a -> (a -> m r) -> m r -> m r

handling_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r #

A version of `catching_`

with the arguments swapped around; useful in
situations where the code for the handler is shorter.

`handling_`

::`MonadError`

e m =>`Prism'`

e a -> m r -> m r -> m r`handling_`

::`MonadError`

e m =>`Lens'`

e a -> m r -> m r -> m r`handling_`

::`MonadError`

e m =>`Traversal'`

e a -> m r -> m r -> m r`handling_`

::`MonadError`

e m =>`Iso'`

e a -> m r -> m r -> m r`handling_`

::`MonadError`

e m =>`ReifiedGetter`

e a -> m r -> m r -> m r`handling_`

::`MonadError`

e m =>`ReifiedFold`

e a -> m r -> m r -> m r

# Trying

trying :: MonadError e m => Getting (First a) e a -> m r -> m (Either a r) #

`trying`

takes a `ReifiedPrism`

(or any `ReifiedGetter`

) to select which exceptions are caught
If the `Exception`

does not match the predicate, it is re-thrown.

`trying`

::`MonadError`

e m =>`Prism'`

e a -> m r -> m (`Either`

a r)`trying`

::`MonadError`

e m =>`Lens'`

e a -> m r -> m (`Either`

a r)`trying`

::`MonadError`

e m =>`Traversal'`

e a -> m r -> m (`Either`

a r)`trying`

::`MonadError`

e m =>`Iso'`

e a -> m r -> m (`Either`

a r)`trying`

::`MonadError`

e m =>`ReifiedGetter`

e a -> m r -> m (`Either`

a r)`trying`

::`MonadError`

e m =>`ReifiedFold`

e a -> m r -> m (`Either`

a r)

# Handlers

catches :: MonadError e m => m a -> [Handler e m a] -> m a #

This function exists to remedy a gap between the functionality of `Control.Exception`

and `Control.Monad.Error`

. `Control.Exception`

supplies `catches`

and
a notion of `Handler`

, which we duplicate here in a form suitable for
working with any `MonadError`

instance.

Sometimes you want to catch two different sorts of error. You could do something like

f =`handling`

_Foo handleFoo (`handling`

_Bar handleBar expr)

However, there are a couple of problems with this approach. The first is
that having two exception handlers is inefficient. However, the more
serious issue is that the second exception handler will catch exceptions
in the first, e.g. in the example above, if `handleFoo`

uses `throwError`

then the second exception handler will catch it.

Instead, we provide a function `catches`

, which would be used thus:

f =`catches`

expr [`handler`

_Foo handleFoo ,`handler`

_Bar handleBar ]

You need this when using `catches`

.

class Handleable e (m :: * -> *) (h :: * -> *) | h -> e m where #

Both `exceptions`

and Control.Exception provide a `Handler`

type.

This lets us write combinators to build handlers that are agnostic about the choice of which of these they use.

handler :: Typeable a => Getting (First a) e a -> (a -> m r) -> h r #

This builds a `Handler`

for just the targets of a given `Prism`

(or any `Getter`

, really).

`catches`

... [`handler`

`_AssertionFailed`

(s ->`$`

"Assertion Failed\n"`++`

s) ,`handler`

`_ErrorCall`

(s ->`$`

"Error\n"`++`

s) ]

This works ith both the `Handler`

type provided by `Control.Exception`

:

`handler`

::`Getter`

`SomeException`

a -> (a ->`IO`

r) ->`Handler`

r`handler`

::`Fold`

`SomeException`

a -> (a ->`IO`

r) ->`Handler`

r`handler`

::`Prism'`

`SomeException`

a -> (a ->`IO`

r) ->`Handler`

r`handler`

::`Lens'`

`SomeException`

a -> (a ->`IO`

r) ->`Handler`

r`handler`

::`Traversal'`

`SomeException`

a -> (a ->`IO`

r) ->`Handler`

r

and with the `Handler`

type provided by `Control.Monad.Catch`

:

`handler`

::`Getter`

`SomeException`

a -> (a -> m r) ->`Handler`

m r`handler`

::`Fold`

`SomeException`

a -> (a -> m r) ->`Handler`

m r`handler`

::`Prism'`

`SomeException`

a -> (a -> m r) ->`Handler`

m r`handler`

::`Lens'`

`SomeException`

a -> (a -> m r) ->`Handler`

m r`handler`

::`Traversal'`

`SomeException`

a -> (a -> m r) ->`Handler`

m r

and with the `Handler`

type provided by `Control.Monad.Error.Lens`

:

`handler`

::`Getter`

e a -> (a -> m r) ->`Handler`

e m r`handler`

::`Fold`

e a -> (a -> m r) ->`Handler`

e m r`handler`

::`Prism'`

e a -> (a -> m r) ->`Handler`

e m r`handler`

::`Lens'`

e a -> (a -> m r) ->`Handler`

e m r`handler`

::`Traversal'`

e a -> (a -> m r) ->`Handler`

e m r

handler_ :: Typeable a => Getting (First a) e a -> m r -> h r #

This builds a `Handler`

for just the targets of a given `Prism`

(or any `Getter`

, really).
that ignores its input and just recovers with the stated monadic action.

`catches`

... [`handler_`

`_NonTermination`

(`return`

"looped") ,`handler_`

`_StackOverflow`

(`return`

"overflow") ]

This works with the `Handler`

type provided by `Control.Exception`

:

`handler_`

::`Getter`

`SomeException`

a ->`IO`

r ->`Handler`

r`handler_`

::`Fold`

`SomeException`

a ->`IO`

r ->`Handler`

r`handler_`

::`Prism'`

`SomeException`

a ->`IO`

r ->`Handler`

r`handler_`

::`Lens'`

`SomeException`

a ->`IO`

r ->`Handler`

r`handler_`

::`Traversal'`

`SomeException`

a ->`IO`

r ->`Handler`

r

and with the `Handler`

type provided by `Control.Monad.Catch`

:

`handler_`

::`Getter`

`SomeException`

a -> m r ->`Handler`

m r`handler_`

::`Fold`

`SomeException`

a -> m r ->`Handler`

m r`handler_`

::`Prism'`

`SomeException`

a -> m r ->`Handler`

m r`handler_`

::`Lens'`

`SomeException`

a -> m r ->`Handler`

m r`handler_`

::`Traversal'`

`SomeException`

a -> m r ->`Handler`

m r

and with the `Handler`

type provided by `Control.Monad.Error.Lens`

:

`handler_`

::`Getter`

e a -> m r ->`Handler`

e m r`handler_`

::`Fold`

e a -> m r ->`Handler`

e m r`handler_`

::`Prism'`

e a -> m r ->`Handler`

e m r`handler_`

::`Lens'`

e a -> m r ->`Handler`

e m r`handler_`

::`Traversal'`

e a -> m r ->`Handler`

e m r

## Instances

Handleable SomeException IO Handler # | |

Typeable m => Handleable SomeException m (Handler m) # | |

Defined in Control.Lens.Internal.Exception | |

Handleable e m (Handler e m) # | |

# Throwing

throwing :: MonadError e m => AReview e t -> t -> m x #

Throw an `Exception`

described by a `ReifiedPrism`

.

`throwing`

l ≡`reviews`

l`throwError`

`throwing`

::`MonadError`

e m =>`Prism'`

e t -> t -> a`throwing`

::`MonadError`

e m =>`Iso'`

e t -> t -> a

throwing_ :: MonadError e m => AReview e () -> m x #

Similar to `throwing`

but specialised for the common case of
error constructors with no arguments.

data MyError = Foo | Bar makePrisms ''MyError`throwing_`

_Foo ::`MonadError`

MyError m => m a