lens-4.15.3: Lenses, Folds and Traversals

Copyright | (C) 2012-16 Edward Kmett
License | BSD-style (see the file LICENSE)
Maintainer | Edward Kmett <ekmett@gmail.com>
Stability | provisional
Portability | Control.Exception
Safe Haskell | Trustworthy
Language | Haskell98

Control.Exception.Lens

Contents

- Handling
- Trying
- Throwing
- Mapping
- Exceptions
- Exception Handlers
  - IOExceptions
  - Arithmetic Exceptions
  - Array Exceptions
  - Assertion Failed
  - Async Exceptions
  - Non-Termination
  - Nested Atomically
  - Blocked Indefinitely
    - on MVar
    - on STM
  - Deadlock
  - No Such Method
  - Pattern Match Failure
  - Record
  - Error Call
  - Allocation Limit Exceeded
  - Type Error
- Handling Exceptions

Description

`Control.Exception` provides an example of a large open hierarchy that we can model with prisms and isomorphisms.

Additional combinators for working with `IOException` results can be found in System.IO.Error.Lens.

The combinators in this module have been generalized to work with `MonadCatch` instead of just `IO`. This enables them to be used more easily in `Monad` transformer stacks.

Synopsis

- catching :: MonadCatch m => Getting (First a) SomeException a -> m r -> (a -> m r) -> m r
- catching_ :: MonadCatch m => Getting (First a) SomeException a -> m r -> m r -> m r
- handling :: MonadCatch m => Getting (First a) SomeException a -> (a -> m r) -> m r -> m r
- handling_ :: MonadCatch m => Getting (First a) SomeException a -> m r -> m r -> m r
- trying :: MonadCatch m => Getting (First a) SomeException a -> m r -> m (Either a r)
- trying_ :: MonadCatch m => Getting (First a) SomeException a -> m r -> m (Maybe r)
- throwing :: AReview SomeException b -> b -> r
- throwing_ :: AReview SomeException () -> m x
- throwingM :: MonadThrow m => AReview SomeException b -> b -> m r
- throwingTo :: MonadIO m => ThreadId -> AReview SomeException b -> b -> m ()
- mappedException :: (Exception e, Exception e') => Setter s s e e'
- mappedException' :: Exception e' => Setter s s SomeException e'
- exception :: Exception a => Prism' SomeException a
- pattern Exception :: forall t. Exception t => t -> SomeException
- class Handleable e m h | h -> e m where
- class AsIOException t where
- pattern IOException_ :: forall s. AsIOException s => IOException -> s
- class AsArithException t where
- _Overflow :: AsArithException t => Prism' t ()
- _Underflow :: AsArithException t => Prism' t ()
- _LossOfPrecision :: AsArithException t => Prism' t ()
- _DivideByZero :: AsArithException t => Prism' t ()
- _Denormal :: AsArithException t => Prism' t ()
- _RatioZeroDenominator :: AsArithException t => Prism' t ()
- pattern ArithException_ :: forall s. AsArithException s => ArithException -> s
- pattern Overflow_ :: forall s. AsArithException s => s
- pattern Underflow_ :: forall s. AsArithException s => s
- pattern LossOfPrecision_ :: forall s. AsArithException s => s
- pattern DivideByZero_ :: forall s. AsArithException s => s
- pattern Denormal_ :: forall s. AsArithException s => s
- pattern RatioZeroDenominator_ :: forall s. AsArithException s => s
- class AsArrayException t where
- _IndexOutOfBounds :: AsArrayException t => Prism' t String
- _UndefinedElement :: AsArrayException t => Prism' t String
- pattern ArrayException_ :: forall s. AsArrayException s => ArrayException -> s
- pattern IndexOutOfBounds_ :: forall s. AsArrayException s => String -> s
- pattern UndefinedElement_ :: forall s. AsArrayException s => String -> s
- class AsAssertionFailed t where
- pattern AssertionFailed_ :: forall s. AsAssertionFailed s => String -> s
- class AsAsyncException t where
- _StackOverflow :: AsAsyncException t => Prism' t ()
- _HeapOverflow :: AsAsyncException t => Prism' t ()
- _ThreadKilled :: AsAsyncException t => Prism' t ()
- _UserInterrupt :: AsAsyncException t => Prism' t ()
- pattern AsyncException_ :: forall s. AsAsyncException s => AsyncException -> s
- pattern StackOverflow_ :: forall s. AsAsyncException s => s
- pattern HeapOverflow_ :: forall s. AsAsyncException s => s
- pattern ThreadKilled_ :: forall s. AsAsyncException s => s
- pattern UserInterrupt_ :: forall s. AsAsyncException s => s
- class AsNonTermination t where
- pattern NonTermination_ :: forall s. AsNonTermination s => s
- class AsNestedAtomically t where
- pattern NestedAtomically_ :: forall s. AsNestedAtomically s => s
- class AsBlockedIndefinitelyOnMVar t where
- pattern BlockedIndefinitelyOnMVar_ :: forall s. AsBlockedIndefinitelyOnMVar s => s
- class AsBlockedIndefinitelyOnSTM t where
- pattern BlockedIndefinitelyOnSTM_ :: forall s. AsBlockedIndefinitelyOnSTM s => s
- class AsDeadlock t where
- pattern Deadlock_ :: forall s. AsDeadlock s => s
- class AsNoMethodError t where
- pattern NoMethodError_ :: forall s. AsNoMethodError s => String -> s
- class AsPatternMatchFail t where
- pattern PatternMatchFail_ :: forall s. AsPatternMatchFail s => String -> s
- class AsRecConError t where
- class AsRecSelError t where
- class AsRecUpdError t where
- pattern RecConError_ :: forall s. AsRecConError s => String -> s
- pattern RecSelError_ :: forall s. AsRecSelError s => String -> s
- pattern RecUpdError_ :: forall s. AsRecUpdError s => String -> s
- class AsErrorCall t where
- pattern ErrorCall_ :: forall s. AsErrorCall s => String -> s
- class AsAllocationLimitExceeded t where
- pattern AllocationLimitExceeded_ :: forall s. AsAllocationLimitExceeded s => s
- class AsTypeError t where
- pattern TypeError_ :: forall s. AsTypeError s => String -> s
- class AsHandlingException t where
- pattern HandlingException_ :: forall s. AsHandlingException s => s

# Handling

catching :: MonadCatch m => Getting (First a) SomeException a -> m r -> (a -> m r) -> m r #

Catch exceptions that match a given `Prism` (or any `Fold`, really).

    >>> catching _AssertionFailed (assert False (return "uncaught")) $ \ _ -> return "caught"
    "caught"

    catching :: MonadCatch m => Prism' SomeException a     -> m r -> (a -> m r) -> m r
    catching :: MonadCatch m => Lens' SomeException a      -> m r -> (a -> m r) -> m r
    catching :: MonadCatch m => Traversal' SomeException a -> m r -> (a -> m r) -> m r
    catching :: MonadCatch m => Iso' SomeException a       -> m r -> (a -> m r) -> m r
    catching :: MonadCatch m => Getter SomeException a     -> m r -> (a -> m r) -> m r
    catching :: MonadCatch m => Fold SomeException a       -> m r -> (a -> m r) -> m r

catching_ :: MonadCatch m => Getting (First a) SomeException a -> m r -> m r -> m r #

Catch exceptions that match a given `Prism` (or any `Getter`), discarding the information about the match. This is particuarly useful when you have a ``Prism'` e ()` where the result of the `Prism` or `Fold` isn't particularly valuable, just the fact that it matches.

    >>> catching_ _AssertionFailed (assert False (return "uncaught")) $ return "caught"
    "caught"

    catching_ :: MonadCatch m => Prism' SomeException a     -> m r -> m r -> m r
    catching_ :: MonadCatch m => Lens' SomeException a      -> m r -> m r -> m r
    catching_ :: MonadCatch m => Traversal' SomeException a -> m r -> m r -> m r
    catching_ :: MonadCatch m => Iso' SomeException a       -> m r -> m r -> m r
    catching_ :: MonadCatch m => Getter SomeException a     -> m r -> m r -> m r
    catching_ :: MonadCatch m => Fold SomeException a       -> m r -> m r -> m r

handling :: MonadCatch m => Getting (First a) SomeException 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 _NonTermination (\_ -> return "caught") $ throwIO NonTermination
    "caught"

    handling :: MonadCatch m => Prism' SomeException a     -> (a -> m r) -> m r -> m r
    handling :: MonadCatch m => Lens' SomeException a      -> (a -> m r) -> m r -> m r
    handling :: MonadCatch m => Traversal' SomeException a -> (a -> m r) -> m r -> m r
    handling :: MonadCatch m => Iso' SomeException a       -> (a -> m r) -> m r -> m r
    handling :: MonadCatch m => Fold SomeException a       -> (a -> m r) -> m r -> m r
    handling :: MonadCatch m => Getter SomeException a     -> (a -> m r) -> m r -> m r

handling_ :: MonadCatch m => Getting (First a) SomeException 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_ _NonTermination (return "caught") $ throwIO NonTermination
    "caught"

    handling_ :: MonadCatch m => Prism' SomeException a     -> m r -> m r -> m r
    handling_ :: MonadCatch m => Lens' SomeException a      -> m r -> m r -> m r
    handling_ :: MonadCatch m => Traversal' SomeException a -> m r -> m r -> m r
    handling_ :: MonadCatch m => Iso' SomeException a       -> m r -> m r -> m r
    handling_ :: MonadCatch m => Getter SomeException a     -> m r -> m r -> m r
    handling_ :: MonadCatch m => Fold SomeException a       -> m r -> m r -> m r

# Trying

trying :: MonadCatch m => Getting (First a) SomeException a -> m r -> m (Either a r) #

A variant of `try` that takes a `Prism` (or any `Fold`) to select which exceptions are caught (c.f. `tryJust`, `catchJust`). If the `Exception` does not match the predicate, it is re-thrown.

    trying :: MonadCatch m => Prism'     SomeException a -> m r -> m (Either a r)
    trying :: MonadCatch m => Lens'      SomeException a -> m r -> m (Either a r)
    trying :: MonadCatch m => Traversal' SomeException a -> m r -> m (Either a r)
    trying :: MonadCatch m => Iso'       SomeException a -> m r -> m (Either a r)
    trying :: MonadCatch m => Getter     SomeException a -> m r -> m (Either a r)
    trying :: MonadCatch m => Fold       SomeException a -> m r -> m (Either a r)

trying_ :: MonadCatch m => Getting (First a) SomeException a -> m r -> m (Maybe r) #

A version of `trying` that discards the specific exception thrown.

    trying_ :: MonadCatch m => Prism'     SomeException a -> m r -> m (Maybe r)
    trying_ :: MonadCatch m => Lens'      SomeException a -> m r -> m (Maybe r)
    trying_ :: MonadCatch m => Traversal' SomeException a -> m r -> m (Maybe r)
    trying_ :: MonadCatch m => Iso'       SomeException a -> m r -> m (Maybe r)
    trying_ :: MonadCatch m => Getter     SomeException a -> m r -> m (Maybe r)
    trying_ :: MonadCatch m => Fold       SomeException a -> m r -> m (Maybe r)

# Throwing

throwing :: AReview SomeException b -> b -> r #

Throw an `Exception` described by a `Prism`. Exceptions may be thrown from purely functional code, but may only be caught within the `IO` `Monad`.

    throwing l ≡ reviews l throw

    throwing :: Prism' SomeException t -> t -> r
    throwing :: Iso' SomeException t   -> t -> r

throwing_ :: AReview SomeException () -> 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

throwingM :: MonadThrow m => AReview SomeException b -> b -> m r #

A variant of `throwing` that can only be used within the `IO` `Monad` (or any other `MonadCatch` instance) to throw an `Exception` described by a `Prism`.

Although `throwingM` has a type that is a specialization of the type of `throwing`, the two functions are subtly different:

    throwing l e `seq` x  ≡ throwing e
    throwingM l e `seq` x ≡ x

The first example will cause the `Exception` `e` to be raised, whereas the second one won't. In fact, `throwingM` will only cause an `Exception` to be raised when it is used within the `MonadCatch` instance. The `throwingM` variant should be used in preference to `throwing` to raise an `Exception` within the `Monad` because it guarantees ordering with respect to other monadic operations, whereas `throwing` does not.

    throwingM l ≡ reviews l throw

    throwingM :: MonadThrow m => Prism' SomeException t -> t -> m r
    throwingM :: MonadThrow m => Iso' SomeException t   -> t -> m r

throwingTo :: MonadIO m => ThreadId -> AReview SomeException b -> b -> m () #

`throwingTo` raises an `Exception` specified by a `Prism` in the target thread.

    throwingTo thread l ≡ reviews l (throwTo thread)

    throwingTo :: ThreadId -> Prism' SomeException t -> t -> m a
    throwingTo :: ThreadId -> Iso' SomeException t   -> t -> m a

# Mapping

mappedException :: (Exception e, Exception e') => Setter s s e e' #

This `Setter` can be used to purely map over the `Exception`s an arbitrary expression might throw; it is a variant of `mapException` in the same way that `mapped` is a variant of `fmap`.

    'mapException' ≡ 'over' 'mappedException'

This view that every Haskell expression can be regarded as carrying a bag of `Exception`s is detailed in “A Semantics for Imprecise Exceptions” by Peyton Jones & al. at PLDI ’99.

The following maps failed assertions to arithmetic overflow:

    >>> handling _Overflow (\_ -> return "caught") $ assert False (return "uncaught") & mappedException %~ \ (AssertionFailed _) -> Overflow
    "caught"

mappedException' :: Exception e' => Setter s s SomeException e' #

This is a type restricted version of `mappedException`, which avoids the type ambiguity in the input `Exception` when using `set`.

The following maps any exception to arithmetic overflow:

    >>> handling _Overflow (\_ -> return "caught") $ assert False (return "uncaught") & mappedException' .~ Overflow
    "caught"

# Exceptions

exception :: Exception a => Prism' SomeException a #

Traverse the strongly typed `Exception` contained in `SomeException` where the type of your function matches the desired `Exception`.

    exception :: (Applicative f, Exception a)
              => (a -> f a) -> SomeException -> f SomeException

pattern Exception :: forall t. Exception t => t -> SomeException #

# Exception Handlers

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.

Minimal complete definition

handler

Methods

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 -> print $ "Assertion Failed\n" ++ s)
                , handler _ErrorCall (s -> print $ "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 # |
Methods handler :: Typeable * a => Getting (First a) SomeException a -> (a -> IO r) -> Handler r # handler_ :: Typeable * a => Getting (First a) SomeException a -> IO r -> Handler r #
Typeable (* -> *) m => Handleable SomeException m (Handler m) # |
Methods handler :: Typeable * a => Getting (First a) SomeException a -> (a -> m r) -> Handler m r # handler_ :: Typeable * a => Getting (First a) SomeException a -> m r -> Handler m r #
Handleable e m (Handler e m) # |
Methods handler :: Typeable * a => Getting (First a) e a -> (a -> m r) -> Handler e m r # handler_ :: Typeable * a => Getting (First a) e a -> m r -> Handler e m r #

## IOExceptions

class AsIOException t where #

Exceptions that occur in the `IO` `Monad`. An `IOException` records a more specific error type, a descriptive string and maybe the handle that was used when the error was flagged.

Due to their richer structure relative to other exceptions, these have a more carefully overloaded signature.

Methods

_IOException :: Prism' t IOException #

Unfortunately the name `ioException` is taken by `base` for throwing IOExceptions.

    _IOException :: Prism' IOException IOException
    _IOException :: Prism' SomeException IOException

Many combinators for working with an `IOException` are available in System.IO.Error.Lens.

Instances

AsIOException IOException # |
Methods _IOException :: Prism' IOException IOException #
AsIOException SomeException # |
Methods _IOException :: Prism' SomeException IOException #

pattern IOException_ :: forall s. AsIOException s => IOException -> s #

## Arithmetic Exceptions

class AsArithException t where #

Arithmetic exceptions.

Methods

_ArithException :: Prism' t ArithException #

Instances

AsArithException ArithException # |
Methods _ArithException :: Prism' ArithException ArithException #
AsArithException SomeException # |
Methods _ArithException :: Prism' SomeException ArithException #

_Overflow :: AsArithException t => Prism' t () #

Handle arithmetic `_Overflow`.

    _Overflow ≡ _ArithException . _Overflow

    _Overflow :: Prism' ArithException ArithException
    _Overflow :: Prism' SomeException  ArithException

_Underflow :: AsArithException t => Prism' t () #

Handle arithmetic `_Underflow`.

    _Underflow ≡ _ArithException . _Underflow

    _Underflow :: Prism' ArithException ArithException
    _Underflow :: Prism' SomeException  ArithException

_LossOfPrecision :: AsArithException t => Prism' t () #

Handle arithmetic loss of precision.

    _LossOfPrecision ≡ _ArithException . _LossOfPrecision

    _LossOfPrecision :: Prism' ArithException ArithException
    _LossOfPrecision :: Prism' SomeException  ArithException

_DivideByZero :: AsArithException t => Prism' t () #

Handle division by zero.

    _DivideByZero ≡ _ArithException . _DivideByZero

    _DivideByZero :: Prism' ArithException ArithException
    _DivideByZero :: Prism' SomeException  ArithException

_Denormal :: AsArithException t => Prism' t () #

Handle exceptional _Denormalized floating pure.

    _Denormal ≡ _ArithException . _Denormal

    _Denormal :: Prism' ArithException ArithException
    _Denormal :: Prism' SomeException  ArithException

_RatioZeroDenominator :: AsArithException t => Prism' t () #

Added in `base` 4.6 in response to this libraries discussion:

http://haskell.1045720.n5.nabble.com/Data-Ratio-and-exceptions-td5711246.html

    _RatioZeroDenominator ≡ _ArithException . _RatioZeroDenominator

    _RatioZeroDenominator :: Prism' ArithException ArithException
    _RatioZeroDenominator :: Prism' SomeException  ArithException

pattern ArithException_ :: forall s. AsArithException s => ArithException -> s #

pattern Overflow_ :: forall s. AsArithException s => s #

pattern Underflow_ :: forall s. AsArithException s => s #

pattern LossOfPrecision_ :: forall s. AsArithException s => s #

pattern DivideByZero_ :: forall s. AsArithException s => s #

pattern Denormal_ :: forall s. AsArithException s => s #

pattern RatioZeroDenominator_ :: forall s. AsArithException s => s #

## Array Exceptions

class AsArrayException t where #

Exceptions generated by array operations.

Methods

_ArrayException :: Prism' t ArrayException #

Extract information about an `ArrayException`.

    _ArrayException :: Prism' ArrayException ArrayException
    _ArrayException :: Prism' SomeException  ArrayException

Instances

AsArrayException ArrayException # |
Methods _ArrayException :: Prism' ArrayException ArrayException #
AsArrayException SomeException # |
Methods _ArrayException :: Prism' SomeException ArrayException #

_IndexOutOfBounds :: AsArrayException t => Prism' t String #

An attempt was made to index an array outside its declared bounds.

    _IndexOutOfBounds ≡ _ArrayException . _IndexOutOfBounds

    _IndexOutOfBounds :: Prism' ArrayException String
    _IndexOutOfBounds :: Prism' SomeException  String

_UndefinedElement :: AsArrayException t => Prism' t String #

An attempt was made to evaluate an element of an array that had not been initialized.

    _UndefinedElement ≡ _ArrayException . _UndefinedElement

    _UndefinedElement :: Prism' ArrayException String
    _UndefinedElement :: Prism' SomeException  String

pattern ArrayException_ :: forall s. AsArrayException s => ArrayException -> s #

pattern IndexOutOfBounds_ :: forall s. AsArrayException s => String -> s #

pattern UndefinedElement_ :: forall s. AsArrayException s => String -> s #

## Assertion Failed

class AsAssertionFailed t where #

`assert` was applied to `False`.

Methods

_AssertionFailed :: Prism' t String #

This `Exception` contains provides information about what assertion failed in the `String`.

    >>> handling _AssertionFailed (\ xs -> "caught" <$ guard ("<interactive>" `isInfixOf` xs) ) $ assert False (return "uncaught")
    "caught"

    _AssertionFailed :: Prism' AssertionFailed String
    _AssertionFailed :: Prism' SomeException   String

Instances

AsAssertionFailed AssertionFailed # |
Methods _AssertionFailed :: Prism' AssertionFailed String #
AsAssertionFailed SomeException # |
Methods _AssertionFailed :: Prism' SomeException String #

pattern AssertionFailed_ :: forall s. AsAssertionFailed s => String -> s #

## Async Exceptions

class AsAsyncException t where #

Asynchronous exceptions.

Methods

_AsyncException :: Prism' t AsyncException #

There are several types of `AsyncException`.

    _AsyncException :: Equality' AsyncException AsyncException
    _AsyncException :: Prism'    SomeException  AsyncException

Instances

AsAsyncException AsyncException # |
Methods _AsyncException :: Prism' AsyncException AsyncException #
AsAsyncException SomeException # |
Methods _AsyncException :: Prism' SomeException AsyncException #

_StackOverflow :: AsAsyncException t => Prism' t () #

The current thread's stack exceeded its limit. Since an `Exception` has been raised, the thread's stack will certainly be below its limit again, but the programmer should take remedial action immediately.

    _StackOverflow :: Prism' AsyncException ()
    _StackOverflow :: Prism' SomeException  ()

_HeapOverflow :: AsAsyncException t => Prism' t () #

The program's heap is reaching its limit, and the program should take action to reduce the amount of live data it has.

Notes:

- It is undefined which thread receives this `Exception`.
- GHC currently does not throw `HeapOverflow` exceptions.

    _HeapOverflow :: Prism' AsyncException ()
    _HeapOverflow :: Prism' SomeException  ()

_ThreadKilled :: AsAsyncException t => Prism' t () #

This `Exception` is raised by another thread calling `killThread`, or by the system if it needs to terminate the thread for some reason.

    _ThreadKilled :: Prism' AsyncException ()
    _ThreadKilled :: Prism' SomeException  ()

_UserInterrupt :: AsAsyncException t => Prism' t () #

This `Exception` is raised by default in the main thread of the program when the user requests to terminate the program via the usual mechanism(s) (e.g. Control-C in the console).

    _UserInterrupt :: Prism' AsyncException ()
    _UserInterrupt :: Prism' SomeException  ()

pattern AsyncException_ :: forall s. AsAsyncException s => AsyncException -> s #

pattern StackOverflow_ :: forall s. AsAsyncException s => s #

pattern HeapOverflow_ :: forall s. AsAsyncException s => s #

pattern ThreadKilled_ :: forall s. AsAsyncException s => s #

pattern UserInterrupt_ :: forall s. AsAsyncException s => s #

## Non-Termination

class AsNonTermination t where #

Thrown when the runtime system detects that the computation is guaranteed not to terminate. Note that there is no guarantee that the runtime system will notice whether any given computation is guaranteed to terminate or not.

Methods

_NonTermination :: Prism' t () #

There is no additional information carried in a `NonTermination` `Exception`.

    _NonTermination :: Prism' NonTermination ()
    _NonTermination :: Prism' SomeException  ()

Instances

AsNonTermination NonTermination # |
Methods _NonTermination :: Prism' NonTermination () #
AsNonTermination SomeException # |
Methods _NonTermination :: Prism' SomeException () #

pattern NonTermination_ :: forall s. AsNonTermination s => s #

## Nested Atomically

class AsNestedAtomically t where #

Thrown when the program attempts to call atomically, from the `STM` package, inside another call to atomically.

Methods

_NestedAtomically :: Prism' t () #

There is no additional information carried in a `NestedAtomically` `Exception`.

    _NestedAtomically :: Prism' NestedAtomically ()
    _NestedAtomically :: Prism' SomeException    ()

Instances

AsNestedAtomically NestedAtomically # |
Methods _NestedAtomically :: Prism' NestedAtomically () #
AsNestedAtomically SomeException # |
Methods _NestedAtomically :: Prism' SomeException () #

pattern NestedAtomically_ :: forall s. AsNestedAtomically s => s #

## Blocked Indefinitely

### on MVar

class AsBlockedIndefinitelyOnMVar t where #

The thread is blocked on an `MVar`, but there are no other references to the `MVar` so it can't ever continue.

Methods

_BlockedIndefinitelyOnMVar :: Prism' t () #

There is no additional information carried in a `BlockedIndefinitelyOnMVar` `Exception`.

    _BlockedIndefinitelyOnMVar :: Prism' BlockedIndefinitelyOnMVar ()
    _BlockedIndefinitelyOnMVar :: Prism' SomeException             ()

Instances

AsBlockedIndefinitelyOnMVar BlockedIndefinitelyOnMVar # |
Methods _BlockedIndefinitelyOnMVar :: Prism' BlockedIndefinitelyOnMVar () #
AsBlockedIndefinitelyOnMVar SomeException # |
Methods _BlockedIndefinitelyOnMVar :: Prism' SomeException () #

pattern BlockedIndefinitelyOnMVar_ :: forall s. AsBlockedIndefinitelyOnMVar s => s #

### on STM

class AsBlockedIndefinitelyOnSTM t where #

The thread is waiting to retry an `STM` transaction, but there are no other references to any TVars involved, so it can't ever continue.

Methods

_BlockedIndefinitelyOnSTM :: Prism' t () #

There is no additional information carried in a `BlockedIndefinitelyOnSTM` `Exception`.

    _BlockedIndefinitelyOnSTM :: Prism' BlockedIndefinitelyOnSTM ()
    _BlockedIndefinitelyOnSTM :: Prism' SomeException            ()

Instances

AsBlockedIndefinitelyOnSTM BlockedIndefinitelyOnSTM # |
Methods _BlockedIndefinitelyOnSTM :: Prism' BlockedIndefinitelyOnSTM () #
AsBlockedIndefinitelyOnSTM SomeException # |
Methods _BlockedIndefinitelyOnSTM :: Prism' SomeException () #

pattern BlockedIndefinitelyOnSTM_ :: forall s. AsBlockedIndefinitelyOnSTM s => s #

## Deadlock

class AsDeadlock t where #

There are no runnable threads, so the program is deadlocked. The `Deadlock` `Exception` is raised in the main thread only.

Methods

_Deadlock :: Prism' t () #

There is no information carried in a `Deadlock` `Exception`.

    _Deadlock :: Prism' Deadlock      ()
    _Deadlock :: Prism' SomeException ()

Instances

AsDeadlock Deadlock # |
Methods _Deadlock :: Prism' Deadlock () #
AsDeadlock SomeException # |
Methods _Deadlock :: Prism' SomeException () #

pattern Deadlock_ :: forall s. AsDeadlock s => s #

## No Such Method

class AsNoMethodError t where #

A class method without a definition (neither a default definition, nor a definition in the appropriate instance) was called.

Methods

_NoMethodError :: Prism' t String #

Extract a description of the missing method.

    _NoMethodError :: Prism' NoMethodError String
    _NoMethodError :: Prism' SomeException String

Instances

AsNoMethodError NoMethodError # |
Methods _NoMethodError :: Prism' NoMethodError String #
AsNoMethodError SomeException # |
Methods _NoMethodError :: Prism' SomeException String #

pattern NoMethodError_ :: forall s. AsNoMethodError s => String -> s #

## Pattern Match Failure

class AsPatternMatchFail t where #

A pattern match failed.

Methods

_PatternMatchFail :: Prism' t String #

Information about the source location of the pattern.

    _PatternMatchFail :: Prism' PatternMatchFail String
    _PatternMatchFail :: Prism' SomeException    String

Instances

AsPatternMatchFail PatternMatchFail # |
Methods _PatternMatchFail :: Prism' PatternMatchFail String #
AsPatternMatchFail SomeException # |
Methods _PatternMatchFail :: Prism' SomeException String #

pattern PatternMatchFail_ :: forall s. AsPatternMatchFail s => String -> s #

## Record

class AsRecConError t where #

An uninitialised record field was used.

Methods

_RecConError :: Prism' t String #

Information about the source location where the record was constructed.

    _RecConError :: Prism' RecConError   String
    _RecConError :: Prism' SomeException String

Instances

AsRecConError RecConError # |
Methods _RecConError :: Prism' RecConError String #
AsRecConError SomeException # |
Methods _RecConError :: Prism' SomeException String #

class AsRecSelError t where #

A record selector was applied to a constructor without the appropriate field. This can only happen with a datatype with multiple constructors, where some fields are in one constructor but not another.

Methods

_RecSelError :: Prism' t String #

Information about the source location where the record selection occurred.

Instances

AsRecSelError RecSelError # |
Methods _RecSelError :: Prism' RecSelError String #
AsRecSelError SomeException # |
Methods _RecSelError :: Prism' SomeException String #

class AsRecUpdError t where #

A record update was performed on a constructor without the appropriate field. This can only happen with a datatype with multiple constructors, where some fields are in one constructor but not another.

Methods

_RecUpdError :: Prism' t String #

Information about the source location where the record was updated.

Instances

AsRecUpdError RecUpdError # |
Methods _RecUpdError :: Prism' RecUpdError String #
AsRecUpdError SomeException # |
Methods _RecUpdError :: Prism' SomeException String #

pattern RecConError_ :: forall s. AsRecConError s => String -> s #

pattern RecSelError_ :: forall s. AsRecSelError s => String -> s #

pattern RecUpdError_ :: forall s. AsRecUpdError s => String -> s #

## Error Call

class AsErrorCall t where #

This is thrown when the user calls `error`.

Methods

_ErrorCall :: Prism' t String #

Retrieve the argument given to `error`.

`ErrorCall` is isomorphic to a `String`.

    >>> catching _ErrorCall (error "touch down!") return
    "touch down!"

Instances

AsErrorCall ErrorCall # |
Methods _ErrorCall :: Prism' ErrorCall String #
AsErrorCall SomeException # |
Methods _ErrorCall :: Prism' SomeException String #

pattern ErrorCall_ :: forall s. AsErrorCall s => String -> s #

## Allocation Limit Exceeded

class AsAllocationLimitExceeded t where #

This thread has exceeded its allocation limit.

Methods

_AllocationLimitExceeded :: Prism' t () #

There is no additional information carried in an `AllocationLimitExceeded` `Exception`.

    _AllocationLimitExceeded :: Prism' AllocationLimitExceeded ()
    _AllocationLimitExceeded :: Prism' SomeException           ()

Instances

AsAllocationLimitExceeded AllocationLimitExceeded # |
Methods _AllocationLimitExceeded :: Prism' AllocationLimitExceeded () #
AsAllocationLimitExceeded SomeException # |
Methods _AllocationLimitExceeded :: Prism' SomeException () #

pattern AllocationLimitExceeded_ :: forall s. AsAllocationLimitExceeded s => s #

## Type Error

class AsTypeError t where #

An expression that didn't typecheck during compile time was called. This is only possible with `-fdefer-type-errors`.

Methods

_TypeError :: Prism' t String #

Details about the failed type check.

    _TypeError :: Prism' TypeError     ()
    _TypeError :: Prism' SomeException ()

Instances

AsTypeError TypeError # |
Methods _TypeError :: Prism' TypeError String #
AsTypeError SomeException # |
Methods _TypeError :: Prism' SomeException String #

pattern TypeError_ :: forall s. AsTypeError s => String -> s #

# Handling Exceptions

class AsHandlingException t where #

This `Exception` is thrown by `lens` when the user somehow manages to rethrow an internal `HandlingException`.

Methods

_HandlingException :: Prism' t () #

There is no information carried in a `HandlingException`.

    _HandlingException :: Prism' HandlingException ()
    _HandlingException :: Prism' SomeException     ()

Instances

AsHandlingException SomeException # |
Methods _HandlingException :: Prism' SomeException () #
AsHandlingException HandlingException # |
Methods _HandlingException :: Prism' HandlingException () #

pattern HandlingException_ :: forall s. AsHandlingException s => s #