| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Control.Effect.ErrorIO
Synopsis
- data ErrorIO (m :: * -> *) a where
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- data SomeException
- throwIO :: (Exception e, Eff ErrorIO m) => e -> m a
- catchIO :: (Exception e, Eff ErrorIO m) => m a -> (e -> m a) -> m a
- errorIOToIO :: (Carrier m, MonadCatch m) => ErrorIOToIOC m a -> m a
- errorIOToError :: Eff (Error SomeException) m => ErrorIOToErrorC m a -> m a
- class MonadThrow m => MonadCatch (m :: Type -> Type)
- type ErrorIOToIOC = CompositionC '[ReinterpretC ErrorIOFinalH ErrorIO '[Optional ((->) SomeException)], InterpretPrimC ErrorIOFinalH (Optional ((->) SomeException))]
- type ErrorIOToErrorC = InterpretC ErrorIOToErrorH ErrorIO
Effects
class (Typeable e, Show e) => Exception e where #
Any type that you wish to throw or catch as an exception must be an
instance of the Exception class. The simplest case is a new exception
type directly below the root:
data MyException = ThisException | ThatException
deriving Show
instance Exception MyExceptionThe default method definitions in the Exception class do what we need
in this case. You can now throw and catch ThisException and
ThatException as exceptions:
*Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException
In more complicated examples, you may wish to define a whole hierarchy of exceptions:
---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler
data SomeCompilerException = forall e . Exception e => SomeCompilerException e
instance Show SomeCompilerException where
show (SomeCompilerException e) = show e
instance Exception SomeCompilerException
compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException
compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
SomeCompilerException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler
data SomeFrontendException = forall e . Exception e => SomeFrontendException e
instance Show SomeFrontendException where
show (SomeFrontendException e) = show e
instance Exception SomeFrontendException where
toException = compilerExceptionToException
fromException = compilerExceptionFromException
frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException
frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
SomeFrontendException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception
data MismatchedParentheses = MismatchedParentheses
deriving Show
instance Exception MismatchedParentheses where
toException = frontendExceptionToException
fromException = frontendExceptionFromExceptionWe can now catch a MismatchedParentheses exception as
MismatchedParentheses, SomeFrontendException or
SomeCompilerException, but not other types, e.g. IOException:
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses
Minimal complete definition
Nothing
Methods
toException :: e -> SomeException #
fromException :: SomeException -> Maybe e #
displayException :: e -> String #
Render this exception value in a human-friendly manner.
Default implementation: show
Since: base-4.8.0.0
Instances
data SomeException #
The SomeException type is the root of the exception type hierarchy.
When an exception of type e is thrown, behind the scenes it is
encapsulated in a SomeException.
Instances
| Show SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
| Exception SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String # | |
Actions
Interpretations
errorIOToIO :: (Carrier m, MonadCatch m) => ErrorIOToIOC m a -> m a Source #
errorIOToError :: Eff (Error SomeException) m => ErrorIOToErrorC m a -> m a Source #
Transform an ErrorIOError SomeException
MonadCatch
class MonadThrow m => MonadCatch (m :: Type -> Type) #
A class for monads which allow exceptions to be caught, in particular
exceptions which were thrown by throwM.
Instances should obey the following law:
catch (throwM e) f = f e
Note that the ability to catch an exception does not guarantee that we can
deal with all possible exit points from a computation. Some monads, such as
continuation-based stacks, allow for more than just a success/failure
strategy, and therefore catch cannot be used by those monads to properly
implement a function such as finally. For more information, see
MonadMask.
Minimal complete definition
Instances
Carriers
type ErrorIOToIOC = CompositionC '[ReinterpretC ErrorIOFinalH ErrorIO '[Optional ((->) SomeException)], InterpretPrimC ErrorIOFinalH (Optional ((->) SomeException))] Source #
type ErrorIOToErrorC = InterpretC ErrorIOToErrorH ErrorIO Source #