Copyright | (c) The University of Glasgow 1998-2002 |
---|---|
License | see libraries/base/LICENSE |
Maintainer | ghc-devs@haskell.org |
Stability | internal |
Portability | non-portable (GHC extensions) |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
Exceptions and exception-handling functions.
The API of this module is unstable and not meant to be consumed by the general public.
If you absolutely must depend on it, make sure to use a tight upper
bound, e.g., base < 4.X
rather than base < 5
, because the interface can
change rapidly without much warning.
Synopsis
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- backtraceDesired :: e -> Bool
- data SomeException = (Exception e, HasExceptionContext) => SomeException e
- someExceptionContext :: SomeException -> ExceptionContext
- addExceptionContext :: ExceptionAnnotation a => a -> SomeException -> SomeException
- mapExceptionContext :: (ExceptionContext -> ExceptionContext) -> SomeException -> SomeException
- newtype NoBacktrace e = NoBacktrace e
- type HasExceptionContext = ?exceptionContext :: ExceptionContext
- data ExceptionContext = ExceptionContext [SomeExceptionAnnotation]
- emptyExceptionContext :: ExceptionContext
- mergeExceptionContext :: ExceptionContext -> ExceptionContext -> ExceptionContext
- data ExceptionWithContext a = ExceptionWithContext ExceptionContext a
- data ArithException
- divZeroException :: SomeException
- overflowException :: SomeException
- ratioZeroDenomException :: SomeException
- underflowException :: SomeException
Documentation
class (Typeable e, Show e) => Exception e where Source #
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 MyException
The 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 = frontendExceptionFromException
We 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
Nothing
toException :: e -> SomeException Source #
toException
should produce a SomeException
with no attached ExceptionContext
.
fromException :: SomeException -> Maybe e Source #
displayException :: e -> String Source #
Render this exception value in a human-friendly manner.
Default implementation:
.show
@since base-4.8.0.0
backtraceDesired :: e -> Bool Source #
Instances
data SomeException Source #
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
.
(Exception e, HasExceptionContext) => SomeException e |
Instances
Exception SomeException Source # | This drops any attached @since base-3.0 |
Defined in GHC.Internal.Exception.Type | |
Show SomeException Source # | Since: ghc-internal-3.0 |
Defined in GHC.Internal.Exception.Type |
someExceptionContext :: SomeException -> ExceptionContext Source #
View the ExceptionContext
of a SomeException
.
addExceptionContext :: ExceptionAnnotation a => a -> SomeException -> SomeException Source #
Add more ExceptionContext
to a SomeException
.
mapExceptionContext :: (ExceptionContext -> ExceptionContext) -> SomeException -> SomeException Source #
newtype NoBacktrace e Source #
Instances
Exception e => Exception (NoBacktrace e) Source # | |
Defined in GHC.Internal.Exception.Type toException :: NoBacktrace e -> SomeException Source # fromException :: SomeException -> Maybe (NoBacktrace e) Source # displayException :: NoBacktrace e -> String Source # backtraceDesired :: NoBacktrace e -> Bool Source # | |
Show e => Show (NoBacktrace e) Source # | |
Defined in GHC.Internal.Exception.Type |
Exception context
type HasExceptionContext = ?exceptionContext :: ExceptionContext Source #
data ExceptionContext Source #
Exception context represents a list of ExceptionAnnotation
s. These are
attached to SomeException
s via addExceptionContext
and
can be used to capture various ad-hoc metadata about the exception including
backtraces and application-specific context.
ExceptionContext
s can be merged via concatenation using the Semigroup
instance or mergeExceptionContext
.
Note that GHC will automatically solve implicit constraints of type ExceptionContext
with emptyExceptionContext
.
Instances
Monoid ExceptionContext Source # | |
Defined in GHC.Internal.Exception.Context | |
Semigroup ExceptionContext Source # | |
Defined in GHC.Internal.Exception.Context (<>) :: ExceptionContext -> ExceptionContext -> ExceptionContext Source # sconcat :: NonEmpty ExceptionContext -> ExceptionContext Source # stimes :: Integral b => b -> ExceptionContext -> ExceptionContext Source # |
emptyExceptionContext :: ExceptionContext Source #
An ExceptionContext
containing no annotations.
@since base-4.20.0.0
mergeExceptionContext :: ExceptionContext -> ExceptionContext -> ExceptionContext Source #
Merge two ExceptionContext
s via concatenation
@since base-4.20.0.0
data ExceptionWithContext a Source #
Wraps a particular exception exposing its ExceptionContext
. Intended to
be used when catch
ing exceptions in cases where access to the context is
desired.
Instances
Exception a => Exception (ExceptionWithContext a) Source # | |
Defined in GHC.Internal.Exception.Type | |
Show a => Show (ExceptionWithContext a) Source # | |
Defined in GHC.Internal.Exception.Type |
Arithmetic exceptions
data ArithException Source #
Arithmetic exceptions.
Overflow | |
Underflow | |
LossOfPrecision | |
DivideByZero | |
Denormal | |
RatioZeroDenominator | @since base-4.6.0.0 |
Instances
Exception ArithException Source # | @since base-4.0.0.0 |
Defined in GHC.Internal.Exception.Type | |
Show ArithException Source # | @since base-4.0.0.0 |
Defined in GHC.Internal.Exception.Type | |
Eq ArithException Source # | @since base-3.0 |
Defined in GHC.Internal.Exception.Type (==) :: ArithException -> ArithException -> Bool Source # (/=) :: ArithException -> ArithException -> Bool Source # | |
Ord ArithException Source # | @since base-3.0 |
Defined in GHC.Internal.Exception.Type compare :: ArithException -> ArithException -> Ordering Source # (<) :: ArithException -> ArithException -> Bool Source # (<=) :: ArithException -> ArithException -> Bool Source # (>) :: ArithException -> ArithException -> Bool Source # (>=) :: ArithException -> ArithException -> Bool Source # max :: ArithException -> ArithException -> ArithException Source # min :: ArithException -> ArithException -> ArithException Source # |