| 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 | Safe |
| Language | Haskell2010 |
GHC.Exception
Description
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
- throw :: forall a e. (?callStack :: CallStack, Exception e) => e -> a
- data ArithException
- divZeroException :: SomeException
- overflowException :: SomeException
- ratioZeroDenomException :: SomeException
- underflowException :: SomeException
- data ErrorCall where
- errorCallException :: String -> SomeException
- errorCallWithCallStackException :: String -> CallStack -> SomeException
- data CallStack
- fromCallSiteList :: [([Char], SrcLoc)] -> CallStack
- getCallStack :: CallStack -> [([Char], SrcLoc)]
- prettyCallStack :: CallStack -> String
- prettyCallStackLines :: CallStack -> [String]
- showCCSStack :: [String] -> [String]
- data SrcLoc = SrcLoc {
- srcLocPackage :: [Char]
- srcLocModule :: [Char]
- srcLocFile :: [Char]
- srcLocStartLine :: Int
- srcLocStartCol :: Int
- srcLocEndLine :: Int
- srcLocEndCol :: Int
- prettySrcLoc :: SrcLoc -> String
Exception class
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 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 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
SomeException
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.
Constructors
| (Exception e, HasExceptionContext) => SomeException e |
Instances
| Exception SomeException | This drops any attached Since: base-3.0 |
Defined in GHC.Internal.Exception.Type Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String Source # backtraceDesired :: SomeException -> Bool Source # | |
| Show SomeException | Since: ghc-internal-3.0 |
Defined in GHC.Internal.Exception.Type | |
Throwing
throw :: forall a e. (?callStack :: CallStack, Exception e) => e -> a Source #
Throw an exception. Exceptions may be thrown from purely
functional code, but may only be caught within the IO monad.
WARNING: You may want to use throwIO instead so that your pure code
stays exception-free.
Concrete exceptions
Arithmetic exceptions
data ArithException Source #
Arithmetic exceptions.
Constructors
| Overflow | |
| Underflow | |
| LossOfPrecision | |
| DivideByZero | |
| Denormal | |
| RatioZeroDenominator | Since: base-4.6.0.0 |
Instances
| Exception ArithException | Since: base-4.0.0.0 |
Defined in GHC.Internal.Exception.Type Methods toException :: ArithException -> SomeException Source # fromException :: SomeException -> Maybe ArithException Source # | |
| Show ArithException | Since: base-4.0.0.0 |
Defined in GHC.Internal.Exception.Type | |
| Eq ArithException | Since: base-3.0 |
Defined in GHC.Internal.Exception.Type Methods (==) :: ArithException -> ArithException -> Bool Source # (/=) :: ArithException -> ArithException -> Bool Source # | |
| Ord ArithException | Since: base-3.0 |
Defined in GHC.Internal.Exception.Type Methods 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 # | |
ErrorCall
This is thrown when the user calls error. The first String is the
argument given to error, second String is the location.
Constructors
| ErrorCallWithLocation String String |
Instances
| Exception ErrorCall | Since: base-4.0.0.0 |
Defined in GHC.Internal.Exception Methods toException :: ErrorCall -> SomeException Source # fromException :: SomeException -> Maybe ErrorCall Source # displayException :: ErrorCall -> String Source # backtraceDesired :: ErrorCall -> Bool Source # | |
| Show ErrorCall | Since: base-4.0.0.0 |
| Eq ErrorCall | Since: base-4.7.0.0 |
| Ord ErrorCall | Since: base-4.7.0.0 |
Defined in GHC.Internal.Exception | |
Reexports
CallStacks are a lightweight method of obtaining a
partial call-stack at any point in the program.
A function can request its call-site with the HasCallStack constraint.
For example, we can define
putStrLnWithCallStack :: HasCallStack => String -> IO ()
as a variant of putStrLn that will get its call-site and print it,
along with the string given as argument. We can access the
call-stack inside putStrLnWithCallStack with callStack.
>>>:{putStrLnWithCallStack :: HasCallStack => String -> IO () putStrLnWithCallStack msg = do putStrLn msg putStrLn (prettyCallStack callStack) :}
Thus, if we call putStrLnWithCallStack we will get a formatted call-stack
alongside our string.
>>>putStrLnWithCallStack "hello"hello CallStack (from HasCallStack): putStrLnWithCallStack, called at <interactive>:... in interactive:Ghci...
GHC solves HasCallStack constraints in three steps:
- If there is a
CallStackin scope -- i.e. the enclosing function has aHasCallStackconstraint -- GHC will append the new call-site to the existingCallStack. - If there is no
CallStackin scope -- e.g. in the GHCi session above -- and the enclosing definition does not have an explicit type signature, GHC will infer aHasCallStackconstraint for the enclosing definition (subject to the monomorphism restriction). - If there is no
CallStackin scope and the enclosing definition has an explicit type signature, GHC will solve theHasCallStackconstraint for the singletonCallStackcontaining just the current call-site.
CallStacks do not interact with the RTS and do not require compilation
with -prof. On the other hand, as they are built up explicitly via the
HasCallStack constraints, they will generally not contain as much
information as the simulated call-stacks maintained by the RTS.
A CallStack is a [(String, SrcLoc)]. The String is the name of
function that was called, the SrcLoc is the call-site. The list is
ordered with the most recently called function at the head.
NOTE: The intrepid user may notice that HasCallStack is just an
alias for an implicit parameter ?callStack :: CallStack. This is an
implementation detail and should not be considered part of the
CallStack API, we may decide to change the implementation in the
future.
Since: base-4.8.1.0
Instances
| IsList CallStack | Be aware that 'fromList . toList = id' only for unfrozen Since: base-4.9.0.0 | ||||
Defined in GHC.Internal.IsList Associated Types
| |||||
| Show CallStack | Since: base-4.9.0.0 | ||||
| type Item CallStack | |||||
Defined in GHC.Internal.IsList | |||||
fromCallSiteList :: [([Char], SrcLoc)] -> CallStack Source #
Convert a list of call-sites to a CallStack.
Since: base-4.9.0.0
getCallStack :: CallStack -> [([Char], SrcLoc)] Source #
Extract a list of call-sites from the CallStack.
The list is ordered by most recent call.
Since: base-4.8.1.0
prettyCallStackLines :: CallStack -> [String] Source #
showCCSStack :: [String] -> [String] Source #
A single location in the source code.
Since: base-4.8.1.0
Constructors
| SrcLoc | |
Fields
| |