HORIZON HASKELLDocslts/ghc-9.10.x248f8f02026-10-05Search names, modules, packages, or :: a typeCtrl K

GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulerelude-1.2.0.0Haskell2010

Relude.Exception

SPDX-License-Identifier : MIT Maintainer : Kowainik xrom.xkov@gmail.com Stability : Stable Portability : Portable

Re-exports most useful functionality from the Control.Exception module. Also provides some convenient utilities to throw and handle exceptions.

  • 2 types
  • 1 class
  • 1 value
  • Packagerelude-1.2.0.0
  • Exports5
  • LanguageHaskell2010
  • LicenceMIT
  • SourceException.hs

Control.Exception reexports

2 declarations
classclass (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 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

Methods

Instances35Exception, …

Bugs

3 declarations
datadata Bug
#

Type that represents exceptions used in cases when a particular codepath is not meant to be ever executed, but happens to be executed anyway.

Constructors

Instances2Show, Exception
  • Show BugDefined in relude-1.2.0.0 · Relude.Exception
  • Exception BugDefined in relude-1.2.0.0 · Relude.Exception
valuebug :: (HasCallStack, Exception e) => e -> a
#

Generate a pure value which, when forced, will synchronously throw the exception wrapped into Bug data type.

patternpattern Exc :: Exception e => e -> SomeException
#

Pattern synonym to easy pattern matching on exceptions. So instead of writing something like this:

isNonCriticalExc :: SomeException -> Bool
isNonCriticalExc e
    | Just (_ :: NodeAttackedError) <- fromException e = True
    | Just DialogUnexpected{} <- fromException e = True
    | otherwise = False

you can use Exc pattern synonym:

isNonCriticalExc :: SomeException -> Bool
isNonCriticalExc = case
    Exc (_ :: NodeAttackedError) -> True  -- matching all exceptions of type NodeAttackedError
    Exc DialogUnexpected{} -> True
    _ -> False

This pattern is bidirectional. You can use Exc e instead of toException e.