base-4.20.0.1: Core data structures and operations
Copyright(c) The University of Glasgow 1998-2002
Licensesee libraries/base/LICENSE
Maintainerghc-devs@haskell.org
Stabilityinternal
Portabilitynon-portable (GHC extensions)
Safe HaskellSafe
LanguageHaskell2010

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

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 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

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

Instances details
Exception Timeout Source #

Since: base-4.7.0.0

Instance details

Defined in System.Timeout

Exception Void

Since: base-4.8.0.0

Instance details

Defined in GHC.Internal.Exception.Type

Exception NestedAtomically

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception NoMatchingContinuationPrompt

Since: base-4.18

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception NoMethodError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception NonTermination

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception PatternMatchFail

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception RecConError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception RecSelError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception RecUpdError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception TypeError

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception Dynamic

Since: base-4.0.0.0

Instance details

Defined in GHC.Internal.Data.Dynamic

Exception ErrorCall

Since: base-4.0.0.0

Instance details

Defined in GHC.Internal.Exception

Exception ArithException

Since: base-4.0.0.0

Instance details

Defined in GHC.Internal.Exception.Type

Exception SomeException

This drops any attached ExceptionContext.

Since: base-3.0

Instance details

Defined in GHC.Internal.Exception.Type

Exception AllocationLimitExceeded

Since: base-4.8.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception ArrayException

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception AssertionFailed

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception AsyncException

Since: base-4.7.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception BlockedIndefinitelyOnMVar

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception BlockedIndefinitelyOnSTM

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception CompactionFailed

Since: base-4.10.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception Deadlock

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception ExitCode

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception FixIOException

Since: base-4.11.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception IOException

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception SomeAsyncException

Since: base-4.7.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception FileLockingNotSupported 
Instance details

Defined in GHC.Internal.IO.Handle.Lock.Common

Exception IOPortException 
Instance details

Defined in GHC.Internal.IOPort

Methods

toException :: IOPortException -> SomeException Source #

fromException :: SomeException -> Maybe IOPortException Source #

displayException :: IOPortException -> String Source #

backtraceDesired :: IOPortException -> Bool Source #

Exception a => Exception (ExceptionWithContext a) 
Instance details

Defined in GHC.Internal.Exception.Type

Exception e => Exception (NoBacktrace e) 
Instance details

Defined in GHC.Internal.Exception.Type

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.

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

ErrorCall

data ErrorCall Source #

This is thrown when the user calls error. The first String is the argument given to error, second String is the location.

Bundled Patterns

pattern ErrorCall :: String -> ErrorCall 

Reexports

data CallStack Source #

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:

  1. If there is a CallStack in scope -- i.e. the enclosing function has a HasCallStack constraint -- GHC will append the new call-site to the existing CallStack.
  2. If there is no CallStack in scope -- e.g. in the GHCi session above -- and the enclosing definition does not have an explicit type signature, GHC will infer a HasCallStack constraint for the enclosing definition (subject to the monomorphism restriction).
  3. If there is no CallStack in scope and the enclosing definition has an explicit type signature, GHC will solve the HasCallStack constraint for the singleton CallStack containing 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

Instances details
IsList CallStack

Be aware that 'fromList . toList = id' only for unfrozen CallStacks, since toList removes frozenness information.

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.IsList

Associated Types

type Item CallStack 
Instance details

Defined in GHC.Internal.IsList

Show CallStack

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Show

type Item CallStack 
Instance details

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

prettyCallStack :: CallStack -> String Source #

Pretty print a CallStack.

Since: ghc-internal-4.9.0.0

data SrcLoc Source #

A single location in the source code.

Since: base-4.8.1.0

Instances

Instances details
Generic SrcLoc 
Instance details

Defined in GHC.Internal.Generics

Associated Types

type Rep SrcLoc

Since: base-4.15.0.0

Instance details

Defined in GHC.Internal.Generics

Show SrcLoc

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Show

Eq SrcLoc

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Stack.Types

type Rep SrcLoc

Since: base-4.15.0.0

Instance details

Defined in GHC.Internal.Generics

prettySrcLoc :: SrcLoc -> String Source #

Pretty print a SrcLoc.

Since: ghc-internal-4.9.0.0