Exception.lhs

\begin{code}
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude
           , ExistentialQuantification
           , MagicHash
           , DeriveDataTypeable
  #-}
{-# OPTIONS_HADDOCK hide #-}
-----------------------------------------------------------------------------
-- |
-- Module      :  GHC.Exception
-- Copyright   :  (c) The University of Glasgow, 1998-2002
-- License     :  see libraries/base/LICENSE
-- 
-- Maintainer  :  cvs-ghc@haskell.org
-- Stability   :  internal
-- Portability :  non-portable (GHC extensions)
--
-- Exceptions and exception-handling functions.
-- 
-----------------------------------------------------------------------------

-- #hide
module GHC.Exception where

import Data.Maybe
import {-# SOURCE #-} Data.Typeable (Typeable, cast)
   -- loop: Data.Typeable -> GHC.Err -> GHC.Exception
import GHC.Base
import GHC.Show
\end{code}

%*********************************************************
%*                                                      *
\subsection{Exceptions}
%*                                                      *
%*********************************************************

\begin{code}
{- |
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@.
-}
data SomeException = forall e . Exception e => SomeException e
    deriving Typeable

instance Show SomeException where
    showsPrec p (SomeException e) = showsPrec p e

{- |
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, Typeable)
>
> 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
>     deriving Typeable
>
> 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
>     deriving Typeable
>
> 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 (Typeable, 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
@

-}
class (Typeable e, Show e) => Exception e where
    toException   :: e -> SomeException
    fromException :: SomeException -> Maybe e

    toException = SomeException
    fromException (SomeException e) = cast e

instance Exception SomeException where
    toException se = se
    fromException = Just
\end{code}

%*********************************************************
%*                                                      *
\subsection{Primitive throw}
%*                                                      *
%*********************************************************

\begin{code}
-- | Throw an exception.  Exceptions may be thrown from purely
-- functional code, but may only be caught within the 'IO' monad.
throw :: Exception e => e -> a
throw e = raise# (toException e)
\end{code}

\begin{code}
-- |This is thrown when the user calls 'error'. The @String@ is the
-- argument given to 'error'.
data ErrorCall = ErrorCall String
    deriving Typeable

instance Exception ErrorCall

instance Show ErrorCall where
    showsPrec _ (ErrorCall err) = showString err

-----

-- |Arithmetic exceptions.
data ArithException
  = Overflow
  | Underflow
  | LossOfPrecision
  | DivideByZero
  | Denormal
  deriving (Eq, Ord, Typeable)

instance Exception ArithException

instance Show ArithException where
  showsPrec _ Overflow        = showString "arithmetic overflow"
  showsPrec _ Underflow       = showString "arithmetic underflow"
  showsPrec _ LossOfPrecision = showString "loss of precision"
  showsPrec _ DivideByZero    = showString "divide by zero"
  showsPrec _ Denormal        = showString "denormal"

\end{code}