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>Control.OldException</TITLE
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>base-4.1.0.0: Basic libraries</TD
><TD CLASS="topbut"
><A HREF="index.html"
>Contents</A
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><A HREF="doc-index.html"
>Index</A
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><FONT SIZE="6"
>Control.OldException</FONT
></TD
><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="infohead"
>Portability</TD
><TD CLASS="infoval"
>non-portable (extended exceptions)</TD
></TR
><TR
><TD CLASS="infohead"
>Stability</TD
><TD CLASS="infoval"
>experimental</TD
></TR
><TR
><TD CLASS="infohead"
>Maintainer</TD
><TD CLASS="infoval"
>libraries@haskell.org</TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="section4"
><B
>Contents</B
></TD
></TR
><TR
><TD
><DL
><DT
><A HREF="#1"
>The Exception type
</A
></DT
><DT
><A HREF="#2"
>Throwing exceptions
</A
></DT
><DT
><A HREF="#3"
>Catching Exceptions
</A
></DT
><DD
><DL
><DT
><A HREF="#4"
>The <TT
>catch</TT
> functions
</A
></DT
><DT
><A HREF="#5"
>The <TT
>handle</TT
> functions
</A
></DT
><DT
><A HREF="#6"
>The <TT
>try</TT
> functions
</A
></DT
><DT
><A HREF="#7"
>The <TT
>evaluate</TT
> function
</A
></DT
><DT
><A HREF="#8"
>The <TT
>mapException</TT
> function
</A
></DT
><DT
><A HREF="#9"
>Exception predicates
</A
></DT
></DL
></DD
><DT
><A HREF="#10"
>Dynamic exceptions
</A
></DT
><DT
><A HREF="#11"
>Asynchronous Exceptions
</A
></DT
><DD
><DL
><DT
><A HREF="#12"
>Asynchronous exception control
</A
></DT
><DD
><DL
><DT
><A HREF="#13"
>Applying <TT
>block</TT
> to an exception handler
</A
></DT
><DT
><A HREF="#14"
>Interruptible operations
</A
></DT
></DL
></DD
></DL
></DD
><DT
><A HREF="#15"
>Assertions
</A
></DT
><DT
><A HREF="#16"
>Utilities
</A
></DT
></DL
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Description</TD
></TR
><TR
><TD CLASS="doc"
><P
>This module provides support for raising and catching both built-in
and user-defined exceptions.
</P
><P
>In addition to exceptions thrown by <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> operations, exceptions may
be thrown by pure code (imprecise exceptions) or by external events
(asynchronous exceptions), but may only be caught in the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad.
For more details, see:
</P
><UL
><LI
> <EM
>A semantics for imprecise exceptions</EM
>, by Simon Peyton Jones,
Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
in <EM
>PLDI'99</EM
>.
</LI
><LI
> <EM
>Asynchronous exceptions in Haskell</EM
>, by Simon Marlow, Simon Peyton
Jones, Andy Moran and John Reppy, in <EM
>PLDI'01</EM
>.
</LI
></UL
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Synopsis</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A HREF="#t%3AException"
>Exception</A
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
>= <A HREF="#v%3AArithException"
>ArithException</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AArrayException"
>ArrayException</A
> <A HREF="Control-OldException.html#t%3AArrayException"
>ArrayException</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AAssertionFailed"
>AssertionFailed</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AAsyncException"
>AsyncException</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ABlockedOnDeadMVar"
>BlockedOnDeadMVar</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ABlockedIndefinitely"
>BlockedIndefinitely</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ANestedAtomically"
>NestedAtomically</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ADeadlock"
>Deadlock</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ADynException"
>DynException</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AErrorCall"
>ErrorCall</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AExitException"
>ExitException</A
> <A HREF="System-Exit.html#t%3AExitCode"
>ExitCode</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AIOException"
>IOException</A
> <A HREF="Control-OldException.html#t%3AIOException"
>IOException</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ANoMethodError"
>NoMethodError</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ANonTermination"
>NonTermination</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3APatternMatchFail"
>PatternMatchFail</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ARecConError"
>RecConError</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ARecSelError"
>RecSelError</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ARecUpdError"
>RecUpdError</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A HREF="#t%3AIOException"
>IOException</A
> </TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A HREF="#t%3AArithException"
>ArithException</A
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
>= <A HREF="#v%3AOverflow"
>Overflow</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AUnderflow"
>Underflow</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ALossOfPrecision"
>LossOfPrecision</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ADivideByZero"
>DivideByZero</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3ADenormal"
>Denormal</A
></TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A HREF="#t%3AArrayException"
>ArrayException</A
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
>= <A HREF="#v%3AIndexOutOfBounds"
>IndexOutOfBounds</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AUndefinedElement"
>UndefinedElement</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A HREF="#t%3AAsyncException"
>AsyncException</A
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
>= <A HREF="#v%3AStackOverflow"
>StackOverflow</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AHeapOverflow"
>HeapOverflow</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AThreadKilled"
>ThreadKilled</A
></TD
></TR
><TR
><TD CLASS="decl"
>| <A HREF="#v%3AUserInterrupt"
>UserInterrupt</A
></TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AthrowIO"
>throwIO</A
> :: <A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> e => e -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Athrow"
>throw</A
> :: <A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> e => e -> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AioError"
>ioError</A
> :: <A HREF="System-IO-Error.html#t%3AIOError"
>IOError</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AthrowTo"
>throwTo</A
> :: <A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> e => <A HREF="Control-Concurrent.html#t%3AThreadId"
>ThreadId</A
> -> e -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Acatch"
>catch</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AcatchJust"
>catchJust</A
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> (b -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Ahandle"
>handle</A
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AhandleJust"
>handleJust</A
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b) -> (b -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Atry"
>try</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-Either.html#t%3AEither"
>Either</A
> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> a)</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AtryJust"
>tryJust</A
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-Either.html#t%3AEither"
>Either</A
> b a)</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Aevaluate"
>evaluate</A
> :: a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AmapException"
>mapException</A
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
>) -> a -> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AioErrors"
>ioErrors</A
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="System-IO-Error.html#t%3AIOError"
>IOError</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AarithExceptions"
>arithExceptions</A
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AerrorCalls"
>errorCalls</A
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AdynExceptions"
>dynExceptions</A
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Aassertions"
>assertions</A
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AasyncExceptions"
>asyncExceptions</A
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AuserErrors"
>userErrors</A
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AthrowDyn"
>throwDyn</A
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> exception => exception -> b</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AthrowDynTo"
>throwDynTo</A
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> exception => <A HREF="Control-Concurrent.html#t%3AThreadId"
>ThreadId</A
> -> exception -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AcatchDyn"
>catchDyn</A
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> exception => <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> (exception -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Ablock"
>block</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Aunblock"
>unblock</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Aassert"
>assert</A
> :: <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool"
>Bool</A
> -> a -> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Abracket"
>bracket</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> (a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b) -> (a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Abracket_"
>bracket_</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AbracketOnError"
>bracketOnError</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> (a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b) -> (a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Afinally"
>finally</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AsetUncaughtExceptionHandler"
>setUncaughtExceptionHandler</A
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
>) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AgetUncaughtExceptionHandler"
>getUncaughtExceptionHandler</A
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
>)</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="1"
><A NAME="1"
>The Exception type
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A NAME="t:Exception"
><A NAME="t%3AException"
></A
></A
><B
>Exception</B
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>The type of exceptions. Every kind of system-generated exception
has a constructor in the <TT
><A HREF="Control-OldException.html#t%3AException"
>Exception</A
></TT
> type, and values of other
types may be injected into <TT
><A HREF="Control-OldException.html#t%3AException"
>Exception</A
></TT
> by coercing them to
<TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
> (see the section on Dynamic Exceptions:
<A HREF="Control-OldException.html#DynamicExceptions"
>Control.OldException</A
>).
</TD
></TR
><TR
><TD CLASS="section4"
>Constructors</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:ArithException"
><A NAME="v%3AArithException"
></A
></A
><B
>ArithException</B
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
><TD CLASS="rdoc"
>Exceptions raised by arithmetic
operations. (NOTE: GHC currently does not throw
<TT
><A HREF="Control-OldException.html#v%3AArithException"
>ArithException</A
></TT
>s except for DivideByZero).
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:ArrayException"
><A NAME="v%3AArrayException"
></A
></A
><B
>ArrayException</B
> <A HREF="Control-OldException.html#t%3AArrayException"
>ArrayException</A
></TD
><TD CLASS="rdoc"
>Exceptions raised by array-related
operations. (NOTE: GHC currently does not throw
<TT
><A HREF="Control-OldException.html#v%3AArrayException"
>ArrayException</A
></TT
>s).
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:AssertionFailed"
><A NAME="v%3AAssertionFailed"
></A
></A
><B
>AssertionFailed</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>This exception is thrown by the
<TT
><A HREF="Control-OldException.html#v%3Aassert"
>assert</A
></TT
> operation when the condition
fails. The <TT
><A HREF="Data-Char.html#t%3AString"
>String</A
></TT
> argument contains the
location of the assertion in the source program.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:AsyncException"
><A NAME="v%3AAsyncException"
></A
></A
><B
>AsyncException</B
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
><TD CLASS="rdoc"
>Asynchronous exceptions (see section on Asynchronous Exceptions: <A HREF="Control-OldException.html#AsynchronousExceptions"
>Control.OldException</A
>).
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:BlockedOnDeadMVar"
><A NAME="v%3ABlockedOnDeadMVar"
></A
></A
><B
>BlockedOnDeadMVar</B
></TD
><TD CLASS="rdoc"
>The current thread was executing a call to
Control.Concurrent.MVar.takeMVar that could never return,
because there are no other references to this <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:BlockedIndefinitely"
><A NAME="v%3ABlockedIndefinitely"
></A
></A
><B
>BlockedIndefinitely</B
></TD
><TD CLASS="rdoc"
>The current thread was waiting to retry an atomic memory transaction
that could never become possible to complete because there are no other
threads referring to any of the TVars involved.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:NestedAtomically"
><A NAME="v%3ANestedAtomically"
></A
></A
><B
>NestedAtomically</B
></TD
><TD CLASS="rdoc"
>The runtime detected an attempt to nest one STM transaction
inside another one, presumably due to the use of
unsafePeformIO with <TT
><A HREF="GHC-Conc.html#v%3Aatomically"
>atomically</A
></TT
>.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:Deadlock"
><A NAME="v%3ADeadlock"
></A
></A
><B
>Deadlock</B
></TD
><TD CLASS="rdoc"
>There are no runnable threads, so the program is
deadlocked. The <TT
><A HREF="Control-OldException.html#v%3ADeadlock"
>Deadlock</A
></TT
> exception is
raised in the main thread only (see also: <A HREF="Control-Concurrent.html"
>Control.Concurrent</A
>).
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:DynException"
><A NAME="v%3ADynException"
></A
></A
><B
>DynException</B
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
><TD CLASS="rdoc"
>Dynamically typed exceptions (see section on Dynamic Exceptions: <A HREF="Control-OldException.html#DynamicExceptions"
>Control.OldException</A
>).
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:ErrorCall"
><A NAME="v%3AErrorCall"
></A
></A
><B
>ErrorCall</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>The <TT
><A HREF="Control-OldException.html#v%3AErrorCall"
>ErrorCall</A
></TT
> exception is thrown by <TT
><A HREF="Prelude.html#v%3Aerror"
>error</A
></TT
>. The <TT
><A HREF="Data-Char.html#t%3AString"
>String</A
></TT
>
argument of <TT
><A HREF="Control-OldException.html#v%3AErrorCall"
>ErrorCall</A
></TT
> is the string passed to <TT
><A HREF="Prelude.html#v%3Aerror"
>error</A
></TT
> when it was
called.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:ExitException"
><A NAME="v%3AExitException"
></A
></A
><B
>ExitException</B
> <A HREF="System-Exit.html#t%3AExitCode"
>ExitCode</A
></TD
><TD CLASS="rdoc"
>The <TT
><A HREF="Control-OldException.html#v%3AExitException"
>ExitException</A
></TT
> exception is thrown by System.Exit.exitWith (and
System.Exit.exitFailure). The ExitCode argument is the value passed
to System.Exit.exitWith. An unhandled <TT
><A HREF="Control-OldException.html#v%3AExitException"
>ExitException</A
></TT
> exception in the
main thread will cause the program to be terminated with the given
exit code.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:IOException"
><A NAME="v%3AIOException"
></A
></A
><B
>IOException</B
> <A HREF="Control-OldException.html#t%3AIOException"
>IOException</A
></TD
><TD CLASS="rdoc"
>These are the standard IO exceptions generated by
Haskell's <TT
>IO</TT
> operations. See also <A HREF="System-IO-Error.html"
>System.IO.Error</A
>.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:NoMethodError"
><A NAME="v%3ANoMethodError"
></A
></A
><B
>NoMethodError</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>An attempt was made to invoke a class method which has
no definition in this instance, and there was no default
definition given in the class declaration. GHC issues a
warning when you compile an instance which has missing
methods.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:NonTermination"
><A NAME="v%3ANonTermination"
></A
></A
><B
>NonTermination</B
></TD
><TD CLASS="rdoc"
>The current thread is stuck in an infinite loop. This
exception may or may not be thrown when the program is
non-terminating.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:PatternMatchFail"
><A NAME="v%3APatternMatchFail"
></A
></A
><B
>PatternMatchFail</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>A pattern matching failure. The <TT
><A HREF="Data-Char.html#t%3AString"
>String</A
></TT
> argument should contain a
descriptive message including the function name, source file
and line number.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:RecConError"
><A NAME="v%3ARecConError"
></A
></A
><B
>RecConError</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>An attempt was made to evaluate a field of a record
for which no value was given at construction time. The
<TT
><A HREF="Data-Char.html#t%3AString"
>String</A
></TT
> argument gives the location of the
record construction in the source program.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:RecSelError"
><A NAME="v%3ARecSelError"
></A
></A
><B
>RecSelError</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>A field selection was attempted on a constructor that
doesn't have the requested field. This can happen with
multi-constructor records when one or more fields are
missing from some of the constructors. The
<TT
><A HREF="Data-Char.html#t%3AString"
>String</A
></TT
> argument gives the location of the
record selection in the source program.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:RecUpdError"
><A NAME="v%3ARecUpdError"
></A
></A
><B
>RecUpdError</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>An attempt was made to update a field in a record,
where the record doesn't have the requested field. This can
only occur with multi-constructor records, when one or more
fields are missing from some of the constructors. The
<TT
><A HREF="Data-Char.html#t%3AString"
>String</A
></TT
> argument gives the location of the
record update in the source program.
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:Exception')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:Exception" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Data-Eq.html#t%3AEq"
>Eq</A
> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Text-Show.html#t%3AShow"
>Show</A
> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A NAME="t:IOException"
><A NAME="t%3AIOException"
></A
></A
><B
>IOException</B
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>Exceptions that occur in the <TT
>IO</TT
> monad.
An <TT
>IOException</TT
> records a more specific error type, a descriptive
string and maybe the handle that was used when the error was
flagged.
</TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:IOException')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:IOException" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Data-Eq.html#t%3AEq"
>Eq</A
> <A HREF="Control-OldException.html#t%3AIOException"
>IOException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Text-Show.html#t%3AShow"
>Show</A
> <A HREF="Control-OldException.html#t%3AIOException"
>IOException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> <A HREF="Control-OldException.html#t%3AIOException"
>IOException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> <A HREF="Control-OldException.html#t%3AIOException"
>IOException</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A NAME="t:ArithException"
><A NAME="t%3AArithException"
></A
></A
><B
>ArithException</B
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>Arithmetic exceptions.
</TD
></TR
><TR
><TD CLASS="section4"
>Constructors</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:Overflow"
><A NAME="v%3AOverflow"
></A
></A
><B
>Overflow</B
></TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:Underflow"
><A NAME="v%3AUnderflow"
></A
></A
><B
>Underflow</B
></TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:LossOfPrecision"
><A NAME="v%3ALossOfPrecision"
></A
></A
><B
>LossOfPrecision</B
></TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:DivideByZero"
><A NAME="v%3ADivideByZero"
></A
></A
><B
>DivideByZero</B
></TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:Denormal"
><A NAME="v%3ADenormal"
></A
></A
><B
>Denormal</B
></TD
><TD CLASS="rdoc"
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:ArithException')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:ArithException" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Data-Eq.html#t%3AEq"
>Eq</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Ord.html#t%3AOrd"
>Ord</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Text-Show.html#t%3AShow"
>Show</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A NAME="t:ArrayException"
><A NAME="t%3AArrayException"
></A
></A
><B
>ArrayException</B
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>Exceptions generated by array operations
</TD
></TR
><TR
><TD CLASS="section4"
>Constructors</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:IndexOutOfBounds"
><A NAME="v%3AIndexOutOfBounds"
></A
></A
><B
>IndexOutOfBounds</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>An attempt was made to index an array outside
its declared bounds.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:UndefinedElement"
><A NAME="v%3AUndefinedElement"
></A
></A
><B
>UndefinedElement</B
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
><TD CLASS="rdoc"
>An attempt was made to evaluate an element of an
array that had not been initialized.
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:ArrayException')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:ArrayException" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Data-Eq.html#t%3AEq"
>Eq</A
> <A HREF="Control-OldException.html#t%3AArrayException"
>ArrayException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Ord.html#t%3AOrd"
>Ord</A
> <A HREF="Control-OldException.html#t%3AArrayException"
>ArrayException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Text-Show.html#t%3AShow"
>Show</A
> <A HREF="Control-OldException.html#t%3AArrayException"
>ArrayException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> <A HREF="Control-OldException.html#t%3AArrayException"
>ArrayException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> <A HREF="Control-OldException.html#t%3AArrayException"
>ArrayException</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
> <A NAME="t:AsyncException"
><A NAME="t%3AAsyncException"
></A
></A
><B
>AsyncException</B
> </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>Asynchronous exceptions.
</TD
></TR
><TR
><TD CLASS="section4"
>Constructors</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:StackOverflow"
><A NAME="v%3AStackOverflow"
></A
></A
><B
>StackOverflow</B
></TD
><TD CLASS="rdoc"
>The current thread's stack exceeded its limit.
Since an exception has been raised, the thread's stack
will certainly be below its limit again, but the
programmer should take remedial action
immediately.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:HeapOverflow"
><A NAME="v%3AHeapOverflow"
></A
></A
><B
>HeapOverflow</B
></TD
><TD CLASS="rdoc"
><P
>The program's heap is reaching its limit, and
the program should take action to reduce the amount of
live data it has. Notes:
</P
><UL
><LI
> It is undefined which thread receives this exception.
</LI
><LI
> GHC currently does not throw <TT
><A HREF="Control-OldException.html#v%3AHeapOverflow"
>HeapOverflow</A
></TT
> exceptions.
</LI
></UL
></TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:ThreadKilled"
><A NAME="v%3AThreadKilled"
></A
></A
><B
>ThreadKilled</B
></TD
><TD CLASS="rdoc"
>This exception is raised by another thread
calling Control.Concurrent.killThread, or by the system
if it needs to terminate the thread for some
reason.
</TD
></TR
><TR
><TD CLASS="arg"
><A NAME="v:UserInterrupt"
><A NAME="v%3AUserInterrupt"
></A
></A
><B
>UserInterrupt</B
></TD
><TD CLASS="rdoc"
>This exception is raised by default in the main thread of
the program when the user requests to terminate the program
via the usual mechanism(s) (e.g. Control-C in the console).
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:AsyncException')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:AsyncException" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Data-Eq.html#t%3AEq"
>Eq</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Ord.html#t%3AOrd"
>Ord</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Text-Show.html#t%3AShow"
>Show</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="2"
><A NAME="2"
>Throwing exceptions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:throwIO"
><A NAME="v%3AthrowIO"
></A
></A
><B
>throwIO</B
> :: <A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> e => e -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>A variant of <TT
><A HREF="Control-OldException.html#v%3Athrow"
>throw</A
></TT
> that can only be used within the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad.
</P
><P
>Although <TT
><A HREF="Control-OldException.html#v%3AthrowIO"
>throwIO</A
></TT
> has a type that is an instance of the type of <TT
><A HREF="Control-OldException.html#v%3Athrow"
>throw</A
></TT
>, the
two functions are subtly different:
</P
><PRE
> throw e `seq` x ===> throw e
throwIO e `seq` x ===> x
</PRE
><P
>The first example will cause the exception <TT
>e</TT
> to be raised,
whereas the second one won't. In fact, <TT
><A HREF="Control-OldException.html#v%3AthrowIO"
>throwIO</A
></TT
> will only cause
an exception to be raised when it is used within the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad.
The <TT
><A HREF="Control-OldException.html#v%3AthrowIO"
>throwIO</A
></TT
> variant should be used in preference to <TT
><A HREF="Control-OldException.html#v%3Athrow"
>throw</A
></TT
> to
raise an exception within the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad because it guarantees
ordering with respect to other <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> operations, whereas <TT
><A HREF="Control-OldException.html#v%3Athrow"
>throw</A
></TT
>
does not.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:throw"
><A NAME="v%3Athrow"
></A
></A
><B
>throw</B
> :: <A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> e => e -> a</TD
></TR
><TR
><TD CLASS="doc"
>Throw an exception. Exceptions may be thrown from purely
functional code, but may only be caught within the IO monad.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:ioError"
><A NAME="v%3AioError"
></A
></A
><B
>ioError</B
> :: <A HREF="System-IO-Error.html#t%3AIOError"
>IOError</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>Raise an <TT
><A HREF="System-IO-Error.html#t%3AIOError"
>IOError</A
></TT
> in the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:throwTo"
><A NAME="v%3AthrowTo"
></A
></A
><B
>throwTo</B
> :: <A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> e => <A HREF="Control-Concurrent.html#t%3AThreadId"
>ThreadId</A
> -> e -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="doc"
><P
><TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> raises an arbitrary exception in the target thread (GHC only).
</P
><P
><TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> does not return until the exception has been raised in the
target thread.
The calling thread can thus be certain that the target
thread has received the exception. This is a useful property to know
when dealing with race conditions: eg. if there are two threads that
can kill each other, it is guaranteed that only one of the threads
will get to kill the other.
</P
><P
>If the target thread is currently making a foreign call, then the
exception will not be raised (and hence <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> will not return)
until the call has completed. This is the case regardless of whether
the call is inside a <TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
> or not.
</P
><P
>Important note: the behaviour of <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> differs from that described in
the paper "Asynchronous exceptions in Haskell"
(<A HREF="http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm"
>http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm</A
>).
In the paper, <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> is non-blocking; but the library implementation adopts
a more synchronous design in which <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> does not return until the exception
is received by the target thread. The trade-off is discussed in Section 9 of the paper.
Like any blocking operation, <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> is therefore interruptible (see Section 5.3 of
the paper).
</P
><P
>There is currently no guarantee that the exception delivered by <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> will be
delivered at the first possible opportunity. In particular, a thread may
unblock and then re-block exceptions (using <TT
><A HREF="Control-OldException.html#v%3Aunblock"
>unblock</A
></TT
> and <TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
>) without receiving
a pending <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
>. This is arguably undesirable behaviour.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="3"
><A NAME="3"
>Catching Exceptions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
>There are several functions for catching and examining
exceptions; all of them may only be used from within the
<TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="4"
><A NAME="4"
>The <TT
>catch</TT
> functions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:catch"
><A NAME="v%3Acatch"
></A
></A
><B
>catch</B
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="arg"
>:: </TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
>=> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
>The computation to run
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
>Handler to invoke if an exception is raised
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="ndoc" COLSPAN="2"
><P
>This is the simplest of the exception-catching functions. It
takes a single argument, runs it, and if an exception is raised
the "handler" is executed, with the value of the exception passed as an
argument. Otherwise, the result is returned as normal. For example:
</P
><PRE
> catch (openFile f ReadMode)
(\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
</PRE
><P
>For catching exceptions in pure (non-<TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
>) expressions, see the
function <TT
><A HREF="Control-OldException.html#v%3Aevaluate"
>evaluate</A
></TT
>.
</P
><P
>Note that due to Haskell's unspecified evaluation order, an
expression may return one of several possible exceptions: consider
the expression <TT
>error "urk" + 1 `div` 0</TT
>. Does
<TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
> execute the handler passing
<TT
>ErrorCall "urk"</TT
>, or <TT
>ArithError DivideByZero</TT
>?
</P
><P
>The answer is "either": <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
> makes a
non-deterministic choice about which exception to catch. If you
call it again, you might get a different exception back. This is
ok, because <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
> is an <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> computation.
</P
><P
>Note that <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
> catches all types of exceptions, and is generally
used for "cleaning up" before passing on the exception using
<TT
><A HREF="Control-OldException.html#v%3AthrowIO"
>throwIO</A
></TT
>. It is not good practice to discard the exception and
continue, without first checking the type of the exception (it
might be a ThreadKilled, for example). In this case it is usually better
to use <TT
><A HREF="Control-OldException.html#v%3AcatchJust"
>catchJust</A
></TT
> and select the kinds of exceptions to catch.
</P
><P
>Also note that the <A HREF="Prelude.html"
>Prelude</A
> also exports a function called
Prelude.catch with a similar type to <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
>,
except that the <A HREF="Prelude.html"
>Prelude</A
> version only catches the IO and user
families of exceptions (as required by Haskell 98).
</P
><P
>We recommend either hiding the <A HREF="Prelude.html"
>Prelude</A
> version of Prelude.catch
when importing <A HREF="Control-OldException.html"
>Control.OldException</A
>:
</P
><PRE
> import Prelude hiding (catch)
</PRE
><P
>or importing <A HREF="Control-OldException.html"
>Control.OldException</A
> qualified, to avoid name-clashes:
</P
><PRE
> import qualified Control.OldException as C
</PRE
><P
>and then using <TT
>C.catch</TT
>
</P
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:catchJust"
><A NAME="v%3AcatchJust"
></A
></A
><B
>catchJust</B
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="arg"
>:: </TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
>=> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b</TD
><TD CLASS="rdoc"
>Predicate to select exceptions
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
>Computation to run
</TD
></TR
><TR
><TD CLASS="arg"
>-> b -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
>Handler
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="ndoc" COLSPAN="2"
><P
>The function <TT
><A HREF="Control-OldException.html#v%3AcatchJust"
>catchJust</A
></TT
> is like <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
>, but it takes an extra
argument which is an <EM
>exception predicate</EM
>, a function which
selects which type of exceptions we're interested in. There are
some predefined exception predicates for useful subsets of
exceptions: <TT
><A HREF="Control-OldException.html#v%3AioErrors"
>ioErrors</A
></TT
>, <TT
><A HREF="Control-OldException.html#v%3AarithExceptions"
>arithExceptions</A
></TT
>, and so on. For example,
to catch just calls to the <TT
><A HREF="Prelude.html#v%3Aerror"
>error</A
></TT
> function, we could use
</P
><PRE
> result <- catchJust errorCalls thing_to_try handler
</PRE
><P
>Any other exceptions which are not matched by the predicate
are re-raised, and may be caught by an enclosing
<TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
> or <TT
><A HREF="Control-OldException.html#v%3AcatchJust"
>catchJust</A
></TT
>.
</P
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="5"
><A NAME="5"
>The <TT
>handle</TT
> functions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:handle"
><A NAME="v%3Ahandle"
></A
></A
><B
>handle</B
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>A version of <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
> with the arguments swapped around; useful in
situations where the code for the handler is shorter. For example:
</P
><PRE
> do handle (\e -> exitWith (ExitFailure 1)) $
...
</PRE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:handleJust"
><A NAME="v%3AhandleJust"
></A
></A
><B
>handleJust</B
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b) -> (b -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>A version of <TT
><A HREF="Control-OldException.html#v%3AcatchJust"
>catchJust</A
></TT
> with the arguments swapped around (see
<TT
><A HREF="Control-OldException.html#v%3Ahandle"
>handle</A
></TT
>).
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="6"
><A NAME="6"
>The <TT
>try</TT
> functions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:try"
><A NAME="v%3Atry"
></A
></A
><B
>try</B
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-Either.html#t%3AEither"
>Either</A
> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> a)</TD
></TR
><TR
><TD CLASS="doc"
><P
>Similar to <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
>, but returns an <TT
><A HREF="Data-Either.html#t%3AEither"
>Either</A
></TT
> result which is
<TT
>(<TT
><A HREF="Data-Either.html#v%3ARight"
>Right</A
></TT
> a)</TT
> if no exception was raised, or <TT
>(<TT
><A HREF="Data-Either.html#v%3ALeft"
>Left</A
></TT
> e)</TT
> if an
exception was raised and its value is <TT
>e</TT
>.
</P
><PRE
> try a = catch (Right `liftM` a) (return . Left)
</PRE
><P
>Note: as with <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
>, it is only polite to use this variant if you intend
to re-throw the exception after performing whatever cleanup is needed.
Otherwise, <TT
><A HREF="Control-OldException.html#v%3AtryJust"
>tryJust</A
></TT
> is generally considered to be better.
</P
><P
>Also note that <A HREF="System-IO-Error.html"
>System.IO.Error</A
> also exports a function called
System.IO.Error.try with a similar type to <TT
><A HREF="Control-OldException.html#v%3Atry"
>try</A
></TT
>,
except that it catches only the IO and user families of exceptions
(as required by the Haskell 98 <TT
>IO</TT
> module).
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:tryJust"
><A NAME="v%3AtryJust"
></A
></A
><B
>tryJust</B
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-Either.html#t%3AEither"
>Either</A
> b a)</TD
></TR
><TR
><TD CLASS="doc"
>A variant of <TT
><A HREF="Control-OldException.html#v%3Atry"
>try</A
></TT
> that takes an exception predicate to select
which exceptions are caught (c.f. <TT
><A HREF="Control-OldException.html#v%3AcatchJust"
>catchJust</A
></TT
>). If the exception
does not match the predicate, it is re-thrown.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="7"
><A NAME="7"
>The <TT
>evaluate</TT
> function
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:evaluate"
><A NAME="v%3Aevaluate"
></A
></A
><B
>evaluate</B
> :: a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>Forces its argument to be evaluated to weak head normal form when
the resultant <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> action is executed. It can be used to order
evaluation with respect to other <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> operations; its semantics are
given by
</P
><PRE
> evaluate x `seq` y ==> y
evaluate x `catch` f ==> (return $! x) `catch` f
evaluate x >>= f ==> (return $! x) >>= f
</PRE
><P
><EM
>Note:</EM
> the first equation implies that <TT
>(evaluate x)</TT
> is <EM
>not</EM
> the
same as <TT
>(return $! x)</TT
>. A correct definition is
</P
><PRE
> evaluate x = (return $! x) >>= return
</PRE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="8"
><A NAME="8"
>The <TT
>mapException</TT
> function
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:mapException"
><A NAME="v%3AmapException"
></A
></A
><B
>mapException</B
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Control-OldException.html#t%3AException"
>Exception</A
>) -> a -> a</TD
></TR
><TR
><TD CLASS="doc"
>This function maps one exception into another as proposed in the
paper "A semantics for imprecise exceptions".
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="9"
><A NAME="9"
>Exception predicates
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
>These pre-defined predicates may be used as the first argument to
<TT
><A HREF="Control-OldException.html#v%3AcatchJust"
>catchJust</A
></TT
>, <TT
><A HREF="Control-OldException.html#v%3AtryJust"
>tryJust</A
></TT
>, or <TT
><A HREF="Control-OldException.html#v%3AhandleJust"
>handleJust</A
></TT
> to select certain common
classes of exceptions.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:ioErrors"
><A NAME="v%3AioErrors"
></A
></A
><B
>ioErrors</B
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="System-IO-Error.html#t%3AIOError"
>IOError</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:arithExceptions"
><A NAME="v%3AarithExceptions"
></A
></A
><B
>arithExceptions</B
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Control-OldException.html#t%3AArithException"
>ArithException</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:errorCalls"
><A NAME="v%3AerrorCalls"
></A
></A
><B
>errorCalls</B
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:dynExceptions"
><A NAME="v%3AdynExceptions"
></A
></A
><B
>dynExceptions</B
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:assertions"
><A NAME="v%3Aassertions"
></A
></A
><B
>assertions</B
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:asyncExceptions"
><A NAME="v%3AasyncExceptions"
></A
></A
><B
>asyncExceptions</B
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Control-OldException.html#t%3AAsyncException"
>AsyncException</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:userErrors"
><A NAME="v%3AuserErrors"
></A
></A
><B
>userErrors</B
> :: <A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Char.html#t%3AString"
>String</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="10"
><A NAME="10"
>Dynamic exceptions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><A NAME="DynamicExceptions"
><A NAME="DynamicExceptions"
></A
></A
> Because the <TT
><A HREF="Control-OldException.html#t%3AException"
>Exception</A
></TT
> datatype is not extensible, there is an
interface for throwing and catching exceptions of type <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
>
(see <A HREF="Data-Dynamic.html"
>Data.Dynamic</A
>) which allows exception values of any type in
the <TT
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
></TT
> class to be thrown and caught.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:throwDyn"
><A NAME="v%3AthrowDyn"
></A
></A
><B
>throwDyn</B
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> exception => exception -> b</TD
></TR
><TR
><TD CLASS="doc"
>Raise any value as an exception, provided it is in the
<TT
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
></TT
> class.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:throwDynTo"
><A NAME="v%3AthrowDynTo"
></A
></A
><B
>throwDynTo</B
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> exception => <A HREF="Control-Concurrent.html#t%3AThreadId"
>ThreadId</A
> -> exception -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="doc"
>A variant of <TT
><A HREF="Control-OldException.html#v%3AthrowDyn"
>throwDyn</A
></TT
> that throws the dynamic exception to an
arbitrary thread (GHC only: c.f. <TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
>).
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:catchDyn"
><A NAME="v%3AcatchDyn"
></A
></A
><B
>catchDyn</B
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> exception => <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> (exception -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>Catch dynamic exceptions of the required type. All other
exceptions are re-thrown, including dynamic exceptions of the wrong
type.
</P
><P
>When using dynamic exceptions it is advisable to define a new
datatype to use for your exception type, to avoid possible clashes
with dynamic exceptions used in other libraries.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="11"
><A NAME="11"
>Asynchronous Exceptions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
><A NAME="AsynchronousExceptions"
><A NAME="AsynchronousExceptions"
></A
></A
> Asynchronous exceptions are so-called because they arise due to
external influences, and can be raised at any point during execution.
StackOverflow and HeapOverflow are two examples of
system-generated asynchronous exceptions.
</P
><P
>The primary source of asynchronous exceptions, however, is
<TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
>:
</P
><PRE
> throwTo :: ThreadId -> Exception -> IO ()
</PRE
><P
><TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
> (also <TT
><A HREF="Control-OldException.html#v%3AthrowDynTo"
>throwDynTo</A
></TT
> and Control.Concurrent.killThread) allows one
running thread to raise an arbitrary exception in another thread. The
exception is therefore asynchronous with respect to the target thread,
which could be doing anything at the time it receives the exception.
Great care should be taken with asynchronous exceptions; it is all too
easy to introduce race conditions by the over zealous use of
<TT
><A HREF="Control-OldException.html#v%3AthrowTo"
>throwTo</A
></TT
>.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="12"
><A NAME="12"
>Asynchronous exception control
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
>The following two functions allow a thread to control delivery of
asynchronous exceptions during a critical region.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:block"
><A NAME="v%3Ablock"
></A
></A
><B
>block</B
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>Applying <TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
> to a computation will
execute that computation with asynchronous exceptions
<EM
>blocked</EM
>. That is, any thread which
attempts to raise an exception in the current thread with Control.Exception.throwTo will be
blocked until asynchronous exceptions are enabled again. There's
no need to worry about re-enabling asynchronous exceptions; that is
done automatically on exiting the scope of
<TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
>.
</P
><P
>Threads created by Control.Concurrent.forkIO inherit the blocked
state from the parent; that is, to start a thread in blocked mode,
use <TT
>block $ forkIO ...</TT
>. This is particularly useful if you need to
establish an exception handler in the forked thread before any
asynchronous exceptions are received.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:unblock"
><A NAME="v%3Aunblock"
></A
></A
><B
>unblock</B
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>To re-enable asynchronous exceptions inside the scope of
<TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
>, <TT
><A HREF="Control-OldException.html#v%3Aunblock"
>unblock</A
></TT
> can be
used. It scopes in exactly the same way, so on exit from
<TT
><A HREF="Control-OldException.html#v%3Aunblock"
>unblock</A
></TT
> asynchronous exception delivery will
be disabled again.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section3"
><A NAME="13"
><A NAME="13"
>Applying <TT
>block</TT
> to an exception handler
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
>There's an implied <TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
> around every exception handler in a call
to one of the <TT
><A HREF="Control-OldException.html#v%3Acatch"
>catch</A
></TT
> family of functions. This is because that is
what you want most of the time - it eliminates a common race condition
in starting an exception handler, because there may be no exception
handler on the stack to handle another exception if one arrives
immediately. If asynchronous exceptions are blocked on entering the
handler, though, we have time to install a new exception handler
before being interrupted. If this weren't the default, one would have
to write something like
</P
><PRE
> block (
catch (unblock (...))
(\e -> handler)
)
</PRE
><P
>If you need to unblock asynchronous exceptions again in the exception
handler, just use <TT
><A HREF="Control-OldException.html#v%3Aunblock"
>unblock</A
></TT
> as normal.
</P
><P
>Note that <TT
><A HREF="Control-OldException.html#v%3Atry"
>try</A
></TT
> and friends <EM
>do not</EM
> have a similar default, because
there is no exception handler in this case. If you want to use <TT
><A HREF="Control-OldException.html#v%3Atry"
>try</A
></TT
>
in an asynchronous-exception-safe way, you will need to use
<TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
>.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section3"
><A NAME="14"
><A NAME="14"
>Interruptible operations
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
>Some operations are <EM
>interruptible</EM
>, which means that they can receive
asynchronous exceptions even in the scope of a <TT
><A HREF="Control-OldException.html#v%3Ablock"
>block</A
></TT
>. Any function
which may itself block is defined as interruptible; this includes
Control.Concurrent.MVar.takeMVar
(but not Control.Concurrent.MVar.tryTakeMVar),
and most operations which perform
some I/O with the outside world. The reason for having
interruptible operations is so that we can write things like
</P
><PRE
> block (
a <- takeMVar m
catch (unblock (...))
(\e -> ...)
)
</PRE
><P
>if the Control.Concurrent.MVar.takeMVar was not interruptible,
then this particular
combination could lead to deadlock, because the thread itself would be
blocked in a state where it can't receive any asynchronous exceptions.
With Control.Concurrent.MVar.takeMVar interruptible, however, we can be
safe in the knowledge that the thread can receive exceptions right up
until the point when the Control.Concurrent.MVar.takeMVar succeeds.
Similar arguments apply for other interruptible operations like
System.IO.openFile.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="15"
><A NAME="15"
>Assertions
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:assert"
><A NAME="v%3Aassert"
></A
></A
><B
>assert</B
> :: <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool"
>Bool</A
> -> a -> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>If the first argument evaluates to <TT
><A HREF="../ghc-prim/GHC-Bool.html#v%3ATrue"
>True</A
></TT
>, then the result is the
second argument. Otherwise an AssertionFailed exception is raised,
containing a <TT
><A HREF="Data-Char.html#t%3AString"
>String</A
></TT
> with the source file and line number of the
call to <TT
><A HREF="Control-OldException.html#v%3Aassert"
>assert</A
></TT
>.
</P
><P
>Assertions can normally be turned on or off with a compiler flag
(for GHC, assertions are normally on unless optimisation is turned on
with <TT
>-O</TT
> or the <TT
>-fignore-asserts</TT
>
option is given). When assertions are turned off, the first
argument to <TT
><A HREF="Control-OldException.html#v%3Aassert"
>assert</A
></TT
> is ignored, and the second argument is
returned as the result.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="16"
><A NAME="16"
>Utilities
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:bracket"
><A NAME="v%3Abracket"
></A
></A
><B
>bracket</B
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="arg"
>:: </TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
>=> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
>computation to run first ("acquire resource")
</TD
></TR
><TR
><TD CLASS="arg"
>-> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
><TD CLASS="rdoc"
>computation to run last ("release resource")
</TD
></TR
><TR
><TD CLASS="arg"
>-> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
><TD CLASS="rdoc"
>computation to run in-between
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="ndoc" COLSPAN="2"
><P
>When you want to acquire a resource, do some work with it, and
then release the resource, it is a good idea to use <TT
><A HREF="Control-OldException.html#v%3Abracket"
>bracket</A
></TT
>,
because <TT
><A HREF="Control-OldException.html#v%3Abracket"
>bracket</A
></TT
> will install the necessary exception handler to
release the resource in the event that an exception is raised
during the computation. If an exception is raised, then <TT
><A HREF="Control-OldException.html#v%3Abracket"
>bracket</A
></TT
> will
re-raise the exception (after performing the release).
</P
><P
>A common example is opening a file:
</P
><PRE
> bracket
(openFile "filename" ReadMode)
(hClose)
(\handle -> do { ... })
</PRE
><P
>The arguments to <TT
><A HREF="Control-OldException.html#v%3Abracket"
>bracket</A
></TT
> are in this order so that we can partially apply
it, e.g.:
</P
><PRE
> withFile name mode = bracket (openFile name mode) hClose
</PRE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:bracket_"
><A NAME="v%3Abracket_"
></A
></A
><B
>bracket_</B
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
></TR
><TR
><TD CLASS="doc"
>A variant of <TT
><A HREF="Control-OldException.html#v%3Abracket"
>bracket</A
></TT
> where the return value from the first computation
is not required.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:bracketOnError"
><A NAME="v%3AbracketOnError"
></A
></A
><B
>bracketOnError</B
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="arg"
>:: </TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
>=> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
>computation to run first ("acquire resource")
</TD
></TR
><TR
><TD CLASS="arg"
>-> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
><TD CLASS="rdoc"
>computation to run last ("release resource")
</TD
></TR
><TR
><TD CLASS="arg"
>-> a -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
><TD CLASS="rdoc"
>computation to run in-between
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="System-IO.html#t%3AIO"
>IO</A
> c</TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="ndoc" COLSPAN="2"
>Like bracket, but only performs the final action if there was an
exception raised by the in-between computation.
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:finally"
><A NAME="v%3Afinally"
></A
></A
><B
>finally</B
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="arg"
>:: </TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
>=> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
>computation to run first
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
><TD CLASS="rdoc"
>computation to run afterward (even if an exception
was raised)
</TD
></TR
><TR
><TD CLASS="arg"
>-> <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="ndoc" COLSPAN="2"
>A specialised variant of <TT
><A HREF="Control-OldException.html#v%3Abracket"
>bracket</A
></TT
> with just a computation to run
afterward.
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:setUncaughtExceptionHandler"
><A NAME="v%3AsetUncaughtExceptionHandler"
></A
></A
><B
>setUncaughtExceptionHandler</B
> :: (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
>) -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:getUncaughtExceptionHandler"
><A NAME="v%3AgetUncaughtExceptionHandler"
></A
></A
><B
>getUncaughtExceptionHandler</B
> :: <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Control-OldException.html#t%3AException"
>Exception</A
> -> <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
>)</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
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>Produced by <A HREF="http://www.haskell.org/haddock/"
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distrib
>
Mandriva
>
2010.0
>
i586
>
media
>
contrib-release
>
by-pkgid
>
70ec89744a04da80369b4702b2c37256
>
files
>
205
