Exceptions

From Rosetta Code

This task is to give an example of an exception handling routine and to "throw" a new exception.

Cf. Exceptions Through Nested Calls

Contents 1 Ada 2 Aikido 3 ALGOL 68 4 AppleScript 5 AutoHotkey 6 C 7 C++ 8 C# 9 Clojure 10 ColdFusion 11 Common Lisp 12 D 13 E 14 Factor 15 Forth 16 Go 17 Haskell 18 J 19 Java 20 JavaScript 21 Logo 22 Make 23 MATLAB 24 Modula-3 25 MOO 26 Objective-C 27 OCaml 28 Oz 29 Perl 30 PHP 31 PicoLisp 32 PL/I 33 Pop11 34 PureBasic 35 Python 36 R 37 Raven 38 Ruby 39 Slate 40 Standard ML 41 Tcl 42 Ursala 43 V 44 Visual Basic .NET 45 Visual Basic for Applications

[edit] Ada

Define an exception

Foo_Error : exception;

Raise an exception

procedure Foo is
begin
   raise Foo_Error;
end Foo;

Re-raising once caught exception:

  ...
exception
   when Foo_Error =>
      if ... then -- Alas, cannot handle it here,
         raise;   -- continue propagation of
      end if;

Handle an exception

procedure Call_Foo is
begin
   Foo;
exception
   when Foo_Error =>
      ... -- do something
   when others =>
      ... -- this catches all other exceptions
end Call_Foo;

Ada.Exceptions
The standard package Ada.Exceptions provides a possibility to attach messages to exceptions, to get exception occurrence information and textual description of exceptions. The following example illustrates basic functionality of:

with Ada.Exceptions;  use Ada.Exceptions;
with Ada.Text_IO;     use Ada.Text_IO;
 
procedure Main is
begin
   ...
   Raise_Exception (Foo_Error'Identity, "This is the exception message");
   ..
exception
   when Error : others =>
      Put_Line ("Something is wrong here" & Exception_Information (Error));
end Main;

[edit] Aikido

Aikido provides try, catch and throw statements.

Catching exceptions
There is one catch clause per try statement. The variable caught is whatever is thrown. It does not have to be a particular type, although there is a System.Exception class defined for system exceptions.

 
try {
    var lines = readfile ("input.txt")
    process (lines)
} catch (e) {
   do_somthing(e)
}
 
 

Throwing exceptions
You can throw any value.

 
if (error) {
    throw "Error"
}
 
if (something) {
   throw new MyException (errorcode, a, b)
}
 
 
 

[edit] ALGOL 68

Define an exception

# a user defined object #
MODE OBJECTFOO = STRUCT ( PROC (REF OBJECTFOO)BOOL foo event mended, ... );
 
PROC on foo event = (REF OBJECTFOO foo, PROC (REF OBJECTFOO)BOOL foo event)VOID: (
  foo event mended OF foo := foo event
);

Raise an exception

OBJECTFOO foo proxy := foo base; # event routines are specific to an foo #
 
on foo event(foo proxy, raise foo event);
 
WHILE TRUE DO
  # now raise example foo event #
  IF NOT (foo event mended OF foo proxy)(foo proxy) THEN undefined # trace back # FI
OD;

Re-raising once caught exception:

...
except foo event:
  IF ... THEN # Alas, cannot handle it here continue propagation of #
    IF NOT (foo event mended OF foo base)(foo base) THEN undefined # trace back # FI
  FI

Handle an exception

PROC raise foo event(REF OBJECTFOO foo)BOOL:
  IF mend foo(foo) THEN
    TRUE # continue #
  ELSE
    except foo event
    FALSE # OR fall back to default event routine #
  FI

Standard Prelude "on event" routines
ALGOL 68 uses event routines extensively in the "standard transput" (stdio) to manage the various events that arise when data is read (or written) to a file or external device. The built in "on event" routines are:

• on char error - if the character transput (input or output) in cannot be converted to the standard character set.
• on format error - if the format specified is incompatible to the data being transput (input or output)
• on line end - if an end of line was read while the program was "transputting" data
• on logical file end - if the end of data was encounted during transput
• on page end - if the end of a page was encounted during transput
• on physical file end - if the end of physical media was encounted during transput
• on value error - if the data transput was incompatibly with the variable being transput, eg a letter when a digit was expected.

All of the above allow the programmer to define a user created event routine when a particular event happens to a particular FILE. When such an event routine is called, then the routine can use any of the standard prelude routine to reposition the FILE and rectify the detected event, eg:

• space or back space
• new line, new page, set or reset.

For example: these may notify the operator to mount a new tape (in the case of physical file end).

The handler is permitted to return TRUE depending on whether the event has been handled and the program can can continue. And FALSE is when event remains unhandled, and the standard prelude event routine should be used. The handler is also permitted to exit to a label (without returning anything) if the user defined event routine determines that processing is complete.

[edit] AppleScript

try

try
    set num to 1 / 0
    --do something that might throw an error
end try

try-on error

try
    set num to 1 / 0
    --do something that might throw an error
on error errMess number errNum
    --errMess and number errNum are optional
    display alert "Error # " & errNum & return & errMess
end try

error

error "Error message." number 2000

[edit] AutoHotkey

foo()
If ErrorLevel
  Msgbox calling foo failed with:  %ErrorLevel%
 
foo()
{
  If success
    Return
  Else
    ErrorLevel = foo_error
  Return
}

[edit] C

The setjmp()/longjmp() functions in the C standard library header <setjmp.h> are typically used for exception handling.

try-catch

#include <setjmp.h>
 
enum { MY_EXCEPTION = 1 }; /* any non-zero number */
 
jmp_buf env;
 
void foo()
{
  longjmp(env, MY_EXCEPTION); /* throw MY_EXCEPTION */
}
 
void call_foo()
{
  switch (setjmp(env)) {
  case 0:                     /* try */
    foo();
    break;
  case MY_EXCEPTION:          /* catch MY_EXCEPTION */
    /* handle exceptions of type MY_EXCEPTION */
    break;
  default:
    /* handle any type of exception not handled by above catches */
    /* note: if this "default" section is not included, that would be equivalent to a blank "default" section */
    /* i.e. any exception not caught above would be caught and ignored */
    /* there is no way to "let the exception through" */
  }
}

[edit] C++

C++ has no finally construct. Instead you can do this in the destructor of an object on the stack, which will be called if an exception is thrown.

The exception can be of any type, this includes int's, other primitives, as well as objects.

Defining exceptions

struct MyException
{
  // data with info about exception
};

There's also a class std::exception which you can, but are not required to derive your exception class from. The advantage of doing so is that you can catch unknown exceptions and still get some meaningful information out. There are also more specific classes like std::runtime_error which derive from std::exception.

#include <exception>
struct MyException: std::exception
{
  char const* what() const throw() { return "description"; }
}

Note that in principle you can throw any copyable type as exception, including built-in types.

Throw exceptions

// this function can throw any type of exception
void foo()
{
  throw MyException();
}
 
// this function can only throw the types of exceptions that are listed
void foo2() throw(MyException)
{
  throw MyException();
}
 
// this function turns any exceptions other than MyException into std::bad_exception
void foo3() throw(MyException, std::bad_exception)
{
  throw MyException();
}

Catching exceptions

try {
  foo();
}
catch (MyException &exc)
{
  // handle exceptions of type MyException and derived
}
catch (std::exception &exc)
{
  // handle exceptions derived from std::exception, which were not handled by above catches
  // e.g.
  std::cerr << exc.what() << std::endl;
}
catch (...)
{
  // handle any type of exception not handled by above catches
}

[edit] C#

Works with: Visual Studio version 2005

Defining exceptions

public class MyException : Exception
{
  // data with info about exception
};

Throw exceptions

void foo()
{
  throw MyException();
}

Catching exceptions

try {
  foo();
}
catch (MyException e)
{
  // handle exceptions of type MyException and derived
}
catch
{
  // handle any type of exception not handled by above catches
}

[edit] Clojure

Expression handling in Clojure is basically like Java in S-expressions:

(try
  (if (> (rand) 0.5)
    (throw (RuntimeException. "oops!"))
  (println "see this half the time")
  (catch RuntimeException e
    (println e)
  (finally
    (println "always see this"))

[edit] ColdFusion

Catch Exceptions
inside <cfscript>:

try {
  foo();
} catch (Any e) {
  // handle exception e
}

otherwise:

<cftry>
<cfcatch type="Database|...">
</cfcatch>
</cftry>

[edit] Common Lisp

The Common Lisp condition system allows much more control over condition signaling and condition handling than many exception-based systems. The following example, however, simply defines a condition type, unexpected-odd-number, defines a function get-number which generates a random number, returning it if it is even, but signaling an unexpected-odd-number condition if it is odd. The function get-even-number uses handler-case to call get-number returning its result if no condition is signaled, and, in the case that an unexpected-odd-number condition is signaled, returning one plus the odd number.

(define-condition unexpected-odd-number (error)
  ((number :reader number :initarg :number))
  (:report (lambda (condition stream)
             (format stream "Unexpected odd number: ~w."
                     (number condition)))))
 
(defun get-number (&aux (n (random 100)))
  (if (not (oddp n)) n
    (error 'unexpected-odd-number :number n)))
 
(defun get-even-number ()
  (handler-case (get-number)
    (unexpected-odd-number (condition)
      (1+ (number condition)))))

A good introduction to Lisp's condition system is the chapter Beyond Exception Handling: Conditions and Restarts from Peter Seibel's Practical Common Lisp.

In Common Lisp, there are functions throw and catch, but these are not related to the condition system. Rather, they provide another mechanism for non-local control transfer.

[edit] D

Throw Exceptions

void test() {
  throw new Exception("Sample Exception");
}

Catch Exceptions

void test2() {
  try test();
  catch (Exception ex) { writefln(ex); throw ex; /* rethrow */ }
}

In debug mode, stack traces can be generated via an external package, but the standard library does not support it by default.

Ways to implement finally

void test3() {
  try test2();
  finally writefln("test3 finally");
}

Or also with scope guards!

void test4() {
  scope(exit) writefln("Test4 done");
  scope(failure) writefln("Test4 exited by exception");
  scope(success) writefln("Test4 exited by return or function end");
  test2();
}

[edit] E

Exceptions

An exception object describes what the problem is and has nothing to do with control flow.

Due to E's ancestry as a JVM scripting language, E does not yet have any standard mechanism for user-defined exception types.

A string provided in place of an exception will be coerced to a generic exception object.

There are two control flow constructs used with exceptions: throw and eject.

Throw and catch

throw is the built-in function which throws exceptions in the conventional sense: control goes to the catch block of the most recently entered try/catch construct.

def nameOf(arg :int) {
    if (arg == 43) {
        return "Bob"
    } else {
        throw("Who?")
    }
}
 
def catching(arg) {
    try {
        return ["ok", nameOf(arg)]
    } catch exceptionObj {
        return ["notok", exceptionObj]
    }
}

? catching(42)
# value: ["not ok", problem: Who?]
 
? catching(43)
# value: ["ok", "Bob"]
 
? catching(45.7)
# value: ["not ok", problem: the float64 45.7 doesn't coerce to an int]

However, there is a problem here: exceptions accidentally produced or uncaught from inside a given module can lead to the calling program getting information about the internals that it shouldn't have (possibly a security problem). As a result of this, we are planning to move to a 'sealed exception' model where throw and catch have the same control flow, but only debuggers can see any information in a caught exception other than "a throw happened". For situations where the caller should have information about what happened, the ejector mechanism will be used.

Ejectors

Ejectors provide the same sort of "exit to catch block" control flow that throw/catch do, but with an explicit path rather than implicitly "nearest enclosing". Ejectors are also used as a general purpose control construct as well as for exceptions.

The escape ej { body } catch pat { catch block } construct creates an ejector object and binds it to ej, which is valid for as long as body is executing. An ejector object is a function; if it is called, then control immediately passes to the catch block, with its argument bound to pat.

The above code rewritten to use ejectors:

def nameOf(arg :int, ejector) {
    if (arg == 43) {
        return "Bob"
    } else {
        ejector("Who?")
    }
}
 
def catching(arg) {
    escape unnamed {
        return ["ok", nameOf(arg, unnamed)]
    } catch exceptionObj {
        return ["notok", exceptionObj]
    }
}

? catching(42)
# value: ["not ok", problem: Who?]
 
? catching(43)
# value: ["ok", "Bob"]
 
? catching(45.7)
# problem: the float64 45.7 doesn't coerce to an int

Note that the escape-catch block does not catch the coercion error resulting from passing a float64 instead of an int, since that is an (implicit) throw.

(One further refinement: While an ejector is an ordinary function, which does not return, it is generally desirable to protect against being supplied a function which unexpectedly does return. For this purpose we have throw.eject which calls the supplied function and throws if that function returns: throw.eject(ejector, "Who?"))

The benefit of using ejectors to communicate exceptions, besides the information-leak prevention described above, is that only exceptions intended to be handled by that catch block will be passed to it; unexpected internal errors will be handled by general try/catch handlers.

For example, suppose we have nameOf written as follows:

var nameTable := null
def nameOf(arg :int, ejector) {
    if (nameTable == null) {
        nameTable := <import:nameTableParser>.parseFile(<file:nameTable.txt>)
    }
    if (nameTable.maps(arg)) {
        return nameTable[arg]
    } else {
        ejector(makeNotFoundException("Who?"))
    }
}

Suppose that loading the parser, or reading the file, throws a NotFoundException (note this exception type was made up for this example). Even though it is of the same type as the "Who?" exception, it will not be caught by the caller's escape/catch block since it was not passed via the ejector, whereas a traditional "try { ... } catch ex :NotFoundException { ... }" as in other languages would, leading to incorrect handling of the error.

[edit] Factor

Throw Exceptions

"Install Linux, Problem Solved" throw
 
TUPLE: velociraptor ;
\ velociraptor new throw

Or a shorthand for this:

ERROR: velociraptor ;
velociraptor

Catch Exceptions

! Preferred exception handling
: try-foo
    [ foo ] [ foo-failed ] recover ;
 
: try-bar
    [ bar ] [ bar-errored ] [ bar-always ] cleanup ;
 
! Used rarely
[ "Fail" throw ] try   ! throws a "Fail"
[ "Fail" throw ] catch ! returns "Fail"
[ "Hi" print ] catch   ! returns f (looks the same as throwing f; don't throw f)
[ f throw ] catch      ! returns f, bad!  use recover or cleanup instead

[edit] Forth

Forth's exception mechanism is, like most things in Forth, very simple but powerful. CATCH captures the data and return stack pointers, then executes an execution token. THROW conditionally throws a value up to the most recent CATCH, restoring the stack pointers.

Throw Exceptions

: f ( -- )  1 throw ." f " ;  \ will throw a "1"
: g ( -- )  0 throw ." g " ;  \ does not throw

Catch Exceptions

: report ( n -- ) ?dup if ." caught " . else ." no throw" then ;
: test ( -- )
  ['] f catch report
  ['] g catch report ;

test example. (Output shown in bold)

cr test
'''caught 1 g no throw ok'''

Note that CATCH only restores the stack pointers, not the stack values, so any values that were changed during the execution of the token will have undefined values. In practice, this means writing code to clean up the stack, like this:

10 ['] myfun catch if drop then

[edit] Go

Go does not have exceptions, but does have built-in functions for error handling. panic(x) "throws" a value, and recover() "catches" it.

recover() needs to be called in a "deferred" function call, otherwise it will have no effect. defer delays the function call until the current function returns (or crashes).

func foo() {
   defer func() {
      if err := recover(); err != nil {
         fmt.Printf("Error: %v\n", err)
      }
   }()
   panic("FAIL!")
}

Outputs:

Error: FAIL!

[edit] Haskell

Exceptions can be implemented using monads; no special syntax is necessary.[1] In GHC, specialized functionality for exceptions are provided by the Control.Exception module.

Defining exceptions
The type "Exception", which contains pre-defined exceptions, cannot be extended. You can however use "dynamic exceptions", which can be of any type that is of "Typeable" class.

Throw exceptions
In the context of the IO monad, use "throwIO" to throw exceptions; the expression will return any type:

do {- ... -}
   throwIO SomeException

In purely functional context, use "throw" to throw exceptions; the expression will match any type:

if condition then 3
else throw SomeException

To throw a user-defined exception, use "throwDyn":

if condition then 3
else throwDyn myException

Catching exceptions
The "catch" function performs the whole try-catch stuff. It is usually used in infix style: pattern-matches on the exception type and argument:

do
  {- do IO computations here -}
`catch` \ex -> do
  {- handle exception "ex" here -}

Note: Control.Exception's "catch" is different than Prelude's "catch".

To catch a user-defined exception, use "catchDyn":

do
  {- do IO computations here -}
`catchDyn` \ex -> do
  {- handle exception "ex" here -}

[edit] J

Tacit
Program u :: v executes u and provides its result as output unless an error occurs. In case of error, the result of v is provided instead.

Explicit
An exception in an explicit definition can be detected with try. and catcht. and can be thrown with throw. as seen below.

   pickyPicky =: verb define
     if. y-:'bad argument' do.
        throw.
     else.
        'thanks!'
     end.
   )
 
   tryThis  =: verb define
     try.
        pickyPicky y
     catcht.
        'Uh oh!'
     end.
   )
 
   tryThis 'bad argument'
Uh oh!

[edit] Java

An exception needs to extend the Exception type.

Defining exceptions

//Checked exception
public class MyException extends Exception {
   //Put specific info in here
}
 
//Unchecked exception
public class MyRuntimeException extends RuntimeException {}

Throw exceptions

public void fooChecked() throws MyException {
   throw new MyException();
}
 
public void fooUnchecked() {
   throw new MyRuntimeException();
}

Catching exceptions

try {
   fooChecked();
}
catch(MyException exc) {
   //Catch only your specified type of exception
}
catch(Exception exc) {
   //Catch any non-system error exception
}
catch(Throwable exc) {
   //Catch everything including system errors (not recommended)
}
finally {
   //This code is always executed after exiting the try block
}

[edit] JavaScript

Throwing exceptions

function doStuff() {
  throw new Error('Not implemented!');
}

Catching exceptions

try {
  element.attachEvent('onclick', doStuff);
}
catch(e if e instanceof TypeError) {
  element.addEventListener('click', doStuff, false);
}
finally {
  eventSetup = true;
}

[edit] Logo

Works with: UCB Logo

to div.checked :a :b
  if :b = 0 [(throw "divzero 0)]
  output :a / :b
end
to div.safely :a :b
  output catch "divzero [div.checked :a :b]
end

There are also some predefined exceptions:

• throw "toplevel returns to the interactive prompt if uncaught (like control-C)
• (throw "error [message]) prints a message like a primitive, bypassing normal catch output
• throw "system immediately exits Logo to the shell
• catch "error will catch any thrown error instead of printing an error message

[edit] Make

In make, an exception is caused when a rule returns a non-zero status i.e the below will fail as false returns 1, (thus raising exception)

fail.mk

all:
     false

Using -@ to ignore the exception.

catch.mk

all:
    -@make -f fail.mk

Using explicit exit 0 to ignore the exception.

catch.mk

all:
    make -f fail.mk; exit 0

[edit] MATLAB

Errors are thrown using the "error" keyword.

Sample usage:

>> error 'Help'
??? Help

[edit] Modula-3

Defining exceptions
Exceptions can only be declared at the "top-level" of a module or interface. Arguments are optional.

EXCEPTION EndOfFile;
EXCEPTION Error(TEXT);

Throw exceptions
Exceptions can be bound to procedures using RAISES:

PROCEDURE Foo() RAISES { EndOfFile } =
  ...
  RAISE EndOfFile;
  ...

Catching exceptions

TRY
  Foo();
EXCEPT
| EndOfFile => HandleFoo();
END;

Modula-3 also has a FINALLY keyword:

TRY
  Foo();
FINALLY
  CleanupFoo(); (* always executed *)
END;

[edit] MOO

Throw exceptions
Values can be raised to exceptions using raise():

raise(E_PERM);

Catching exceptions

try
  this:foo();
except e (ANY)
  this:bar(e);
endtry

MOO also has a finally statement:

try
  this:foo();
finally
  this:bar();
endtry

Shorthand

`this:foo()!ANY=>this:bar()';

[edit] Objective-C

Defining exceptions
Exceptions can be any Objective-C object, though they are usually instances of NSException. You can create a subclass of NSException if necessary:

@interface MyException : NSException {
  //Put specific info in here
}
@end

Throw exceptions

- (void)foo {
  @throw [NSException exceptionWithName:@"TerribleException"
                                 reason:@"OMGWTFBBQ111!1"  userInfo:nil];
}

Catching exceptions

@try {
  [self foo];
}
@catch (MyException *exc) {
  //Catch only your specified type of exception
}
@catch (NSException *exc) {
  //Catch any NSException or subclass
  NSLog(@"caught exception named %@, with reason: %@", [exc name], [exc reason]);  
}
@catch (id exc) {
  //Catch any kind of object
}
@finally {
  //This code is always executed after exiting the try block
}

[edit] OCaml

Defining exceptions
Like constructors, exceptions may or may not have an argument:

exception My_Exception;;
exception Another_Exception of string;;

Throw exceptions
Throw exceptions with the "raise" function; the expression will match any type:

let foo x =
  match x with
    1 -> raise My_Exception
  | 2 -> raise (Another_Exception "hi mom")
  | _ -> 5
;;

Catching exceptions
The "with" syntax pattern-matches on the exception type and argument:

try
  string_of_int (foo 2)
with
  My_Exception        -> "got my exception"
| Another_Exception s -> s
| _                   -> "unknown exception"

[edit] Oz

Throw exceptions
Any value can be thrown as an exception. Typically record values are used.

raise sillyError end
raise slightlyLessSilly(data:42 reason:outOfMemory) end

By using a record value with a feature debug set to unit you can indicate that the exception shall have debug information (including a stack trace).

try
   raise someError(debug:unit) end
catch someError(debug:d(stack:ST ...)...) then
   {Inspect ST}
end

See also: Exceptions in the Oz documentation.

Catching exceptions
Exception are caught with pattern matching. Ellipsis indicating additional optional fields are often useful here.

try
   {Foo}
catch sillyError then
   {Bar}
[] slightlyLessSilly(data:D ...) then
   {Quux D}
[] _ then %% an unknown type of exception was thrown
   {Baz}
finally
   {Fin}
end

[edit] Perl

Using eval
Expeptions using the core eval function:

# throw an exception
die "Danger, danger, Will Robinson!";
 
# catch an exception and show it
eval {
    die "this could go wrong mightily";
};
print $@ if $@;
 
# rethrow
die $@;

See http://perldoc.perl.org/perlvar.html#%24EVAL_ERROR for the meaning of the special variable $@. See http://search.cpan.org/dist/Error for advanced object based-exception handling.

Using Try::Tiny
The same using the Try::Tiny module:

# throw an exception
die "Danger, danger, Will Robinson!";

# catch an exception and show it
try {
    die "this could go wrong mightily";
} catch {
    print;
};

# rethrow (inside of catch)
die $_;

Other styles
More complicated exception handling can be achieved in Perl using TryCatch or Exception::Class modules.

[edit] PHP

Works with: PHP version 5.0+

Exceptions were not available prior to PHP 5.0

Define exceptions

class MyException extends Exception
{
    //  Custom exception attributes & methods
}

Throwing exceptions

function throwsException()
{
    throw new Exception('Exception message');
}

Catching Exceptions

try {
    throwsException();
} catch (Exception $e) {
    echo 'Caught exception: ' . $e->getMessage();
}

[edit] PicoLisp

catch, throw (and finally) can be used for exception handling. 'throw' will transfer control to a 'catch' environment that was set up with the given label.

(catch 'thisLabel          # Catch this label
   (println 1)             # Do some processing (print '1')
   (throw 'thisLabel 2)    # Abort processing and return '2'
   (println 3) )           # This is never reached

Output:

1        # '1' is printed
-> 2     # '2' is returned

[edit] PL/I

 
/* Define a new exception, called "my_condition". */
on condition (my_condition) snap begin;
   put skip list ('My condition raised.');
end;
 
/* Raise that exception */
signal condition (my_condition);
 
/* Raising that exception causes the message "My condition raised" */
/* to be printed, and execution then resumes at the statement      */
/* following the SIGNAL statement.                                 */
 

[edit] Pop11

Throwing exceptions

define throw_exception();
   throw([my_exception my_data]);
enddefine;

Catching exceptions

define main();
   vars cargo;
   define catcher();
      ;;; print exception data
      cargo =>
   enddefine;
   catch(throw_exception, catcher, [my_exception ?cargo]);
enddefine;
 
main();

[edit] PureBasic

Procedure ErrorHandler()
  MessageRequester("Exception test", "The following error happened: " + ErrorMessage())
EndProcedure
 
MessageRequester("Exception test", "Test start")
 
OnErrorCall(@ErrorHandler())
 
RaiseError(#PB_OnError_InvalidMemory) ;a custom error# can also be used here depending on the OS being compiled for

[edit] Python

Defining an exception

import exceptions
class SillyError(exceptions.Exception):
    def __init__(self,args=None):
         self.args=args

Note: In most cases new exceptions are defined simply using the pass statement. For example:

class MyInvalidArgument(ValueError):
   pass

This example makes "MyInvalidArgument" an type of ValueError (one of the built-in exceptions). It's simply declared as a subclass of the existing exception and no over-riding is necessary. (An except clause for ValueError would catch MyInvalidArgument exceptions ... but one's code could insert a more specific exception handler for the more specific type of exception).

Throwing an exception
Works with: Python version 2.x and 3.x

Creating an exception using the default constructor of an exception class:

def spam():
    raise SillyError # equivalent to raise SillyError()

Works with: Python version 2.5

Passing an argument to the constructor of an exception class:

def spam():
    raise SillyError, 'egg' # equivalent to raise SillyError('egg')

The above syntax is removed in Python 3.0; but the following syntax works in Python 2.x and 3.x, so should be preferred.

Works with: Python version 2.x and 3.x

def spam():
    raise SillyError('egg')

Handling an exception
Works with: Python version 2.5

try-except-else-finally

try:
   foo()
except SillyError, se:
   print se.args
   bar()
else:
   # no exception occurred
   quux()
finally:
   baz()

Before Python 2.5 it was not possible to use finally and except together. (It was necessary to nest a separate try...except block inside of your try...finally block).

Works with: Python version 3.0

Note: Python3 will change the syntax of except slightly, but in a way that is not backwards compatible. In Python 2.x and earlier the except statement could list a single exception or a tuple/list of exceptions and optionally a name to which the exception object will be bound. In the old versions the exception's name followed a comma (as in the foregoing example). In Python3 the syntax will become: except Exception1 [,Exception2 ...] as ExceptionName

try:
   foo()
except SillyError as se:
   print(se.args)
   bar()
else:
   # no exception occurred
   quux()
finally:
   baz()

[edit] R

Define an exception

 
e <- simpleError("This is a simpleError")
 

Raise an exception

 
stop("An error has occured")
stop(e)                      #where e is a simpleError, as above
 

Handle an exception

 
tryCatch(
  {
    if(runif(1) > 0.5)
    {
      message("This doesn't throw an error")  
    } else
    {
      stop("This is an error")
    }
  },
  error = function(e) message(paste("An error occured", e$message, sep = ": ")),
  finally = message("This is called whether or not an exception occured")
)
 

[edit] Raven

42 as custom_error
 
define foo
    custom_error throw
 
try
    foo
catch
    custom_error =
    if  'oops' print

[edit] Ruby

Defining an exception

class SillyError < Exception
    def initialize(args = nil)
         @args = args
    end
end

Note: Often new exceptions are defined simply with no body. For example:

class MyInvalidArgument < ArgumentError
end

This example makes "MyInvalidArgument" an type of ArgumentError (one of the built-in exceptions). It's simply declared as a subclass of the existing exception and no over-riding is necessary. (A rescue clause for ArgumentError would catch MyInvalidArgument exceptions ... but one's code could insert a more specific exception handler for the more specific type of exception).

Throwing an exception

def spam
    raise SillyError, 'egg'
end

Handling an exception

rescue/else/ensure

begin
    foo
rescue SillyError, se
    print se.args
    bar()
else
    # no exception occurred
    quux
ensure
    baz
end

The "rescue" clause is like the "catch" clause in other languages. The "ensure" clause is like the "finally" clause in other languages.

Catch/Throw
Ruby has a separate exception-like system that is meant to be used to exit out of deep executions that are not errors.

def foo
    throw :done
end
 
catch :done do
    foo
end

You can only "throw" and "catch" symbols. Like exceptions, the throw can be made from a function defined elsewhere from the catch block.

[edit] Slate

Handling Exceptions

se@(SceneElement traits) doWithRestart: block
[
  block handlingCases: {Abort -> [| :_ | ^ Nil]}
].

Define Exceptions

conditions define: #Abort &parents: {Restart}.
"An Abort is a Restart which exits the computation, unwinding the stack."
 
_@lobby abort
[
  Abort signal
].
_@(Abort traits) describeOn: console
[
  console ; 'Abort evaluation of expression\n'
].
 
"This will call:"
c@(Condition traits) signal
"Signalling a Condition."
[
  c tryHandlers
].

Throwing Exceptions

Some lines in this example are too long (more than 80 characters). Please fix the code if it's possible and remove this message.

(fileName endsWith: '.image') ifTrue: [error: 'Image filename specified where Slate source expected. Make sure you run slate with the -i flag to specify an image.'].

[edit] Standard ML

Define Exceptions

exception MyException;
exception MyDataException of int; (* can be any first-class type, not just int *)

Throw Exceptions

fun f() = raise MyException;
fun g() = raise MyDataException 22;

Catch Exceptions

val x = f() handle MyException => 22;
val y = f() handle MyDataException x => x;

[edit] Tcl

package require Tcl 8.5
 
# Throw
proc e {args} {
    error "error message" "error message for stack trace" {errorCode list}
}
 
# Catch and rethrow
proc f {} {
    if {[catch {e 1 2 3 4} errMsg options] != 0} {
        return -options $options $errMsg 
    }
}
 
f

This creates the stack trace

error message for stack trace
    (procedure "e" line 1)
    invoked from within
"e 1 2 3 4"
    (procedure "f" line 2)
    invoked from within
"f"

[edit] Ursala

In this program fragment, a function named thrower returns the string 'success' if its argument is non-empty, but otherwise raises an exception with the diagnostic message 'epic fail'. (The diagnostic message can also be made to depend on the input.)

#import std
 
thrower = ~&?/'success'! -[epic fail]-!%
 
catcher = guard(thrower,---[someone failed]-)

If the exception is not caught, the program terminates immediately and the diagnostic is written to stderr. Alternatively, a calling function or any caller thereof can be defined to catch an exception as shown. The exception handler may inspect and arbitrarily modify the diagnostic message, but normal execution may not be resumed. In this example, the exception handler appends some additional verbiage to the message.

[edit] V

throwing exceptions

[myproc
  ['new error' 1 2 3] throw
  'should not come here' puts
].

catching them

[myproc] [puts] catch
=[new error 1 2 3]

[edit] Visual Basic .NET

Defining exceptions

Class MyException 
  Inherits Exception
  'data with info about exception
End Class

Throw exceptions

Sub foo()
    Throw New MyException
End Sub

Catching exceptions

Sub bar()
    Try
        foo()
    Catch e As MyException When e.Data.Contains("Foo")
        ' handle exceptions of type MyException when the exception contains specific data
    Catch e As MyException
        ' handle exceptions of type MyException and derived exceptions
    Catch e As Exception
        ' handle any type of exception not handled by above catches
    Finally
        'code here occurs whether or not there was an exception
    End Try
End Sub

[edit] Visual Basic for Applications

For historical reasons, Exceptions are called 'Errors' in VBA and VB Classic. VBA inherited several distinct exception handling models, which may be freely mixed and matched. The major limitations are that nested Try/Catch blocks must be constructed by the user, and that the User Defined Labels required for the Catch/Finally blocks may not be reused within a subroutine. For these reasons, it is conventional to only have only 1 Try/Catch block per subroutine.

Throw exceptions

Sub foo1()
    err.raise(vbObjectError + 1050)
End Sub
 
Sub foo2()
    Error vbObjectError + 1051
End Sub
 

Catching exceptions

Sub bar1()
'by convention, a simple handler
    On Error Goto Catch
    fooX
    Exit Sub
Catch:
    'handle all exceptions
    Exit Sub
 
Sub bar2()
'a more complex handler, illustrating some of the flexibility of VBA exception handling
    on error goto catch
100     fooX
200     fooY
    'finally block may be placed anywhere: this is complexity for it's own sake:
    goto finally
 
catch:
    if erl= 100 then
        ' handle exception at first line: in this case, by ignoring it:
        resume next
    else
        select case err.nummber
        case vbObjectError + 1050
            ' handle exceptions of type 1050
        case vbObjectError + 1051
            ' handle exceptions of type 1051
        case else
            ' handle any type of exception not handled by above catches or line numbers
    resume finally
 
finally:
    'code here occurs whether or not there was an exception
    'block may be placed anywhere
    'by convention, often just a drop through to an Exit Sub, rather tnan a code block
    Goto end_try:
 
end_try:
    'by convention, often just a drop through from the catch block
exit sub