Exceptions: Difference between revisions
| Line 1,629: | Line 1,629: | ||
Here is a complicated exceptions example straight from the manual. |
Here is a complicated exceptions example straight from the manual. |
||
This is a |
This is a deliberately convoluted way to process input consisting of lines which have the form: <pre>{monkey | gorilla | human} <name></pre> |
||
Some custom exceptions are defined, and arranged into a hierarchy via @(defex) directives. An exception precedence hierarchy is established. A gorilla is a kind of ape, and an ape is a kind of primate. A monkey is a kind of primate, and so is a human. |
Some custom exceptions are defined, and arranged into a hierarchy via @(defex) directives. An exception precedence hierarchy is established. A gorilla is a kind of ape, and an ape is a kind of primate. A monkey is a kind of primate, and so is a human. |
||
Revision as of 01:30, 20 September 2011
You are encouraged to solve this task according to the task description, using any language you may know.
These are examples of control structures. You may also be interested in:
This task is to give an example of an exception handling routine and to "throw" a new exception.
Cf. Exceptions Through Nested Calls
Ada
Define an exception <lang ada>Foo_Error : exception;</lang>
Raise an exception <lang ada>procedure Foo is begin
raise Foo_Error;
end Foo;</lang> Re-raising once caught exception: <lang ada> ... exception
when Foo_Error =>
if ... then -- Alas, cannot handle it here,
raise; -- continue propagation of
end if;</lang>
Handle an exception <lang ada>procedure Call_Foo is begin
Foo;
exception
when Foo_Error =>
... -- do something
when others =>
... -- this catches all other exceptions
end Call_Foo;</lang>
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:
<lang ada>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;</lang>
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.
<lang aikido>
try {
var lines = readfile ("input.txt")
process (lines)
} catch (e) {
do_somthing(e)
}
</lang>
Throwing exceptions
You can throw any value.
<lang aikido>
if (error) {
throw "Error"
}
if (something) {
throw new MyException (errorcode, a, b)
}
</lang>
ALGOL 68
Define an exception <lang algol68># 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
);</lang>
Raise an exception <lang algol68>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;</lang> Re-raising once caught exception: <lang algol68>... 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</lang>
Handle an exception <lang algol68>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</lang>
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.
AppleScript
try <lang applescript>try
set num to 1 / 0 --do something that might throw an error
end try</lang>
try-on error <lang applescript>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</lang>
error <lang applescript>error "Error message." number 2000</lang>
AutoHotkey
<lang AutoHotkey>foo() If ErrorLevel
Msgbox calling foo failed with: %ErrorLevel%
foo() {
If success Return Else ErrorLevel = foo_error Return
}</lang>
C
The setjmp()/longjmp() functions in the C standard library header <setjmp.h> are typically used for exception handling.
try-catch
<lang c>#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" */
}
}</lang>
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 <lang cpp>struct MyException {
// data with info about exception
};</lang>
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.
<lang cpp>#include <exception> struct MyException: std::exception {
char const* what() const throw() { return "description"; }
}</lang>
Note that in principle you can throw any copyable type as exception, including built-in types.
Throw exceptions <lang cpp>// 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();
}</lang>
Catching exceptions <lang cpp>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
}</lang>
C#
Defining exceptions <lang csharp>public class MyException : Exception {
// data with info about exception
};</lang>
Throw exceptions <lang csharp>void foo() {
throw MyException();
}</lang>
Catching exceptions <lang csharp>try {
foo();
} catch (MyException e) {
// handle exceptions of type MyException and derived
} catch {
// handle any type of exception not handled by above catches
}</lang>
Clojure
Expression handling in Clojure is basically like Java in S-expressions: <lang clojure>(try
(if (> (rand) 0.5) (throw (RuntimeException. "oops!")) (println "see this half the time") (catch RuntimeException e (println e) (finally (println "always see this"))</lang>
ColdFusion
Catch Exceptions
inside <cfscript>:
<lang cfm>try {
foo();
} catch (Any e) {
// handle exception e
}</lang>
otherwise: <lang cfm><cftry> <cfcatch type="Database|..."> </cfcatch> </cftry></lang>
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.
<lang lisp>(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)))))</lang>
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.
D
Throw Exceptions <lang d>void test() {
throw new Exception("Sample Exception");
}</lang>
Catch Exceptions <lang d>void test2() {
try test();
catch (Exception ex) { writefln(ex); throw ex; /* rethrow */ }
}</lang>
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 <lang d>void test3() {
try test2();
finally writefln("test3 finally");
}</lang> Or also with scope guards! <lang d>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();
}</lang>
Delphi
Throw Exceptions <lang delphi>procedure test; begin
raise Exception.Create('Sample Exception');
end;</lang>
Catch Exceptions <lang delphi>procedure test2; begin
try test; except ShowMessage(Exception(ExceptObject).Message); // Showing exception message raise; // Rethrowing end;
end;</lang>
Ways to implement finally <lang delphi>procedure test3; begin
try
test2;
finally
ShowMessage('test3 finally');
end;
end;</lang>
DWScript
Throw Exceptions <lang delphi>procedure Test; begin
raise Exception.Create('Sample Exception');
end;</lang>
Catch Exceptions <lang delphi>procedure Test2; begin
try
test;
except
on E: Exception do begin // Filter by exception class
PrintLn(E.Message); // Showing exception message
raise; // Rethrowing
end;
end;
end;</lang>
Ways to implement finally <lang delphi>procedure Test3; begin
try
test2;
finally
PrintLn('Test3 finally');
end;
end;</lang>
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.
<lang e>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]
}
}</lang>
<lang e>? 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]</lang>
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:
<lang e>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]
}
}</lang>
<lang e>? 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</lang>
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:
<lang e>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?"))
}
}</lang>
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.
Factor
Throw Exceptions <lang factor>"Install Linux, Problem Solved" throw
TUPLE: velociraptor ; \ velociraptor new throw</lang>
Or a shorthand for this: <lang factor>ERROR: velociraptor ; velociraptor</lang>
Catch Exceptions <lang factor>! 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</lang>
Fancy
<lang fancy># define custom exception class
- StdError is base class for all exception classes
def class MyError : StdError {
def initialize: message {
# forward to StdError's initialize method
super initialize: message
}
}
try {
# raises/throws a new MyError exception within try-block MyError new: "my message" . raise!
} catch MyError => e {
# catch exception # this will print "my message" e message println
} finally {
# this will always be executed (as in e.g. Java) "This is how exception handling in Fancy works :)" println
}</lang>
Fantom
<lang fantom> // Create a new error class by subclassing sys::Err const class SpecialErr : Err {
// you must provide some message about the error
// to the parent class, for reporting
new make () : super ("special error") {}
}
class Main {
static Void fn ()
{
throw SpecialErr ()
}
public static Void main ()
{
try
fn()
catch (SpecialErr e)
echo ("Caught " + e)
}
} </lang>
Output:
$ fan exceptions.fan Caught exceptions_0::SpecialErr: special error
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 <lang forth>: f ( -- ) 1 throw ." f " ; \ will throw a "1"
- g ( -- ) 0 throw ." g " ; \ does not throw</lang>
Catch Exceptions <lang forth>: report ( n -- ) ?dup if ." caught " . else ." no throw" then ;
- test ( -- )
['] f catch report ['] g catch report ;</lang>
test example. (Output shown in bold) <lang forth>cr test caught 1 g no throw ok</lang>
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: <lang forth>10 ['] myfun catch if drop then</lang>
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).
<lang go>func foo() {
defer func() {
if err := recover(); err != nil {
fmt.Printf("Error: %v\n", err)
}
}()
panic("FAIL!")
}</lang>
Outputs:
Error: FAIL!
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:
<lang haskell>do {- ... -}
throwIO SomeException</lang>
In purely functional context, use "throw" to throw exceptions; the expression will match any type: <lang haskell>if condition then 3 else throw SomeException</lang>
To throw a user-defined exception, use "throwDyn": <lang haskell>if condition then 3 else throwDyn myException</lang>
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:
<lang haskell>do
{- do IO computations here -}
`catch` \ex -> do
{- handle exception "ex" here -}</lang>
Note: Control.Exception's "catch" is different than Prelude's "catch".
To catch a user-defined exception, use "catchDyn": <lang haskell>do
{- do IO computations here -}
`catchDyn` \ex -> do
{- handle exception "ex" here -}</lang>
Icon and Unicon
The following Unicon example makes use of support for exceptions found in the The Unicon Code Library. Since exception support is not built into Unicon, but rather implemented as Unicon code, there are limitations not found in languages that natively support exceptions.
<lang Unicon>import Exceptions
procedure main(A)
every i := !A do {
case Try().call{ write(g(i)) } of {
Try().catch(): {
x := Try().getException()
write(x.getMessage(), ":\n", x.getLocation())
}
}
}
end
procedure g(i)
if numeric(i) = 3 then Exception().throw("bad value of "||i)
return i
end</lang>
A sample run is:
-> ExceptionTest 1 2 3 4 5
1
2
Exception: bad value of 3:
procedure g [ExceptionTest.icn:15]
procedure main [ExceptionTest.icn:5]
4
5
->
Note: it may be possible to implement exceptions in Icon; however, it would require a major rework and would likely be inelegant.
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.
<lang j> 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!</lang>
Java
An exception needs to extend the Exception type.
Defining exceptions <lang java>//Checked exception public class MyException extends Exception {
//Put specific info in here
}
//Unchecked exception public class MyRuntimeException extends RuntimeException {}</lang>
Throw exceptions <lang java>public void fooChecked() throws MyException {
throw new MyException();
}
public void fooUnchecked() {
throw new MyRuntimeException();
}</lang>
Catching exceptions <lang java>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
}</lang>
Java 7 added "multicatch" and "smart rethrow". <lang java5>public void foo() throws UnsupportedDataTypeException{
try{
throwsNumberFormatException();
//the following methods throw exceptions which extend IOException
throwsUnsupportedDataTypeException();
throwsFileNotFoundException();
}catch(FileNotFoundException | NumberFormatException ex){
//deal with these two Exceptions without duplicating code
}catch(IOException e){
//deal with the UnsupportedDataTypeException as well as any other unchecked IOExceptions
throw e;
}
}</lang>
In previous versions of Java, foo() would have to declare that it throws an IOException. The "smart rethrow" recognizes that the only checked exception that can result in the rethrow ("throw e;") is an UnsupportedDataTypeException. The last catch block will still catch any other unchecked IOExceptions and rethrow them, but foo() only needs to declare that UnsupportedDataTypeExceptions are thrown from it since that's the only checked exception that can cause a rethrow.
The other catch block uses the same code to handle both FileNotFoundExceptions and NumberFormatExceptions by adding a | between the exception types that are declared.
JavaScript
Throwing exceptions
<lang javascript>function doStuff() {
throw new Error('Not implemented!');
}</lang>
Catching exceptions
<lang javascript>try {
element.attachEvent('onclick', doStuff);
} catch(e if e instanceof TypeError) {
element.addEventListener('click', doStuff, false);
} finally {
eventSetup = true;
}</lang>
Logo
<lang 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</lang> 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
Lua
<lang Lua> error("Error message") </lang>
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
<lang make>all:
false</lang>
Using -@ to ignore the exception.
catch.mk
<lang make>all:
-@make -f fail.mk</lang>
Using explicit exit 0 to ignore the exception.
catch.mk
<lang make>all:
make -f fail.mk; exit 0</lang>
MATLAB
Errors are thrown using the "error" keyword.
Sample usage: <lang MATLAB>>> error 'Help' ??? Help</lang>
Modula-3
Defining exceptions
Exceptions can only be declared at the "top-level" of a module or interface. Arguments are optional.
<lang modula3>EXCEPTION EndOfFile;
EXCEPTION Error(TEXT);</lang>
Throw exceptions
Exceptions can be bound to procedures using RAISES:
<lang modula3>PROCEDURE Foo() RAISES { EndOfFile } =
... RAISE EndOfFile; ...</lang>
Catching exceptions <lang modula3>TRY
Foo();
EXCEPT | EndOfFile => HandleFoo(); END;</lang>
Modula-3 also has a FINALLY keyword: <lang modula3>TRY
Foo();
FINALLY
CleanupFoo(); (* always executed *)
END;</lang>
MOO
Throw exceptions
Values can be raised to exceptions using raise():
<lang moo>raise(E_PERM);</lang>
Catching exceptions <lang moo>try
this:foo();
except e (ANY)
this:bar(e);
endtry</lang>
MOO also has a finally statement: <lang moo>try
this:foo();
finally
this:bar();
endtry</lang>
Shorthand <lang moo>`this:foo()!ANY=>this:bar()';</lang>
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:
<lang objc>@interface MyException : NSException {
//Put specific info in here
} @end</lang>
Throw exceptions <lang objc>- (void)foo {
@throw [NSException exceptionWithName:@"TerribleException"
reason:@"OMGWTFBBQ111!1" userInfo:nil];
}</lang>
Catching exceptions <lang objc>@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
}</lang>
OCaml
Defining exceptions
Like constructors, exceptions may or may not have an argument:
<lang ocaml>exception My_Exception;;
exception Another_Exception of string;;</lang>
Throw exceptions
Throw exceptions with the "raise" function; the expression will match any type:
<lang ocaml>let foo x =
match x with 1 -> raise My_Exception | 2 -> raise (Another_Exception "hi mom") | _ -> 5
- </lang>
Catching exceptions
The "with" syntax pattern-matches on the exception type and argument:
<lang ocaml>try
string_of_int (foo 2)
with
My_Exception -> "got my exception"
| Another_Exception s -> s | _ -> "unknown exception"</lang>
Oz
Throw exceptions
Any value can be thrown as an exception. Typically record values are used.
<lang oz>raise sillyError end
raise slightlyLessSilly(data:42 reason:outOfMemory) end</lang>
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).
<lang oz>try
raise someError(debug:unit) end
catch someError(debug:d(stack:ST ...)...) then
{Inspect ST}
end</lang>
See also: Exceptions in the Oz documentation.
Catching exceptions
Exception are caught with pattern matching. Ellipsis indicating additional optional fields are often useful here.
<lang oz>try
{Foo}
catch sillyError then
{Bar}
[] slightlyLessSilly(data:D ...) then
{Quux D}
[] _ then %% an unknown type of exception was thrown
{Baz}
finally
{Fin}
end</lang>
PARI/GP
Catching errors in GP
The errors that can be trapped in GP are:
alarmer
|
generic error |
gdiver
|
division by 0 |
invmoder
|
impossible modular inverse |
archer
|
not available on this architecture or operating system |
typeer
|
wrong type |
errpile
|
the PARI stack overflows |
talker
|
generic error |
user
|
User-initiated error |
<lang parigp>trap(/* specific error can be given here, or leave blank to catch all */,
"caught"
,
error("bad stuff")
)</lang>
Throwing errors in GP
The only error that can be thrown in GP is user error: <lang parigp>error("Text of error here")</lang>
Throwing errors in PARI
Many more errors can be caught and thrown directly in PARI:
<lang C>pari_err(arither1, "functionName"); // Gives "*** functionName: not an integer argument in an arithmetic function"</lang>
Catching errors in PARI
It is rare that this mechanism needs to be used in PARI, rather than standard C methods, but the function closure_trapgen (similar to closure_evalgen) is available:
<lang C>GEN x = closure_trapgen(arither1, f); // Executes the function f, catching "not an integer argument in an arithmetic function" errors
if (x == (GEN)1L) // Was there an error?
pari_printf("Don't do that!\n"); // Recover</lang>
Perl
Using eval
Exceptions using the core eval function:
<lang perl># 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 $@;</lang>
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:
<lang perl># throw an exception die "Danger, danger, Will Robinson!";</lang> <lang perl># catch an exception and show it try {
die "this could go wrong mightily";
} catch {
print;
};</lang> <lang perl># rethrow (inside of catch) die $_;</lang>
Other styles
More complicated exception handling can be achieved in Perl using TryCatch or Exception::Class modules.
Perl 6
The Perl 6 equivalent to Perl 5's eval {...} is try {...}. A try block by default has a CATCH block that handles all fatal exceptions by ignoring them. If you define a CATCH block within the try, it replaces the default CATCH. It also makes the try keyword redundant, because any block can function as a try block if you put a CATCH block within it. For furthur information see: http://perlcabal.org/syn/S04.html#Exception_handlers
<lang perl6>try { die "Help I'm dieing!"; CATCH { note $_.uc; say "Cough, Cough, Aiee!!" } }
CATCH { note "No you're not."; say $_; }
say "Yay. I'm alive.";
die "I'm dead.";
say "Arrgh.";</lang>
HELP I'M DIEING! Cough, Cough, Aiee!! Yay. I'm alive. No you're not. I'm dead.
PHP
Exceptions were not available prior to PHP 5.0
Define exceptions <lang php>class MyException extends Exception {
// Custom exception attributes & methods
}</lang>
Throwing exceptions <lang php>function throwsException() {
throw new Exception('Exception message');
}</lang>
Catching Exceptions <lang php>try {
throwsException();
} catch (Exception $e) {
echo 'Caught exception: ' . $e->getMessage();
}</lang>
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. <lang PicoLisp>(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</lang>
Output:
1 # '1' is printed -> 2 # '2' is returned
PL/I
<lang> /* 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. */ </lang>
Pop11
Throwing exceptions
<lang pop11>define throw_exception();
throw([my_exception my_data]);
enddefine;</lang>
Catching exceptions
<lang pop11>define main();
vars cargo;
define catcher();
;;; print exception data
cargo =>
enddefine;
catch(throw_exception, catcher, [my_exception ?cargo]);
enddefine;
main();</lang>
PureBasic
<lang 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</lang>
Python
Defining an exception
<lang python>import exceptions class SillyError(exceptions.Exception):
def __init__(self,args=None):
self.args=args</lang>
Note: In most cases new exceptions are defined simply using the pass statement. For example:
<lang python>class MyInvalidArgument(ValueError):
pass</lang>
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
Creating an exception using the default constructor of an exception class: <lang python>def spam():
raise SillyError # equivalent to raise SillyError()</lang>
Passing an argument to the constructor of an exception class: <lang python>def spam():
raise SillyError, 'egg' # equivalent to raise SillyError('egg')</lang>
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.
<lang python>def spam():
raise SillyError('egg')</lang>
Handling an exception
try-except-else-finally
<lang python>try:
foo()
except SillyError, se:
print se.args bar()
else:
# no exception occurred quux()
finally:
baz()</lang>
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).
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 <lang python>try:
foo()
except SillyError as se:
print(se.args) bar()
else:
# no exception occurred quux()
finally:
baz()</lang>
R
Define an exception <lang r> e <- simpleError("This is a simpleError") </lang>
Raise an exception <lang r> stop("An error has occured") stop(e) #where e is a simpleError, as above </lang>
Handle an exception <lang r> 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")
) </lang>
Raven
<lang raven>42 as custom_error
define foo
custom_error throw
try
foo
catch
custom_error = if 'oops' print</lang>
Ruby
Defining an exception
<lang ruby># define an exception class SillyError < Exception end</lang>
SillyError is simply declared as a subclass of Exception. No over-riding is necessary.
<lang ruby>class MyInvalidArgument < ArgumentError end</lang>
MyInvalidArgument is a type of ArgumentError (a built-in class). 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.
Handling an exception
<lang ruby>
- raise (throw) an exception
def spam
raise SillyError, 'egg'
end
- rescue (catch) an exception
begin
spam
rescue SillyError => se
puts se # writes 'egg' to stdout
end</lang>
<lang ruby>begin
foo
rescue ArgumentError => e
# rescues a MyInvalidArgument or any other ArgumentError bar
rescue => e
# rescues a StandardError quack
else
# runs if no exception occurred quux
ensure
# always runs baz
end</lang>
ArgumentError is a type of StandardError, but Ruby uses the first matching "rescue" clause. So we never "quack" for an ArgumentError, but we only "bar" for it.
The "rescue" clause is like the "catch" clause in other languages. The "ensure" clause is like the "finally" clause in other languages.
<lang ruby># short way to rescue any StandardError quotient = 1 / 0 rescue "sorry"</lang>
The short form "a rescue b" returns a, but if a raises a StandardError, then it returns b. (ZeroDivisionError is a subclass of StandardError.)
Catch and throw
Ruby has a separate exception-like system that is meant to be used to exit out of deep executions that are not errors.
<lang ruby>def foo
throw :done
end
catch :done do
foo
end</lang>
With Ruby 1.8, you can only "throw" and "catch" symbols. With Ruby 1.9, you can throw and catch any object. Like exceptions, the throw can be made from a function defined elsewhere from the catch block.
Scheme
Exception handling can be created with any language supporting continuations, using as few primitves as possible, exception handling in Scheme can look like this. (But anyone wishing to continue using exceptions will abstract them into macros).
<lang scheme> (define (me-errors xx exception)
(if (even? xx)
xx
(exception)))
- example that does nothing special on exception
(call/cc
(lambda (exception) (me-errors 222 exception) (display "I guess everything is alright")))
- example that laments oddness on exception
(call/cc
(lambda (all-ok) ;used to "jump" over exception handling
(call/cc
(lambda (exception-handle)
(me-errors 333 exception-handle)
(display "I guess everything is alright")
(all-ok)))
(display "oh my god it is ODD!")))
</lang>
Slate
Handling Exceptions
<lang slate>se@(SceneElement traits) doWithRestart: block [
block handlingCases: {Abort -> [| :_ | ^ Nil]}
].</lang>
Define Exceptions
<lang slate>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
].</lang>
Throwing Exceptions
<lang slate>(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.'].</lang>
Smalltalk
Throwing an Exception
<lang smalltalk>"exec" "gst" "-f" "$0" "$0" "$*" "exit"
Transcript show: 'Throwing yawp'; cr. self error: 'Yawp!'.</lang>
<lang shell>$ ./yawp.st Throwing yawp Object: nil error: Yawp! Error(Exception)>>signal (AnsiExcept.st:216) Error(Exception)>>signal: (AnsiExcept.st:226) UndefinedObject(Object)>>error: (AnsiExcept.st:1565) UndefinedObject>>executeStatements (yawp.st:5)</lang>
Handling an Exception
<lang smalltalk>"exec" "gst" "-f" "$0" "$0" "$*" "exit"
[ Transcript show: 'Throwing yawp'; cr. self error: 'Yawp!'. ] on: Error do: [ :e | Transcript show: 'Caught yawp'; cr. ].</lang>
<lang shell>$ ./yawp.st Throwing yawp Caught yawp</lang>
Standard ML
Define Exceptions <lang sml>exception MyException; exception MyDataException of int; (* can be any first-class type, not just int *)</lang>
Throw Exceptions <lang sml>fun f() = raise MyException; fun g() = raise MyDataException 22;</lang>
Catch Exceptions <lang sml>val x = f() handle MyException => 22; val y = f() handle MyDataException x => x;</lang>
Tcl
<lang 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</lang> 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"
Txr
Here is a complicated exceptions example straight from the manual.
This is a deliberately convoluted way to process input consisting of lines which have the form:
{monkey | gorilla | human} <name>
Some custom exceptions are defined, and arranged into a hierarchy via @(defex) directives. An exception precedence hierarchy is established. A gorilla is a kind of ape, and an ape is a kind of primate. A monkey is a kind of primate, and so is a human.
In the main @(collect) clause, we have a try protect block in which we collect three different cases of primate. For each one, we throw an exception with the primate type symbol, and its name. This is caught in the catch clause as the argument "name". The catch clause performs another pattern match, @kind @name. This match is being applied to exactly the same line of data for which the exception was thrown (backtracking!). Therefore the @kind variable will collect the primate type. However @name already has a binding since it is the argument of the catch. Since it has a value already, that value has to match what is in the data. Of course, it does since it was derived from that data. The data and the variable unify against each other.
<lang txr>@(defex gorilla ape primate) @(defex monkey primate) @(defex human primate) @(collect) @(try) @(cases) gorilla @name @(throw gorilla name) @(or) monkey @name @(throw monkey name) @(or) human @name @(throw human name) @(end)@#cases @(catch primate (name)) @kind @name @(output) we have a primate @name of kind @kind @(end)@#output @(end)@#try @(end)@#collect</lang>
Sample interactive run. Here the input is typed into standard input from the tty. The output is interleaved with the input, since TXR doesn't reads ahead only as much data as it needs.
$ txr primates.txr - [TTY]human Harry [TTY]gorilla Gordon [OUT]we have a primate Harry of kind human [TTY]monkey Mike [OUT]we have a primate Gordon of kind gorilla [TTY][Ctrl-D/EOF] [OUT]we have a primate Mike of kind monkey
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.) <lang Ursala>#import std
thrower = ~&?/'success'! -[epic fail]-!%
catcher = guard(thrower,---[someone failed]-)</lang>
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.
V
throwing exceptions <lang v>[myproc
['new error' 1 2 3] throw 'should not come here' puts
].</lang>
catching them
<lang v>[myproc] [puts] catch =[new error 1 2 3]</lang>
Visual Basic .NET
Defining exceptions <lang vbnet>Class MyException
Inherits Exception 'data with info about exception
End Class</lang>
Throw exceptions
<lang vbnet>Sub foo()
Throw New MyException
End Sub</lang>
Catching exceptions <lang vbnet>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</lang>
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 <lang vba>Sub foo1()
err.raise(vbObjectError + 1050)
End Sub
Sub foo2()
Error vbObjectError + 1051
End Sub </lang>
Catching exceptions <lang vba>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</lang>
- Programming Tasks
- Control Structures
- Ada
- Aikido
- ALGOL 68
- AppleScript
- AutoHotkey
- C
- C++
- C sharp
- Clojure
- ColdFusion
- Common Lisp
- D
- Delphi
- DWScript
- E
- Factor
- Fancy
- Fantom
- Forth
- Go
- Haskell
- Unicon
- J
- Java
- JavaScript
- Logo
- Lua
- Make
- MATLAB
- Modula-3
- MOO
- Objective-C
- OCaml
- Oz
- PARI/GP
- Perl
- Perl 6
- PHP
- PicoLisp
- PL/I
- Pop11
- PureBasic
- Python
- R
- Raven
- Ruby
- Scheme
- Slate
- Examples needing attention
- Smalltalk
- Standard ML
- Tcl
- Txr
- Ursala
- V
- Visual Basic .NET
- Visual Basic
- M4/Omit
- Retro/Omit