各种语言版本的抛出异常Exceptions

Exceptions

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

Cf. Exceptions Through Nested Calls

Contents

 [hide] 

• 1 Ada
• 2 Aikido
• 3 Aime
• 4 ALGOL 68
• 5 AppleScript
• 6 AutoHotkey
  • 6.1 True exceptions
  • 6.2 ErrorLevel-based exceptions
• 7 BBC BASIC
• 8 Bracmat
• 9 C
• 10 C++
• 11 C#
• 12 Clojure
• 13 ColdFusion
• 14 Common Lisp
• 15 D
• 16 Delphi
• 17 Déjà Vu
• 18 DWScript
• 19 E
• 20 Erlang
• 21 Factor
• 22 Fancy
• 23 Fantom
• 24 Forth
• 25 Go
• 26 Haskell
• 27 Icon and Unicon
• 28 J
• 29 Java
• 30 JavaScript
• 31 jq
• 32 Lasso
• 33 Logo
• 34 Logtalk
• 35 Lua
• 36 Make
• 37 Mathematica / Wolfram Language
• 38 MATLAB
• 39 Modula-3
• 40 MOO
• 41 Nemerle
• 42 NetRexx
• 43 Nim
• 44 Objective-C
• 45 OCaml
• 46 Oforth
• 47 Oz
• 48 PARI/GP
  • 48.1 Catching errors in GP
  • 48.2 Throwing errors in GP
  • 48.3 Throwing errors in PARI
  • 48.4 Catching errors in PARI
• 49 Pascal
• 50 Perl
• 51 Perl 6
• 52 PHL
• 53 PHP
• 54 PicoLisp
• 55 PL/I
• 56 PL/pgSQL
• 57 Pop11
• 58 PureBasic
• 59 Python
• 60 R
• 61 Racket
• 62 Raven
• 63 REXX
• 64 Ruby
• 65 Scala
• 66 Scheme
• 67 Seed7
• 68 Sidef
• 69 Slate
• 70 Smalltalk
• 71 Standard ML
• 72 Swift
• 73 Tcl
• 74 TXR
• 75 Ursala
• 76 V
• 77 Visual Basic .NET
• 78 Visual Basic for Applications
• 79 zkl

Ada[edit]

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;

Aikido[edit]

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.Exceptionclass 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)
}
 
 
 

Aime[edit]

Simple Exception Throwing

void
throwing(void)
{
    o_text("throwing...\n");
    error("now!");
}
 
void
catching(void)
{
    o_text("ready to catch\n");
    if (trap(throwing)) {
o_text("caught!\n");
    } else {
# nothing was thrown
    }
}
 
integer
main(void)
{
    catching();
 
    return 0;
}

Output:

ready to catchthrowing...aime: tmp/et: 5: now!caught!

Exception Types

void
ft(integer a, text &s)
{
    if (a & 1) {
s = "odd";
error("bad number");
    } elif (a & a - 1) {
s = "not a power of two";
error("bad number");
    }
}
 
void
fc(integer a)
{
    text e;
 
    if (trap(ft, a, e)) {
v_text("exception of type `");
v_text(e);
v_text("' thrown for ");
v_integer(a);
v_newline();
    } else {
v_text("no exception thrown for ");
v_integer(a);
v_newline();
    }
}
 
integer
main(void)
{
    fc(5);
    fc(6);
    fc(8);
 
    return 0;
}

Output:

aime: tmp/et1: 6: bad numberexception of type `odd' thrown for 5aime: tmp/et1: 9: bad numberexception of type `not a power of two' thrown for 6no exception thrown for 8

ALGOL 68[edit]

Typically: In the Algol68 transput an attempt is first made to "mend" an "event" on an object. However: After a mend has failed, the event may be "sent" (via a GO TO) then "caught" by in an outside scope.

The "GOST 27975-88 Programming language ALGOL 68 extended" Soviet standard - (Язык программирования АЛГОЛ 68 расширенный (PDF)) had the addition mechanisms, e.g.: on, exception & raise.

Works with: ALGOL 68 version Revision 1 - one extension to language used - PRAGMA READ - a non standard feature similar to C's #include directive.

Works with: ALGOL 68G version Any - tested with release algol68g-2.8.

File: prelude/event_base(obj).a68

COMMENT
  Define an general event handling mechanism on MODE OBJ: 
  * try to parallel pythons exception handling flexibility
END COMMENT
 
COMMENT
  REQUIRES:
    MODE OBJ # These can be a UNION of REF types #
    OP OBJIS
  PROVIDES:
    OP ON, RAISE, RESET
    PROC obj on, obj raise, obj reset
END COMMENT
 
# define object related to OBJ EVENTS #
MODE
  RAISEOBJ = PROC(OBJ)VOID, RAWMENDOBJ = PROC(OBJ)BOOL,
  MENDOBJ = UNION(RAWMENDOBJ, PROC VOID), # Generalise: Allow PROC VOID (a GOTO) as a short hand #
  NEWSCOPEOBJ = STRUCT(REF NEWSCOPEOBJ up, FLEXOBJ obj flex, FLEXEVENTOBJ event flex, MENDOBJ mended),
  SCOPEOBJ = REF NEWSCOPEOBJ;
 
MODE FLEXOBJ=FLEX[0]OBJ;
 
# Provide an INIT to convert a GO TO to a MEND ... useful for direct aborts #
OP INITMENDOBJ = (PROC VOID go to)MENDOBJ: (go to; SKIP);
 
SCOPEOBJ obj scope end = NIL;
SCOPEOBJ obj scope begin := obj scope end; # INITialise stack #
OBJ obj any = EMPTY;
EVENTOBJ obj event any = NIL;
 
# Some crude Singly Linked-List manipulations of the scopes, aka stack ... #
# An event/mended can be shared for all OBJ of the same type: #
PRIO INITAB = 1, +=: = 1;
OP INITAB = (SCOPEOBJ lhs, MENDOBJ obj mend)SCOPEOBJ:
  lhs := (obj scope end, obj any, obj event any, obj mend);
 
OP INITSCOPE = (MENDOBJ obj mend)SCOPEOBJ: HEAP NEWSCOPEOBJ INITAB obj mend;
OP +=: = (SCOPEOBJ item, REF SCOPEOBJ rhs)SCOPEOBJ: ( up OF item := rhs; rhs := item );
OP +=: = (MENDOBJ mend, REF SCOPEOBJ rhs)SCOPEOBJ: INITSCOPE mend +=: rhs;
#OP -=: = (REF SCOPEOBJ scope)SCOPEOBJ: scope := up OF scope;#
 
COMMENT Restore the prio event scope: ~ END COMMENT
PROC obj reset = (SCOPEOBJ up scope)VOID: obj scope begin := up scope;
MENDOBJ obj unmendable = (OBJ obj)BOOL: FALSE;
 
MODE NEWEVENTOBJ = STRUCT( # the is simple a typed place holder #
  SCOPEOBJ scope,
  STRING description,
  PROC (OBJ #obj#, MENDOBJ #obj mend#)SCOPEOBJ on,
  PROC (OBJ #obj#, STRING #msg#)VOID raise
), EVENTOBJ = REF NEWEVENTOBJ;
 
MODE FLEXEVENTOBJ = FLEX[0]EVENTOBJ;
 
COMMENT Define how to catch an event:
    obj - IF obj IS NIL then mend event on all OBJects
    obj mend - PROC to call to repair the object
    return the prior event scope
END COMMENT
PROC obj on = (FLEXOBJ obj flex, FLEXEVENTOBJ event flex, MENDOBJ mend)SCOPEOBJ: (
  mend +=: obj scope begin;
  IF obj any ISNTIN obj flex THEN obj flex OF obj scope begin := obj flex FI;
  IF obj event any ISNTIN event flex THEN event flex OF obj scope begin := event flex FI;
  up OF obj scope begin
);
 
PRIO OBJIS = 4, OBJISNT = 4; # pick the same PRIOrity as EQ and NE #
OP OBJISNT = (OBJ a,b)BOOL: NOT(a OBJIS b);
 
PRIO ISIN = 4, ISNTIN = 4;
OP ISNTIN = (OBJ obj, FLEXOBJ obj flex)BOOL: (
  BOOL isnt in := FALSE;
  FOR i TO UPB obj flex WHILE isnt in := obj OBJISNT obj flex[i] DO SKIP OD;
  isnt in
);
OP ISIN = (OBJ obj, FLEXOBJ obj flex)BOOL: NOT(obj ISNTIN obj flex);
 
OP ISNTIN = (EVENTOBJ event, FLEXEVENTOBJ event flex)BOOL: (
  BOOL isnt in := TRUE;
  FOR i TO UPB event flex WHILE isnt in := event ISNT event flex[i] DO SKIP OD;
  isnt in
);
OP ISIN = (EVENTOBJ event, FLEXEVENTOBJ event flex)BOOL: NOT(event ISNTIN event flex);
 
COMMENT Define how to raise an event, once it is raised try and mend it:
  if all else fails produce an error message and stop
END COMMENT
PROC obj raise = (OBJ obj, EVENTOBJ event, STRING msg)VOID:(
  SCOPEOBJ this scope := obj scope begin;
# until mended this event should cascade through scope event handlers/members #
  FOR i WHILE this scope ISNT SCOPEOBJ(obj scope end) DO
    IF (obj any ISIN obj flex OF this scope OR obj ISIN obj flex OF this scope ) AND
       (obj event any ISIN event flex OF this scope OR event ISIN event flex OF this scope)
    THEN
      CASE mended OF this scope IN
        (RAWMENDOBJ mend):IF mend(obj) THEN break mended FI,
        (PROC VOID go to): (go to; stop)
      OUT put(stand error, "undefined: raise stop"); stop
      ESAC
    FI;
    this scope := up OF this scope
  OD;
  put(stand error, ("OBJ event: ",msg)); stop; FALSE
EXIT
  break mended: TRUE
);
 
CO define ON and some useful(?) RAISE OPs CO
PRIO ON = 1, RAISE = 1;
OP ON = (MENDOBJ mend, EVENTOBJ event)SCOPEOBJ: obj on(obj any, event, mend),
   RAISE = (OBJ obj, EVENTOBJ event)VOID: obj raise(obj, event, "unnamed event"),
   RAISE = (OBJ obj, MENDOBJ mend)VOID: ( mend ON obj event any; obj RAISE obj event any),
   RAISE = (EVENTOBJ event)VOID: obj raise(obj any, event, "unnamed event"),
   RAISE = (MENDOBJ mend)VOID: ( mend ON obj event any; RAISE obj event any),
   RAISE = (STRING msg, EVENTOBJ event)VOID: obj raise(obj any, event, msg);
OP (SCOPEOBJ #up scope#)VOID RESET = obj reset;
 
SKIP

File: test/event.a68

#!/usr/bin/a68g --script #
 
MODE OBJ=UNION(REF INT, REF REAL, REF STRING,# etc # VOID);
 
OP OBJIS = (OBJ a,b)BOOL: # Are a and b at the same address? #
  CASE a IN # Ironically Algol68's STRONG typing means we cannot simply compare addresses #
    (REF INT a): a IS (b|(REF INT b):b|NIL),
    (REF REAL a): a IS (b|(REF REAL b):b|NIL),
    (REF STRING a): a IS (b|(REF STRING b):b|NIL)
  OUT FALSE
  ESAC;
 
PR READ "prelude/event_base(obj).a68" PR;
NEWEVENTOBJ obj eventa := SKIP;
NEWEVENTOBJ obj eventb := SKIP;
NEWEVENTOBJ user defined exception := SKIP;
 
# An event can be continued "mended" or break "unmended" #
PROC found sum sqs continue = (OBJ obj)BOOL: ( print("."); TRUE); # mended #
PROC found sum sqs break = (OBJ obj)BOOL: (found sq sum sqs; FALSE); # unmended #
 
INT sum sqs:=0;
REAL x:=111, y:=222, z:=333;
 
SCOPEOBJ obj scope reset := obj on((sum sqs, x,y,z), (obj eventa,obj eventb), VOID:found sq sum sqs);
 
# An event handler specific to the specific object instance: #
#SCOPEOBJ obj scope reset := obj on eventb(sum sqs, VOID:found sq sum sqs);#
 
# Or... An "obj any" event handler: #
# SCOPEOBJ obj scope reset := found sum sqs break ON obj eventb; #
 
# Raise the "event eventb" on an object: #
  FOR i DO
    sum sqs +:= i*i;
    IF sum sqs = 70*70 THEN # 1st try to use an instance specific mend on the object #
      obj raise(sum sqs, obj eventb, "Found a sq sum of sqs") FI; # OR ... #
    IF sum sqs = 70*70 THEN "Found a sq sum of sqs" RAISE obj eventb FI; # OR ... #
    IF sum sqs = 70*70 THEN RAISE found sum sqs break FI # simplest #
  OD;
  RESET obj scope reset # need to manually reset back to prior handlers #
 
# Catch "event eventb": #
EXIT found sq sum sqs:
  print(("sum sqs:",sum sqs, new line)); # event eventb caught code here ... #
  RESET obj scope reset;
 
  "finally: raise the base unmendable event" RAISE obj eventb

Output:

sum sqs:      +4900OBJ event: finally: raise the base unmendable event

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 encountered during transput
• on page end - if the end of a page was encountered during transput
• on physical file end - if the end of physical media was encountered 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, e.g.:

• 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 un-handled, 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.

See also

• StackOverflow: What language was the first to implement exception handling?

AppleScript[edit]

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

AutoHotkey[edit]

True exceptions[edit]

Works with: AutoHotkey_L

In AutoHotkey_L Try, Catch, and Throw are available to handle exceptions.
From the Throw documentation:

try
    BadlyCodedFunc()
catch e
    MsgBox % "Error in " e.What ", which was called at line " e.Line 
 
BadlyCodedFunc() {
    throw Exception("Fail", -1)
}

ErrorLevel-based exceptions[edit]

In AutoHotkey_Basic, the only option for error-handling is using ErrorLevel

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

BBC BASIC[edit]

      ON ERROR PROCerror(ERR, REPORT$) : END
 
      ERROR 100, "User-generated exception"
      END
 
      DEF PROCerror(er%, rpt$)
      PRINT "Exception occurred"
      PRINT "Error number was " ; er%
      PRINT "Error string was " rpt$
      ENDPROC

Output:

Exception occurredError number was 100Error string was User-generated exception

Bracmat[edit]

After completed evaluation, each Bracmat expression not only has a value, but also a success or failure status attached to it. Pattern matching expressions are the most common expressions to test some condition, but also other expressions can obtain a status different from 'success'. Here are some situations where an expression fails:

• pattern matching fails if the pattern does not match the subject
• reading the contents of a file fails if the file cannot be opened for reading or if the file has not the expected format
• retrieving a value bound to a symbol that has no value bound to it fails
• accessing an object member that does not exist fails
• division by zero fails
• an operation headed by the 'and then' operator & fails if either its left hand side or its right hand side fails
• an operation headed by the 'or else' operator | fails if both its left hand side and its right hand side fail

Rather than writing statements delimited by ; characters, which do not have usefull succes/failure states, you write a Bracmat program with 'and then' and 'or else' as connectives between expressions. A Bracmat program therefore needs only consist of a single statement. In many cases a series of expressions connected with the 'and then' operator is meant to succeed, evaluating each expression in turn with success. In an exceptional case, however, such as a file that cannot be opened, the series is interrupted and control can be taken over by an outer 'or else' control structure that evaluates its right hand side.

To 'throw and exception' you can use the always failing expression ~. In the example, the ~ ensures that the call to the contemplate function is never attempted if something went wrong with reading the files. This failure percolates further up, so calling the function MyFunction fails as well, making the whole program fail.

( ( MyFunction
  =   someText XMLstuff
    .   (   get$!arg:?someText
          & get$("CorporateData.xml",X,ML):?XMLstuff
        |     out
            $ ( str
              $ ( "Something went wrong when reading your file \""
                  !arg
                  "\". Or was it the Corporate Data? Hard to say. Anyhow, now I throw you out."
                )
              )
          & ~
        )
      & contemplate$(!someText,!XMLstuff)
  )
& MyFunction$"Tralula.txt"
);

If you copy/paste this code to the Bracmat prompt without the statement delimiter ;, you will see an 'F' after the output, indicating that your input failed to evaluate successfully.

Something went wrong when reading your file "Tralula.txt". Or was it the Corporate Data? Hard to say. Anyhow, now I throw you out.    F

C[edit]

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" */
  }
}

With multi-thread support and nested exceptions

 
#include <stdio.h>
#include <setjmp.h>
#include <stdlib.h>
 
enum exceptions {
        EXCEPTION_1 = 1,
        EXCEPTION_2,
        EXCEPTION_3
};
 
 
#define throw(exception) do {                                   \
                if (exp)                                        \
                        longjmp(*exp, exception);               \
                printf("uncaught exception %d\n", exception);   \
                exit(exception);                                \
        } while (0)
 
#define try(block, catch_block)                 \
{                                               \
        jmp_buf *exception_outer = exp;         \
        jmp_buf exception_inner;                \
        exp = &exception_inner;                 \
        int exception = setjmp(*exp);           \
        if (!exception) {                       \
                do block while(0);              \
                exp = exception_outer;          \
        } else {                                \
                exp = exception_outer;          \
                switch(exception) {             \
                catch_block                     \
                default:                        \
                        throw(exception);       \
                }                               \
        }                                       \
}
 
#define catch(exception, block) \
        case exception: do block while (0); break;
 
 
#define throws  jmp_buf* exp
 
// define a throwing function
void g(throws) {
        printf("g !\n");
        throw(EXCEPTION_1);
        printf("shouldnt b here\n");
}
 
void h(throws, int a)
{
        printf("h %d!\n", a);
        throw(EXCEPTION_2);
}
 
void f(throws) {
        try({
                g(exp); // call g with intention to catch exceptions
        },
        catch(EXCEPTION_1, {
                printf("exception 1\n");
                h(exp, 50); // will throw exception 2 inside this catch block
        }))
}
 
int main(int argc, char* argv[])
{
        throws = NULL; // define exception stack base
        try({
                f(exp);
        },
        catch(EXCEPTION_2, {
                printf("exception 2\n");
        })
        catch(EXCEPTION_3, {
                printf("exception 3\n");
        }))
 
        h(exp, 60); // will result in "uncaught exception"
        return 0;
}
 
 

Now all we need to do is add a finally block :) hint: it is possible

C++[edit]

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
}

C#[edit]

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
}

Clojure[edit]

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"))

ColdFusion[edit]

Catch Exceptions
inside <cfscript>:

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

otherwise:

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

Common Lisp[edit]

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.

D[edit]

import std.stdio;
 
/// Throw Exceptions
/// Stack traces are generated compiling with the -g switch.
void test1() {
  throw new Exception("Sample Exception");
}
 
/// Catch Exceptions
void test2() {
    try {
        test1();
    } catch (Exception ex) {
        writeln(ex);
        throw ex; // rethrow
    }
}
 
/// Ways to implement finally
void test3() {
    try test2();
    finally writeln("test3 finally");
}
 
/// Or also with scope guards
void test4() {
    scope(exit) writeln("Test4 done");
    scope(failure) writeln("Test4 exited by exception");
    scope(success) writeln("Test4 exited by return or function end");
    test2();
}
 
void main() {
    test4();
}

Delphi[edit]

Throw Exceptions

procedure test;
begin
  raise Exception.Create('Sample Exception');
end;

Catch Exceptions

procedure test2;
begin
  try
    test;
  except
    ShowMessage(Exception(ExceptObject).Message); // Showing exception message
    raise; // Rethrowing
  end;
end;

Ways to implement finally

procedure test3;
begin
  try
    test2;
  finally
    ShowMessage('test3 finally');
  end;
end;

Déjà Vu[edit]

stuff-going-wrong:
raise :value-error
 
try:
stuff-going-wrong
catch value-error:
!print "Whoops!"

Output:

Whoops!

DWScript[edit]

Throw Exceptions

procedure Test;
begin
   raise Exception.Create('Sample Exception');
end;

Catch Exceptions

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;

Ways to implement finally

procedure Test3;
begin
   try
      test2;
   finally
      PrintLn('Test3 finally');
   end;
end;

E[edit]

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.

Erlang[edit]

 
-module( exceptions ).
 
-export( [task/0] ).
 
task() ->
    try    
    erlang:throw( new_exception )
 
    catch
    _:Exception -> io:fwrite( "Catched ~p~n", [Exception] )
 
    end.
 

Output:

14> exceptions:task().Catched new_exception

Factor[edit]

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

Fancy[edit]

# define custom exception class
# StandardError is base class for all exception classes
class MyError : StandardError {
  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
}

Fantom[edit]

 
// 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)
  }
}
 

Output:

$ fan exceptions.fan Caught exceptions_0::SpecialErr: special error

Forth[edit]

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

Go[edit]

Execution errors such as attempting to index an array out of bounds trigger a run-time panic equivalent to a call of the built-in function panic() with a value of the implementation-defined interface type runtime.Error.

panic(x) "throws" a value (of any type), 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 fails).

package main
 
import "fmt"
 
func foo() int {
fmt.Println("let's foo...")
defer func() {
if e := recover(); e != nil {
fmt.Println("Recovered from", e)
}
}()
var a []int
a[12] = 0
fmt.Println("there's no point in going on.")
panic("never reached")
panic(fmt.Scan) // Can use any value, here a function!
}
 
func main() {
foo()
fmt.Println("glad that's over.")
}

Run in the Go Playground.

Output:

let's foo...Recovered from runtime error: index out of rangeglad that's over.

Haskell[edit]

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

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 -}

Icon and Unicon[edit]

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.

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

A sample run is:

-> ExceptionTest 1 2 3 4 512Exception: bad value of 3:    procedure g [ExceptionTest.icn:15]    procedure main [ExceptionTest.icn:5]45->

Note: it may be possible to implement exceptions in Icon; however, it would require a major rework and would likely be inelegant.

J[edit]

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!

Java[edit]

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
}

Works with: Java version 7+

Java 7 added "multicatch" and "smart rethrow".

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;
    }
}

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[edit]

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;
}

jq[edit]

Works with: jq version >1.4

The ability to "catch" exceptions was introduced after jq version 1.4 was released and so a brief explanation is included here.

Exceptions, as before, can be raised by the execution of an error statement: error(STRING)

The "try" clause takes the form:

try FILTER catch CATCHER

where FILTER and CATCHER may be any jq expressions.

Within a "try" clause, . is the value of the STRING of the exception that has been caught.

Example:

def division(a;b):
  def abs: if . < 0 then -. else . end;
  if a == 0 and b == 0 then error("0/0")
  elif b == 0 then error("division by 0")
  elif (a|abs|log) - (b|abs|log) > 700 then error("OOB")
  else a/b
  end;
 
def test(a;b):
  try division(a;b)
  catch if . == "0/0" then 0
        elif . == "division by 0" then null
        else "\(.): \(a) / \(b)"
        end;

1. test(0;0) # produces 0

1. test(1e300; 1e-300) # produces "OOB: 1e+300 / 1e-300"</lang>

Lasso[edit]

protect => {
   handle_error => {
        // do something else
   }
   fail(-1,'Oops')
}

Logo[edit]

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

Logtalk[edit]

Logtalk exception-handling mechanism is based on the catch/3 and throw/1 predicates inherited from Prolog:

 
:- object(exceptions).
 
    :- public(double/2).
    double(X, Y) :-
        catch(double_it(X,Y), Error, handler(Error, Y)).
 
    handler(error(not_a_number(X), logtalk(This::double(X,Y), Sender)), Y) :-
        % try to fix the error and resume computation;
        % if not possible, rethrow the exception
        (   catch(number_codes(Nx, X), _, fail) ->
            double_it(Nx, Y)
        ;   throw(error(not_a_number(X), logtalk(This::double(X,Y), Sender)))
        ).
 
    double_it(X, Y) :-
        (   number(X) ->
            Y is 2*X
        ;   this(This),
            sender(Sender),
            throw(error(not_a_number(X), logtalk(This::double(X,Y), Sender)))
        ).
 
:- end_object.
 

Output:

| ?- exceptions::double(1, Double).Double = 2yes| ?- exceptions::double("1", Double).Double = 2yes| ?- exceptions::double(a, Double).uncaught exception: error(not_a_number(a),logtalk(exceptions::double(a,_),user))

Lua[edit]

Throwing an Exception

 
error("Something bad happened!")
 

Catching Exceptions

 
function throw_error()
    error("Whoops")
    -- won't ever appear, due to previous error() call
    return "hello!"
end
 
-- 'status' is false if 'throw_error' threw an error
-- otherwise, when everything went well, it will be true.
-- 'errmsg' contains the error message, plus filename and line number of where the error occured
status, errmsg = pcall(throw_error)
print("errmsg = ", errmsg)
 

Note that `pcall` passes every argument after the function object or function name to said function:

 
function throw_error_with_argment(argument)
    error(string.format("Whoops! argument = %s", argument))
    -- won't ever appear, due to previous error() call
    return "hello!"
end
 
status, errmsg = pcall(throw_error_with_argment, "foobar 123")
print("errmsg = ", errmsg)
 

If a function does not throw an error, 'errmsg' (which might be called 'returned' as well) contains the value(s) returned from the function:

 
function throw_error_with_argment(argument)
    return "hello!"
end
 
status, errmsg = pcall(throw_error_with_argment, "foobar 123")
print("errmsg = ", errmsg)
 

Make[edit]

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

Mathematica / Wolfram Language[edit]

f[x_] := If[x > 10, Throw[overflow], x!]
 
Example usage : 
Catch[f[2] + f[11]]
-> overflow
 
Catch[f[2] + f[3]]
-> 8

MATLAB[edit]

Errors are thrown using the "error" keyword.

Sample usage:

>> error 'Help'
??? Help

Modula-3[edit]

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;

MOO[edit]

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()';

Nemerle[edit]

// define a new exception
class MyException : Exception
{
    ...
}
 
// throw an exception
Foo() : void
{
    throw MyException();
}
 
// catching exceptions
try {
    Foo();
}
catch { // catch block uses pattern matching syntax
    |e is MyException => ... // handle exception
    |_ => throw e // rethrow unhandled exception
}
finally {
    ... // code executes whether or not exception was thrown
}

NetRexx[edit]

As NetRexx runs under the control of a JVM it has the same exception model as Java.

/* NetRexx */
options replace format comments java crossref symbols nobinary
 
-- =============================================================================
class RExceptions public
 
  -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  method test() public signals RExceptions.TakeException
    if (1 == 1) then signal RExceptions.TakeException()
    return
 
  -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  method main(args = String[]) public static
    do
      RExceptions().test()
    catch ex = Exception
      say ex.toString()
    end
 
    return;
 
-- =============================================================================
class RExceptions.TakeException public extends Exception
 
  -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  method TakeException() public
    super('I resent that!')
    return
 

Output:

RExceptions$TakeException: I resent that!

Nim[edit]

Defining exceptions

type SillyError = object of Exception

Throwing an exception

proc spam() =
  raise newException(SillyError, "Some error")

Handling an exception

try:
  spam()
except SillyError:
  echo "Got SillyError with message: ", getCurrentExceptionMsg()
except:
  echo "Got another exception"
finally:
  echo "Finally"

Objective-C[edit]

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
}

OCaml[edit]

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"

Oforth[edit]

Oforth uses try/when blocks to trap exceptions and throw to throw an execption

It is also possible to create new exception classes (see Exception.of).

func: iwillThrowAnException { Exception throw("A new exception") }
 
func: iwillCatch {
| e |
   try: e [ iwillThrowAnException ] when: [ System.Out "Exception catched : " << e << cr ]
   try: e [ 1 dup last ] when: [ System.Out "Exception catched : " << e << cr ]
   "Done" println
}

Output:

Exception catched : A new exceptionException catched : Integer <1> does not understand #lastDone

Oz[edit]

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

PARI/GP[edit]

Catching errors in GP[edit]

The errors that can be trapped in GP are:

alarmergeneric errorgdiverdivision by 0invmoderimpossible modular inversearchernot available on this architecture or operating systemtypeerwrong typeerrpilethe PARI stack overflowstalkergeneric erroruserUser-initiated error

trap(/* specific error can be given here, or leave blank to catch all */,
  "caught"
,
  error("bad stuff")
)

Throwing errors in GP[edit]

The only error that can be thrown in GP is user error:

error("Text of error here")

Throwing errors in PARI[edit]

Many more errors can be caught and thrown directly in PARI:

0Generic errortalker2?bugparierBug, please reportalarmerGeneric erroropenfilerFile I/OtalkerGeneric errorflagerrInvalid flagimplNot implementedarcherNot available on this systemnotfuncerNot a function in function callprecerPrecision too lowtypeerIncorrect typeconsisterInconsistent datauserUser-initiated errorerrpileStack overflowoverflowerOverflowmatinv1Non-invertible matrix (in gauss)mattype1Not a square matrixarither1Not an integer argument in an arithmetic functionprimer1Not enough precomputed primesinvmoderImpossible inverseconstpolerConstant polynomialnotpolerNot a polynomialredpolerReducible polynomialzeropolerZero polynomialoperi"Impossible"operf"Forbidden"gdiverDivision by zeromemerNot enough memorynegexperNegative exponentsqrter5Non quadratic residue (in gsqrt)noerNot an error...

pari_err(arither1, "functionName"); // Gives "*** functionName: not an integer argument in an arithmetic function"

Catching errors in PARI[edit]

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:

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

Pascal[edit]

See Delphi

Perl[edit]

Using eval
Exceptions 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.

Perl 6[edit]

Works with: rakudo version 2015-09-10

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. The inside of a CATCH functions as a switch statement on the current exception.

try {
    die "Help I'm dieing!";
    CATCH {
        when X::AdHoc { note .Str.uc; say "Cough, Cough, Aiee!!" }
        default { note "Unexpected exception, $_!" }
    }
}
 
say "Yay. I'm alive.";
 
die "I'm dead.";
 
say "Arrgh.";
 
CATCH {
    default { note "No you're not."; say $_.Str; }
}

HELP I'M DIEING!Cough, Cough, Aiee!!Yay. I'm alive.No you're not.I'm dead.

Perl 6 comes with phasers, that are called when certain conditions in the life of a program, routine or block are met. CATCH is one of them and works nicely together withLEAVE that is called even if an exception would force the current block to be left immediately. It's a nice place to put your cleanup code.

sub f(){
        ENTER { note '1) f has been entered' }
        LEAVE { note '2) f has been left' }
        say '3) here be dragons';
        die '4) that happend to be deadly';
}
 
f();
say '5) am I alive?';
 
CATCH {
        when X::AdHoc { note q{6) no, I'm dead}; }
}

1) f has been entered3) here be dragons6) no, I'm dead2) f has been left

PHL[edit]

PHL does not support multiple catch-clauses.

module exceptions;
 
extern printf;
 
struct @MyException : @Exception {
 
};
 
@Void func throws ex [
throw new @MyException;
]
 
@Integer main [
try func();
catch (e) {
if (e::getType == "MyException") {
printf("MyException thrown!\n");
} else {
printf("Unhandled exception!\n");
}
}
return 0;
]

PHP[edit]

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();
}

PicoLisp[edit]

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

PL/I[edit]

 
/* 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.                                 */
 

PL/pgSQL[edit]

Raise an exception

 
BEGIN
   raise exception 'this is a generic user exception';
   raise exception division_by_zero;
END;
 

Handle an exception

Hande division by zero and re-raising once caught other exception:

 
CREATE FUNCTION special_division(p_num DOUBLE PRECISION, p_den DOUBLE PRECISION) RETURNS text
AS $body$
BEGIN
   RETURN p_num/p_den::text;
EXCEPTION
   WHEN division_by_zero THEN
      IF p_num>0 THEN
         RETURN 'Inf';
      ELSIF p_num<0 THEN
         RETURN '-Inf';
      ELSE 
         RETURN 'INDEF';
      END IF;
   WHEN others THEN
      raise;
END; 
 

Pop11[edit]

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();

PureBasic[edit]

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

Python[edit]

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()

R[edit]

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")
)
 

Racket[edit]

 
#lang racket
 
;; define a new exception type
(struct exn:my-exception exn ())
 
;; handler that prints the message ("Hi!")
(define (handler exn)
  (displayln (exn-message exn)))
 
;; install exception handlers
(with-handlers ([exn:my-exception? handler])
  ;; raise the exception
  (raise (exn:my-exception "Hi!" (current-continuation-marks))))
 

Raven[edit]

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

REXX[edit]

While the REXX language doesn't have a throw capability per se, it does have the ability to catch exceptions (by label).
This type of exception handling (in REXX) has its limitation (the label is local to the program, not external subroutines).

/*REXX program demonstrates handling an exception;   catching is via a label. */
          do j=9  by -5
          say  'square root of'  j  "is"  sqrt(j)
          end   /*j*/
exit                                   /*stick a fork in it,  we're all done. */
 
.sqrtNeg: say  'illegal SQRT argument (argument is negative):'  x
exit                                   /*stick a fork in it,  we're all done. */
/*────────────────────────────────────────────────────────────────────────────*/
sqrt: procedure; parse arg x;   if x=0  then return 0;  d=digits();  m.=9
      numeric digits 9; numeric form; h=d+6;        if x<0  then signal .sqrtNeg
      parse value format(x,2,1,,0) 'E0'  with  g 'E' _ .;       g=g*.5'e'_%2
         do j=0  while h>9;      m.j=h;              h=h%2+1;         end  /*j*/
         do k=j+5  to 0  by -1;  numeric digits m.k; g=(g+x/g)*.5;    end  /*k*/
      numeric digits d;     return (g/1)             /*make complex if  X < 0.*/

output

square root of 9 is 3square root of 4 is 2illegal SQRT argument (argument is negative): -1

Ruby[edit]

Defining an exception

# define an exception
class SillyError < Exception
end

SillyError is simply declared as a subclass of Exception. No over-riding is necessary.

class MyInvalidArgument < ArgumentError
end

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

 
# 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

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

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.

# short way to rescue any StandardError
quotient = 1 / 0 rescue "sorry"

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.

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

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.

Scala[edit]

Library: Scala

In Scala there is, thank heavens, no "checked exception" doctrine. Exceptions can be freely implemented, it's the skill of the programmer to use them right. This example is bad example of using exceptions, only to show that they can be used.

In there are 3 main entries: object CheckingAccount, CheckingBlockingAccount and NotImplementedErrorTest to selective start this solution and demonstrate the working of exceptions and handling.

//Defining exceptions
class AccountBlockException extends Exception
class InsufficientFundsException(val amount: Double) extends Exception
 
class CheckingAccount(number: Int, var blocked: Boolean = false, var balance: Double = 0.0) {
  def deposit(amount: Double) { // Throwing an exception 1
    if (blocked) throw new AccountBlockException
    balance += amount
  }
 
  def withdraw(amount: Double) { // Throwing an exception 2
    if (blocked) throw new AccountBlockException
    if (amount <= balance) balance -= amount
    else throw new InsufficientFundsException(amount - balance)
  }
}
 
object CheckingAccount extends App {
 
  class ExampleException1 extends Exception
 
  val c = new CheckingAccount(101)
  println("Depositing $500...")
  try {
    c.deposit(500.00)
    println("\nWithdrawing $100...")
    c.withdraw(100.00)
    println("\nWithdrawing $600...")
    c.withdraw(600.00)
  } catch { // Exception handler
    case ac: InsufficientFundsException => println(s"Sorry, but you are short ${'$'} ${ac.amount}")
    case ac: AccountBlockException      => println("Account blocked.")
 
    ///////////////////////////// An example of multiple exception handler ////////////////////////
    case e@(_: ExampleException1 |
      _: InterruptedException) => println(s"Out of memory or something else.")
 
    case e: Exception => e.printStackTrace()
    case _: Throwable => // Exception cached without any action
  } finally println("Have a nice day")
}
 
object CheckingBlockingAccount extends App {
  val c = new CheckingAccount(102, true)
  println("Depositing $500...")
  try {
    c.deposit(500.00)
    println("\nWithdrawing $100...")
    c.withdraw(100.00)
    println("\nWithdrawing $600...")
    c.withdraw(600.00)
  } catch { // Exception handler
    case ac: InsufficientFundsException => println(s"Sorry, but you are short ${'$'} ${ac.amount}")
    case ac: AccountBlockException      => println("Account blocked.")
    case e: Exception                   => e.printStackTrace()
    case _: Throwable                   =>
  } finally println("Have a nice day")
}
 
object NotImplementedErrorTest extends App {
  ??? // Throws  scala.NotImplementedError: an implementation is missing
}

Output:

Running entry point CheckingAccount

Depositing $500...Withdrawing $100...Withdrawing $600...Sorry, but you are short $ 200.0Have a nice day

Output:

Running entry point CheckingBlockingAccount

Depositing $500...Account blocked.Have a nice day

Output:

Running entry point NotImplementedErrorTest

Exception in thread "main" scala.NotImplementedError: an implementation is missing  at scala.Predef$.$qmark$qmark$qmark(Predef.scala:252)  at NotImplementedErrorTest$delayedInit$body.apply(CheckingAccount.scala:53)  .....

Scheme[edit]

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).

(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!")))

Seed7[edit]

Raise an exception

const proc: foo is func
  begin
    raise RANGE_ERROR;
  end func;

Handle an exception

const proc: main is func
  begin
    block
      foo;
    exception
      catch RANGE_ERROR:
        writeln("catched RANGE_ERROR");
    end block;
  end func;

Sidef[edit]

An exception is thrown by the die keyword, which, if not caught, it terminates the program with an appropriate exit code.

try  {
    die "I'm dead!";        # throws an exception of type 'error'
}
catch { |type, msg|
    say "type: #{type}";    # type: error
    say "msg: #{msg}";      # msg: I'm dead! at test.sf line 2.
};
 
say "I'm alive...";
die "Now I'm dead!";        # this line terminates the program
say "Or am I?";             # Yes, you are!

Output:

type: errormsg: I'm dead! at test.sf line 2.I'm alive...Now I'm dead! at test.sf line 10.

Slate[edit]

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.'].

Smalltalk[edit]

Throwing an Exception

"exec" "gst" "-f" "$0" "$0" "$*"
"exit"
 
Transcript show: 'Throwing yawp'; cr.
self error: 'Yawp!'.

$ ./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)

Handling an Exception

"exec" "gst" "-f" "$0" "$0" "$*"
"exit"
 
[
Transcript show: 'Throwing yawp'; cr.
self error: 'Yawp!'.
] on: Error do: [ :e |
Transcript show: 'Caught yawp'; cr.
].

$ ./yawp.st 
Throwing yawp
Caught yawp

Standard ML[edit]

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;

Swift[edit]

Works with: Swift version 2.x+

Defining exceptions
Exceptions can be of any type that conforms to the ErrorType protocol.

enum MyException : ErrorType {
  case TerribleException
}

Throw exceptions
A function that throws an exception must be explicitly declared so:

func foo() throws {
  throw MyException.TerribleException
}

Catching exceptions

do {
  try foo()
} catch MyException.TerribleException { // this can be any pattern
  //Catch a specific case of exception
} catch {
  //Catch any exception
}

Tcl[edit]

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"

TXR[edit]

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.

@(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

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[edit]

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.

V[edit]

throwing exceptions

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

catching them

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

Visual Basic .NET[edit]

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

Visual Basic for Applications[edit]

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

zkl[edit]

try{ throw(Exception.BadDay) }
catch { println(__exception," was thrown") }
fallthrough {  println("No exception was thrown") }
println("OK");

Output:

BadDay(I'm having a bad day) was thrownOK

If you want "finally" functionality, use onExit or onExitBlock:

fcn f(b){
   try{ 
      onExitBlock("Exit code".println);
      if (b) throw(Exception.BadDay) 
   }
   catch{ println(__exception," was thrown") }
   fallthrough{  println("No exception was thrown") }
   println("OK");
}
f(False); println("--------");
f(True);

Output:

Exit codeNo exception was thrownOK--------Exit codeBadDay(I'm having a bad day) was thrownOK

from: http://rosettacode.org/wiki/Exceptions