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Revision as of 10:52, 10 March 2020
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.
- Related task
11l
<lang 11l>T SillyError
String message
F (message)
.message = message
X.try
X SillyError(‘egg’)
X.catch SillyError se
print(se.message)</lang>
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>
Aime
Simple Exception Throwing <lang aime>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;
}</lang>
- Output:
ready to catch throwing... aime: tmp/et: 5: now! caught!
Exception Types <lang aime>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;
}</lang>
- Output:
aime: tmp/et1: 6: bad number exception of type `odd' thrown for 5 aime: tmp/et1: 9: bad number exception of type `not a power of two' thrown for 6 no exception thrown for 8
ALGOL 68
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.
File: prelude/event_base(obj).a68<lang algol68>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</lang>File: test/event.a68<lang algol68>#!/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</lang>
- Output:
sum sqs: +4900 OBJ 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
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
True exceptions
In AutoHotkey_L Try, Catch, and Throw are available to handle exceptions.
From the Throw documentation:
<lang AHK>try
BadlyCodedFunc()
catch e
MsgBox % "Error in " e.What ", which was called at line " e.Line
BadlyCodedFunc() {
throw Exception("Fail", -1)
}</lang>
ErrorLevel-based exceptions
In AutoHotkey_Basic, the only option for error-handling is using ErrorLevel <lang AutoHotkey>foo() If ErrorLevel
Msgbox calling foo failed with: %ErrorLevel%
foo() {
If success Return Else ErrorLevel = foo_error Return
}</lang>
BBC BASIC
<lang bbcbasic> 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</lang>
Output:
Exception occurred Error number was 100 Error string was User-generated exception
blz
<lang blz> try
1 / 0 # Throw an exception
print("unreachable code")
catch
print("An error occured!")
end </lang>
Bracmat
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.
<lang bracmat>( ( 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"
);</lang>
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
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>
With multi-thread support and nested exceptions <lang c>
- 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;
}
</lang> Now all we need to do is add a finally block :) hint: it is possible
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
<lang d>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();
}</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>
Déjà Vu
<lang dejavu>stuff-going-wrong: raise :value-error
try: stuff-going-wrong catch value-error: !print "Whoops!"</lang>
- Output:
Whoops!
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.
Elena
Defining exceptions <lang elena>class MyException : Exception {
constructor new()
<= new("MyException raised");
}</lang>
Throw exceptions <lang elena>foo() {
MyException.raise()
} </lang>
Catching exceptions <lang elena>try {
o.foo()
} catch:(MyException e) {
// handle exceptions of type MyException and derived
}</lang>
Catching any exception <lang elena>o.foo() | on:(e) {
// handle any type of exception
};</lang>
Erlang
<lang Erlang> -module( exceptions ).
-export( [task/0] ).
task() ->
try erlang:throw( new_exception )
catch _:Exception -> io:fwrite( "Catched ~p~n", [Exception] )
end.
</lang>
- Output:
14> exceptions:task(). Catched new_exception
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
- 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
}</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>
FreeBASIC
FreeBASIC does not support exceptions or the Try/Catch/Finally statement, as such. However, you can use the Err() function, together with a Switch statement, to provide somewhat similar functionality: <lang freebasic>' FB 1.05.0 Win64
Enum ErrorType
myError = 1000
End Enum
Sub foo()
Err = 1000 ' raise a user-defined error
End Sub
Sub callFoo()
foo()
Dim As Long errNo = Err ' cache Err in case it's reset by a different function
Select Case errNo
Case 0
' No error (system defined)
Case 1 To 17
' System defined runtime errors
Case myError: ' catch myError
Print "Caught myError : Error number"; errNo
Case Else
' catch any other type of errors here
End Select
' add any clean-up code here
End Sub
callfoo() Print Print "Press any key to quit" Sleep</lang>
- Output:
Caught myError : Error number 1000
Gambas
Click this link to run this code <lang gambas>Public Sub Main() Dim iInteger As Integer
MakeError DivError
iInteger = "2.54"
Catch
Print Error.Text
End '______________________ Public Sub DivError()
Print 10 / 0
Catch
Print Error.Text
End '______________________ Public Sub MakeError()
Error.Raise("My Error")
Catch
Print Error.Text
End</lang> Output:
My Error Division by zero Type mismatch: wanted Integer, got String instead
Go
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).
<lang go>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.") }</lang>Run in the Go Playground.
- Output:
let's foo... Recovered from runtime error: index out of range glad that's over.
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>
HolyC
HolyC has an exception handling mechanism using try/throw/catch. Exception names are limited to a maximum of 8 bytes in length.
The catch block does not have the capability to differentiate between specific exceptions. Instead, all exceptions for a try block are handled by a single catch block.
<lang holyc>try {
U8 *err = 'Error'; throw(err); // throw exception
} catch {
if (err == 'Error')
Print("Raised 'Error'");
PutExcept; // print the exception and stack trace
}</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>
jq
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: <lang jq>try FILTER catch CATCHER</lang> 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: <lang jq>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;</lang>
- test(0;0) # produces 0
- test(1e300; 1e-300) # produces "OOB: 1e+300 / 1e-300"
Julia
<lang julia>function extendedsqrt(x)
try sqrt(x)
catch
if x isa Number
sqrt(complex(x, 0))
else
throw(DomainError())
end
end
end
@show extendedsqrt(1) # 1 @show extendedsqrt(-1) # 0.0 + 1.0im @show extendedsqrt('x') # ERROR: DomainError</lang>
Kotlin
<lang scala>// version 1.0.6
// In Kotlin all Exception classes derive from Throwable and, by convention, end with the word 'Exception' class MyException (override val message: String?): Throwable(message)
fun foo() {
throw MyException("Bad foo!")
}
fun goo() {
try {
foo()
}
catch (me: MyException) {
println("Caught MyException due to '${me.message}'")
println("\nThe stack trace is:\n")
me.printStackTrace()
}
}
fun main(args: Array<String>) {
goo()
}</lang>
- Output:
Caught MyException due to 'Bad foo!'
The stack trace is:
MyException: Bad foo!
at ExceptionKt.foo(exception.kt:7)
at ExceptionKt.goo(exception.kt:12)
at ExceptionKt.main(exception.kt:22)
langur
Exceptions in langur are hashes guaranteed to contain certain fields, even if they're empty.
A catch causes all the statements preceding it within a block to be the implicit try block.
<lang langur># do something throw "not a math exception"
catch .e {
if .e["cat"] == "math" {
# change result...
} else {
# rethrow the exception
throw
}
} else {
# no exception ...
}</lang>
An else section on a catch is optional. As of 0.7, you can also use else if on a catch. The parser wraps it into the else block of the catch, so that it looks the same to the compiler and VM.
shortened catch
A catch can be shortened by using a single expression that does not start with a variable name. This uses the implicit _err exception variable. Prior to 0.7, the implicit exception variable was .err.
A shortened catch does not allow an else section (action for no exception).
<lang langur>100 / 0
catch if _err["cat"] == "math" {
# change result 123
} else {
throw
}</lang>
<lang langur>val .safediv = f { .x / .y ; catch 0 } .safediv(7, 7) # 1 .safediv(7, 0) # 0</lang>
Lasso
<lang Lasso>protect => {
handle_error => {
// do something else
}
fail(-1,'Oops')
}</lang>
Lingo
Lingo has no try...catch mechanism. A script error will always end execution of the current call stack. There is however a mechanism that prevents that script errors quit the execution of the current movie/projector: you can set up an "alertHook" that is called when such errors occur. This alertHook can then e.g. log the error to a file or database, and if it returns 1, the movie/projector continues to play:
<lang lingo>-- parent script "ErrorHandler"
on alertHook (me, errorType, errorMessage, alertType)
if alertType=#alert then return 0 -- ignore programmatic alerts
-- log error in file "error.log"
fn = _movie.path&"error.log"
fp = xtra("fileIO").new()
fp.openFile(fn, 2)
if fp.status() = -37 then
fp.createFile(fn)
fp.openFile(fn, 2)
end if
fp.setPosition(fp.getLength())
fp.writeString(_system.date() && _system.time() && errorType & ": " & errorMessage & RETURN)
fp.closeFile()
return 1 -- continues movie playback, no error dialog
end</lang>
<lang lingo>-- in a movie script on prepareMovie
_player.alertHook = script("ErrorHandler")
end</lang>
In terms of the behavior described above, a "throw" command triggering custom errors that behave exactly like real script errors can be implemented like this:
<lang lingo>-- in a movie script -- usage: throw("Custom error 23") on throw (msg)
_player.alertHook.alertHook("Script runtime error", msg, #script)
abort() -- exits call stack
end</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
Logtalk
Logtalk exception-handling mechanism is based on the catch/3 and throw/1 predicates inherited from Prolog: <lang logtalk>
- - 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.
</lang>
- Output:
| ?- exceptions::double(1, Double).
Double = 2
yes
| ?- exceptions::double("1", Double).
Double = 2
yes
| ?- exceptions::double(a, Double).
uncaught exception: error(not_a_number(a),logtalk(exceptions::double(a,_),user))
Lua
Throwing an Exception
<lang Lua>
error("Something bad happened!")
</lang>
Catching Exceptions
<lang Lua>
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) </lang>
Note that `pcall` passes every argument after the function object or function name to said function:
<lang Lua>
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) </lang>
If a function does not throw an error, 'errmsg' (which might be called 'returned' as well) contains the value(s) returned from the function:
<lang Lua>
function throw_error_with_argment(argument)
return "hello!"
end
status, errmsg = pcall(throw_error_with_argment, "foobar 123") print("errmsg = ", errmsg) </lang>
M2000 Interpreter
<lang M2000 Interpreter> Module Errors {
Module Check {
Module Error1 {
A=1/0
}
Try ok {
Error1
}
' we get an Error, and Error$ print division by zero in module Error1
If Error or not ok then Print Error$
Error "New Error"
}
Try {
Check
}
Print Error=0 ' no Error return
Print Error$ ' but Error message isn't clear
' Error$ used one time, then cleared automatic
} Errors Print Error$="" </lang>
- Output:
division by zero in module ERROR1
True
New Error in module CHECK
True
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>
Maple
<lang Maple> errorproc:=proc(n) local a; try a:=1/n; catch "numeric exception: division by zero": error "Something went wrong when dividing." end try; end proc; </lang>
Mathematica / Wolfram Language
<lang Mathematica>f[x_] := If[x > 10, Throw[overflow], x!]
Example usage : Catch[f[2] + f[11]] -> overflow
Catch[f[2] + f[3]] -> 8</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>
Nanoquery
<lang nanoquery>try
invalid "this statement will fail"
catch e
println "caught an exception"
println e
end try</lang> Throwing exceptions: <lang nanoquery>throw new(Exception, "exception reason as string")</lang>
Nemerle
<lang Nemerle>// 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
}</lang>
NetRexx
As NetRexx runs under the control of a JVM it has the same exception model as Java. <lang NetRexx>/* 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
</lang> Output:
RExceptions$TakeException: I resent that!
Nim
Defining exceptions <lang nim>type SillyError = object of Exception</lang> Throwing an exception <lang nim>proc spam() =
raise newException(SillyError, "Some error")</lang>
Handling an exception <lang nim>try:
spam()
except SillyError:
echo "Got SillyError with message: ", getCurrentExceptionMsg()
except:
echo "Got another exception"
finally:
echo "Finally"</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>
Oforth
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).
<lang Oforth>: iwillThrowAnException "A new exception" Exception throw ;
- iwillCatch
| e |
try: e [ iwillThrowAnException ] when: [ "Exception catched :" . e .cr ] try: e [ 1 2 over last ] when: [ "Exception catched :" . e .cr ] "Done" println ;</lang>
- Output:
Exception catched : A new exception Exception catched : 1 does not understand #last Done
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>
Pascal
See Delphi
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. The inside of a CATCH functions as a switch statement on the current exception. <lang perl6>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; }
}</lang>
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 with LEAVE 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.
<lang perl6>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}; }
}</lang>
1) f has been entered 3) here be dragons 6) no, I'm dead 2) f has been left
Phix
Phix provides try/catch and throw statements.
Throwing exceptions
You can throw any string (on it's own) or any integer, optionally with any (deeply nested) user_data that you like.
<lang Phix>throw("oh no")
throw(1)
throw(501,{"she",made[me],Do(it)})</lang>
Catching exceptions
There is one and only one non-optional catch clause per try statement.
The variable caught is a sequence, augmented with run-time diagnostics, with whatever was thrown in e[E_CODE] and/or e[E_USER].
<lang Phix>try
one_of(these)
catch e
if e[E_CODE]=501 then
puts(1,"that's no excuse!\n")
else
throw(e)
end if
end try</lang>
An uncaught exception terminates the program in error, otherwise control resumes in the catch clause or after the end try,
with no means (apart from some really nasty inline assembly) of resuming any half-finished block of code, and indeed any
call stack entries between a throw and a catch will already have been torn down and thrown away.
Traditionally fatal errors are re-routed via throw() when the presence of an exception handler is detected.
There is no finally construct (trivial to mimic with the introduction of a single simple boolean flag anyway).
PHL
PHL does not support multiple catch-clauses.
<lang phl>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; ]</lang>
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>
PL/pgSQL
Raise an exception <lang SQL> begin
raise exception 'this is a generic user exception'; raise exception division_by_zero;
end; </lang>
Handle an exception
Hande division by zero and re-raising once caught other exception: <lang SQL> 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; </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>
PowerShell
Throw an exception: <lang PowerShell> throw "Any error message." </lang>
- Output:
Any error message.
At line:1 char:1
+ throw "Any error message."
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : OperationStopped: (Any error message.:String) [], RuntimeException
+ FullyQualifiedErrorId : Any error message.
Throw a more specific exception: <lang PowerShell> throw [System.IO.FileNotFoundException] ".\temp.txt not found." </lang>
- Output:
.\temp.txt not found.
At line:1 char:1
+ throw [System.IO.FileNotFoundException] ".\temp.txt not found."
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : OperationStopped: (:) [], FileNotFoundException
+ FullyQualifiedErrorId : .\temp.txt not found.
Using try {} catch {} is better for more complex error checking because you can test specific errors:
<lang PowerShell>
try
{
Get-Content -Path .\temp.txt
} catch [System.IO.FileNotFoundException] {
Write-Host "File not found exception"
} catch [System.Exception] {
Write-Host "Other exception"
} </lang>
- Output:
Get-Content : Cannot find path 'C:\Users\Owner\temp.txt' because it does not exist.
At line:4 char:5
+ Get-Content -Path .\temp.txt
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ObjectNotFound: (C:\Users\Owner\temp.txt:String) [Get-Content], ItemNotFoundException
+ FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.GetContentCommand
Errors are objects like any other in PowerShell, so you may capture any detail of it: <lang PowerShell> $Error[0] | Get-Member </lang>
- Output:
TypeName: System.Management.Automation.ErrorRecord
Name MemberType Definition
---- ---------- ----------
Equals Method bool Equals(System.Object obj)
GetHashCode Method int GetHashCode()
GetObjectData Method void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serial...
GetType Method type GetType()
ToString Method string ToString()
writeErrorStream NoteProperty bool writeErrorStream=True
CategoryInfo Property System.Management.Automation.ErrorCategoryInfo CategoryInfo {get;}
ErrorDetails Property System.Management.Automation.ErrorDetails ErrorDetails {get;set;}
Exception Property System.Exception Exception {get;}
FullyQualifiedErrorId Property string FullyQualifiedErrorId {get;}
InvocationInfo Property System.Management.Automation.InvocationInfo InvocationInfo {get;}
PipelineIterationInfo Property System.Collections.ObjectModel.ReadOnlyCollection[int] PipelineIterationInfo {get;}
ScriptStackTrace Property string ScriptStackTrace {get;}
TargetObject Property System.Object TargetObject {get;}
PSMessageDetails ScriptProperty System.Object PSMessageDetails {get=& { Set-StrictMode -Version 1; $this.Exception.InnerExcep...
Prolog
<lang prolog>foo(X) :-
\+ integer(X),
throw(b('not even an int')).
foo(X) :-
\+ between(1,10,X),
throw(a('must be between 1 & 10')).
foo(X) :-
format('~p is a valid number~n', X).
go(X) :-
catch(
foo(X),
E,
handle(E)).
handle(a(Msg)) :-
format('~w~n', Msg),
!.
handle(X) :- throw(X).</lang>
- Output:
?- go(1).
1 is a valid number
true.
?- go(5).
5 is a valid number
true.
?- go(11).
must be between 1 & 10
true.
?- go(test).
ERROR: Unhandled exception: b('not even an int')
?-
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>
Racket
<lang racket>
- 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))))
</lang>
Raven
<lang raven>42 as custom_error
define foo
custom_error throw
try
foo
catch
custom_error = if 'oops' print</lang>
REXX
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).
<lang rexx>/*REXX program demonstrates handling an exception (negative #); catching is via a label.*/
do j=9 by -5
say 'the square root of ' j " is " sqrt(j)
end /*j*/
exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ sqrt: procedure; parse arg x; if x=0 then return 0; d=digits(); h=d+6; m.=9
numeric digits; numeric form; 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
.sqrtNeg: say 'illegal SQRT argument (argument is negative):' x; exit 13</lang> output
the square root of 9 is 3 the square root of 4 is 2 illegal SQRT argument (argument is negative): -1
Ring
<lang ring> Try
see 1/0
Catch
raise("Sorry we can't divide 1/0 + nl)
Done </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.
Rust
In Rust, there is no concept of "exception" per se.
Functions return a Result that can be either an error or a success, and the programmer can chose to panic on an error or not. It is illustrated by the code below:
<lang Rust>// IO error is used here just as an example of an already existing // Error use std::io::{Error, ErrorKind};
// Rust technically doesn't have exception, but different // types of error handling. Here are two examples of results.
fn valid_function() -> Result<usize, Error> {
Ok(100)
}
fn errored_function() -> Result<usize, Error> {
Err(Error::new(ErrorKind::Other, "Something wrong happened."))
}
// This should happen only when an unrecoverable error happened fn panicking_function() {
panic!("Unrecoverable state reached");
}
- [cfg(test)]
mod tests {
use super::*;
#[test]
fn test_valid_function() {
let result = match valid_function() {
Ok(number) => number,
Err(_) => panic!("This is not going to happen"),
};
assert_eq!(result, 100);
}
#[test]
fn test_errored_function() {
let result = match errored_function() {
Ok(_) => panic!("This is not going to happen"),
Err(e) => {
assert_eq!(e.to_string(), "Something wrong happened.");
0
}
};
assert_eq!(result, 0);
}
#[test]
#[should_panic]
fn test_panicking_function() {
panicking_function();
}
}</lang>
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.
<lang Scala>//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
}</lang>
- Output:
Running entry point CheckingAccount
Depositing $500... Withdrawing $100... Withdrawing $600... Sorry, but you are short $ 200.0 Have 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
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>
Seed7
Raise an exception<lang seed7>const proc: foo is func
begin raise RANGE_ERROR; end func;</lang>
Handle an exception <lang seed7>const proc: main is func
begin
block
foo;
exception
catch RANGE_ERROR:
writeln("catched RANGE_ERROR");
end block;
end func;</lang>
Sidef
An exception is thrown by the die keyword, which, if not caught, it terminates the program with an appropriate exit code. <lang ruby>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!</lang>
- Output:
type: error msg: I'm dead! at test.sf line 2. I'm alive... Now I'm dead! at test.sf line 10.
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>
SQL PL
With SQL PL: <lang sql pl> --#SET TERMINATOR @
BEGIN
DECLARE numerator INTEGER DEFAULT 12; DECLARE denominator INTEGER DEFAULT 0; DECLARE result INTEGER; DECLARE overflow CONDITION for SQLSTATE '22003' ; DECLARE CONTINUE HANDLER FOR overflow RESIGNAL SQLSTATE '22375' SET MESSAGE_TEXT = 'Zero division'; IF denominator = 0 THEN SIGNAL overflow; ELSE SET result = numerator / denominator; END IF;
END @ </lang> The next example just raise an exception, does not wrap a raised one. <lang sql pl> BEGIN
SIGNAL SQLSTATE '75002' SET MESSAGE_TEXT = 'Customer number is not known';
END @ </lang> Output:
$ db2 -td@ db2 => BEGIN ... db2 (cont.) => END @ DB21034E The command was processed as an SQL statement because it was not a valid Command Line Processor command. During SQL processing it returned: SQL0438N Application raised error or warning with diagnostic text: "Zero division". SQLSTATE=21543 db2 => BEGIN db2 (cont.) => SIGNAL SQLSTATE '75001' db2 (cont.) => SET MESSAGE_TEXT = 'Customer number is not known'; db2 (cont.) => END @ DB21034E The command was processed as an SQL statement because it was not a valid Command Line Processor command. During SQL processing it returned: SQL0438N Application raised error or warning with diagnostic text: "Customer number is not known". SQLSTATE=75001
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>
Stata
In Stata, one can trap errors with the capture command.
Without capture, if an error happens in a program, the execution of the current process terminates and the control is returned to the interactive mode. Execution may be terminated by the error or exit commands, or conditionnally using assert or confirm.
Many other commands may terminate the program if some error occurs during execution: for instance regress will return code 2000 if a regressor has only missing values.
Example of usage:
<lang stata>capture confirm file titanium.dta if _rc { if _rc==601 { display "the file does not exist" } else { * all other cases display "there was an error with return code " _rc } }</lang>
Similarly, Mata has functions error and exit to terminate execution, as well as assert and asserteq.
Swift
Defining exceptions
Exceptions can be of any type that conforms to the ErrorType protocol.
<lang swift>enum MyException : ErrorType {
case TerribleException
}</lang>
Throw exceptions
A function that throws an exception must be explicitly declared so:
<lang swift>func foo() throws {
throw MyException.TerribleException
}</lang>
Catching exceptions <lang swift>do {
try foo()
} catch MyException.TerribleException { // this can be any pattern
//Catch a specific case of exception
} catch {
//Catch any exception
}</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
Ursa
Catching exceptions: <lang ursa>try invalid "this statement will fail" catch syntaxerror # console.err is optional here out "caught an exception" endl console.err end try</lang> Throwing exceptions: <lang ursa>throw (new ursa.exceptions.exception)</lang>
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>
VBA
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 vb>Sub foo1()
err.raise(vbObjectError + 1050)
End Sub
Sub foo2()
Error vbObjectError + 1051
End Sub </lang>
Catching exceptions <lang vb>Sub bar1() 'by convention, a simple handler
On Error GoTo catch foo1 MsgBox " No Error" Exit Sub
catch:
'handle all exceptions MsgBox Err.Number & vbCrLf & Err.Description Exit Sub
End Sub
Sub bar2() 'a more complex handler, illustrating some of the flexibility of VBA exception handling
On Error GoTo catch
100 foo1 200 foo2
'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.Number
Case vbObjectError + 1050
' handle exceptions of type 1050
MsgBox "type 1050"
Case vbObjectError + 1051
' handle exceptions of type 1051
MsgBox "type 1051"
Case Else
' handle any type of exception not handled by above catches or line numbers
MsgBox Err.Number & vbCrLf & Err.Description
End Select
Resume finally
End If
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
End Sub</lang>
zkl
<lang zkl>try{ throw(Exception.BadDay) } catch { println(__exception," was thrown") } fallthrough { println("No exception was thrown") } println("OK");</lang>
- Output:
BadDay(I'm having a bad day) was thrown OK
If you want "finally" functionality, use onExit or onExitBlock: <lang zkl>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);</lang>
- Output:
Exit code No exception was thrown OK -------- Exit code BadDay(I'm having a bad day) was thrown OK
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