Related
We know that a Failure can be handled by a CATCH block.
In the following example we create an 'AdHoc' Failure (in other-sub) and we handle the Exception in a CATCH block (in my-sub)
sub my-sub {
try {
CATCH {
when X::AdHoc { say 'AdHoc Exception handled here'; .resume }
default {say 'Other Exception'; .resume}
}
my $b = other-sub();
$b.so ?? $b.say !! 'This was a Failure'.say;
}
}
sub other-sub { fail 'Failure_X' }
my-sub();
The output is the following:
AdHoc Exception handled here
This was a Failure
My question is though: How can we distinguish between Failure and a "normal" Exception in the CATCH block in order to differentiate between the two cases?
The relationship between Failure and Exception is that a Failure has an Exception - that is to say, it holds the exception object as part of its state. Something like this:
class Failure {
has Exception $.exception;
# ...
}
When a Failure "explodes", it does so by throwing the Exception that is inside of it. Thus, what reaches the CATCH block is the Exception object, and there's no link back to the enclosing Failure. (In fact, a given Exception object could in principle be held by many Failures.)
Therefore, there's no direct way to detect this. From a design perspective, you probably shouldn't be, and should find a different way to solve your problem. A Failure is just a way to defer the throwing of an exception and allowing for it to be treated as a value; it's not intended that the nature of the underlying problem changes because it's conveyed as a value rather than as an immediate transfer of control flow. Unfortunately, the original goal wasn't stated in the question; you may find it useful to look at control exceptions, but otherwise perhaps post another question about the underlying problem you're trying to solve. There's probably a better way.
For completeness, I'll note that there are indirect ways that one may detect that the Exception was thrown by a Failure. For example, if you obtain the .backtrace of the exception object and look at the top frame's package, it's possible to determine that it comes from the Failure:
sub foo() { fail X::AdHoc.new(message => "foo") }
try {
foo();
CATCH {
note do { no fatal; .backtrace[0].code.package ~~ Failure };
.resume
}
}
However, this is heavily dependent on implementation details that could easily change, so I'd not rely on it.
Just remove the try wrapper:
sub my-sub {
# try { <--- remove this line...
CATCH {
when X::AdHoc { say 'AdHoc Exception handled here'; .resume }
default {say 'Other Exception'; .resume}
}
my $b = other-sub();
$b.so ?? $b.say !! 'This was a Failure'.say;
# } <--- ...and this one
}
sub other-sub { fail 'Failure_X' }
my-sub();
You used try. A try does a few things, but the pertinent thing here is that it tells Raku to immediately promote any Failures in its scope to exceptions -- which is what you say you don't want. So the simplest solution is to just stop doing that.
This answer just verbosely repeats part of jnthn's explanation (see in particular comments he wrote below his answer). But I wasn't convinced all readers would spot/understand this aspect, and didn't think a comment or two on jnthn's answer would help, hence this answer.
I've written this as a community answer to ensure I won't benefit from any upvotes because it obviously doesn't warrant that. If it gets enough downvotes we'll just delete it.
I understand, Assertions are used in tests to check if the programmer's pre- and post- conditions are true and if the assertions fail, there is/are bug/bugs that need to debugged and fixed.
An assertion is a software construct where the developer states ("asserts") a condition that he believes will always be true. If the condition evaluates to false in some languages an exception is thrown, in others a message is printed, and in others the program ceases to operate.
"A message is printed" highlighted in the above definition caught my attention.
My question is, can we extend the use of assert statements in the actual code itself and catch AssertionErrors to log the messages and not just tests?
Basically to avoid too much verbose with if-else statements(I hate if-else btw, personally).
For example, all of below
String response = aMethod(sendSomething);
if (response!=null && response!="") {
//blah blah
} else {
log.error("Response is null");
throw new NoDataFoundException();
}
could be replaced with the below that provides more detailed messages.
try{
assertThat(aMethod(sendSomething), notNullValue());
} catch (AssertionError e) {
log.error(e.getMessage());
}
You don't want to catch an assertion failure(*). By definition, the program is operating outside its design perimeter. The only safe thing to do is get out.
(*) OK, you can catch it if the point of catching it is to log whatever you anticipating you'll need to diagnose the problem, and then get out quickly.
In your example, it appears that after testing for a response and logging the failure, you'll carry on inline. In general, that would be a bad idea, but perhaps this was just a quick illustration on your part and should not be taken too literally.
As to what I take to be your overall point: yes, if you're checking pre/postconditions at all, leave the checks in shipping code.
My code would look like:
response = aMethod(sendSomething);
if (response == null || response.isEmpty())
throw new InternalError("....");
and there would be something catching the error at some outermost layer, a point of exception handling being that you can transfer control far away rather than having to deal with everything at the point it occurs -- especially if "it can't happen".
First of all, I know the standard answer will be that exceptions are never to be used for flow control. While I perfectly agree with this, I've been thinking a long time about something I sometimes did, which I'll describe with the following pseudo-code:
try
string keyboardInput = read()
int number = int.parse(keyboardInput)
//the conversion succeeds
if(number >= 1000)
//That's not what I asked for. The message to display to the user
//is already in the catch-block below.
throw new NumberFormatException() //well, there IS something wrong with the number...
catch(NumberFormatException ex) //the user entered text
print("Please enter a valid number below 1000.")
First of all, take this example in a very abstract way. This does not necessarily have to happen. The situation simply is:
A user input needs to be constrained and can go wrong in 2 ways,
either
by a thrown exception the language defines, or by a check. Both errors
are reported by the user in the same way, because they do not need to know
the technical difference of what caused it.
I have thought of several ways to solve it. To begin with, it would be better to throw a custom made exception. The problem I then face is, if I catch it locally, what to do with the other exception? In se, the custom exception would be cause for a second catch-block, in which the message would be copied into just as well. My solution:
//number is wrong
throw new MyException()
catch(NumberFormatException ex)
throw new MyException()
catch(MyException ex) {
print("Please enter...")
The meaning of the exceptions' names is everything here. This application of custom-made exceptions is widely accepted, but essentially I didn't do anything different from the first way: I forced to go into a catch-block, albeit by throwing a custom exception rather than a standard-library one.
The same way applied to throwing the exception on to the calling method (thus not having a catch block for the custom exception) seems to make more sense. My method can go wrong in what is technically two ways, but essentially one way: wrong user input. Therefore, one would write a UserInputException and make the method throw this. New problem: what if this is the main method of an application?
I'm not currently struggling with a specific application to implement this kind of behaviour, my question is purely theoretical and non-language specific.
What is the best way to approach this?
I would consider the first exception to be low-level, and I would handle it (by translation in this case) at the point of call. I find that this leads to code that is easier to maintain and refactor later, as you have less types of exceptions to handle.
try
string keyboardInput = read()
try
int number = int.parse(keyboardInput)
catch(NumberFormatException ex)
throw MyException("Input value was not a number")
//the conversion succeeds
if(number >= 1000)
throw MyException("Input value was out of range")
catch(MyException ex) //the user entered text
print( ex.ToString() )
print("Please enter a valid number below 1000.")
I think you have essentially a few ways to go about it with minimal code duplication in mind:
Use a boolean variable/store the exception: If there was an error anywhere in the the general logic of the specific task you are performing, you exit on the first sign of error and handle that in a separate error handling branch.
Advantages: only one place to handle the error; you can use any custom exception/error condition you like.
Disadvantages: the logic of what you are trying to achieve might be hard to discover.
Create a general function that you can use to inform the user about the error (pre-calculating/storing all information that describes the general error, e.g. the message to display the user), so you can just make one function call when an error condition happens.
Advantages: the logic of your intent might be clearer for readers of the code; you can use anu custom exception/error conditon you like.
Disadvantages: the error will have to be handled in separate places (although with the pre-computed/stored values, there is not much copy-paste, however complex the informing the user part).
If the intent is clear, I don't think throwing exceptions from within your try block explicitly is a bad idea. If you do not want to throw one of the system provided exceptions, you can always create your own that derives from one of them, so you only need a minimal number (preferably one) of catch blocks.
Advantages: only one place to handle error condition -- if there is essentially only one type of exception thrown in try-block.
Disadvantages: if more than one type of exception is thrown, you need nested try-catch blocks (to propagate the exceptions to the most outward one) or a very general (e.g. Exception) catch block to avoid having to duplicate error reporting.
The way I see it is this:
Assuming there's no other way to parse your int that doesn't throw an exception, your code as it is now, is correct and elegant.
The only issue would be if your code was in some kind of loop, in which case you might worry about the overhead of throwing and catching unnecessary exceptions. In that case, you will have to compromise some of your code's beauty in favor of only handling exceptions whenever necessary.
error=false;
try {
string keyboardInput = read();
int number = int.parse(keyboardInput);
//the conversion succeeds
if(number >= 1000) {
//That's not what I asked for. The message to display to the user
//is already in the catch-block below.
error=true;
} catch(NumberFormatException ex) { //the user entered text
error=true;
}
if (error)
print("Please enter a valid number below 1000.");
Also you can think about why you're trying to aggregate two errors into one.
Instead you could inform the user as to what error they did, which might be more helpful in some cases:
try {
string keyboardInput = read();
int number = int.parse(keyboardInput);
//the conversion succeeds
if(number >= 1000) {
//That's not what I asked for. The message to display to the user
//is already in the catch-block below.
print("Please enter a number below 1000.");
} catch(NumberFormatException ex) { //the user entered text
print("Please enter a valid number.");
}
You do not need any exceptions in this particular example.
int number;
if (int.TryParse(keyboardInput, out number) && number < 1000) // success
else // error
However, the situation you describe is common in business software, and throwing an exception to reach a uniform handler is quite common.
One such pattern is XML validation followed by XSLT. In some systems, invalid XML is handled through catching validation exceptions. In these systems, it is pretty natural to reuse the existing exception handling in XSLT (which can naturally detect some classes of data errors that a particular validation language cannot):
<xsl:if test="#required = 'yes' and #prohibited = 'yes'>
<xsl:message terminate='yes'>Error message</xsl:message>
</xsl:if>
It is important to see that if such conditions are extremely rare (expected to occur only during early integration testing, and disappear as defects in other modules get fixed), most of the typical concerns around not using exceptions for flow control do not really apply.
What about approaching this validation problem by writing several validator classes that take in an input and return errors, or no errors. As far as your struggle with exceptions: put that logic into each validator and deal with it there on a case by case basis.
after that you figure out the correct validators to use for your input, collect their errors and handle them.
the benefits of this are:
Validators do one thing, validate a single case
Its up to the validation function to decide how to handle the errors. Do you break on first validation error or do you collect them all and then deal with them?
You can write your code is such a way that the main validation function can validate different types of input using the same code, just picking the correct validators using your favorite technique.
and disadvantages:
You will end up writing more code (but if you are using java, this should be put into the 'benefits' bucket)
here is some example pseudo-code:
validate(input):
validators = Validator.for(input.type)
errors = []
for validator in validators:
errors.push(validator.validate(input))
if errors:
throw PoopException
and some validators:
MaxValidator extends IntValidator:
validate(input):
errors = []
errors.push(super.validate(input))
if input > 1000:
errors.push("bleee!!!! to big!")
return errors
IntValidator:
validate(input):
try:
int.parse(input)
catch NumberFormatException:
return ['not an int']
return []
of course you would need to do some trickery to make the parent validator possibly return you a valid version of the input, in this case string "123" converted to an int so the max validator can handle it, but this can be easily accomplished by making the validators statefull or some other magic.
I can't see this answer anywhere in here, so I'll just post it as another point of view.
As we all know, you can actually break the rules if you know them well enough, so you can use throwing an Exception for flow control if you know it's the best solution for your situation. From what I've seen, it happens usually with some dumb frameworks...
That said, before Java 7 (which brought us the mighty multicatch construct), this was my approach to avoid code repetition:
try {
someOffendingMethod();
} catch (Exception e) {
if (e instanceof NumberFormatException || e instanceof MyException) {
System.out.println("Please enter a valid number.");
}
}
It's a valid technique in C#, too.
//
// To Throw
void PrintType(object obj)
{
if(obj == null)
{
throw new ArgumentNullException("obj")
}
Console.WriteLine(obj.GetType().Name);
}
//
// Not to Throw
void PrintType(object obj)
{
if(obj != null)
{
Console.WriteLine(obj.GetType().Name);
}
}
What principle to keep?
Personally I prefer the first one its say developer-friendly(notified about each "anomaly").
The second one its say user-friendly(let user continue work even if "inside" not everything does right).
I think that is more complicated to find errors and bugs in the maintenance phase when you silently let the things to go on. If something goes wrong you are not notified at once, and sometimes have errors far away from the main error cause, and spend a lot of time to find it.
What do you think?
The second one is lethal. Failing silently is always the wrong thing to do. Suppose this were the banking system in the bank that holds your account. Would you like it if there was a problem paying in your salary and the system silently ignored it?
If the method body handles the null obj properly (in other words, obj != null is not a requirement), then there's no need to throw an exception.
In all other cases: Throw. Let the client take responsibility for their flawed input.
Throwing an exception (if null is an error) seems far better than silently ignoring an error.
There is a third option you can consider:
void PrintType(object obj)
{
Console.WriteLine(obj.GetType().Name);
}
This also throws an exception when obj is null. The advantage of this, is that less code is involved. The disadvantage of this approach is that it is more difficult to tell whether obj can be null.
Throw.
Let the caller of a function determine if it is important enough to throw an exception to the user on a null value, but the function itself should throw because of the invalid argument.
I'd say that it depends on your (developer) preference. From the user perspective, he should never see an unhandled exception, but it does not mean you cannot use exceptions.
I prefer the first one, because I find null to be a totally unnecessary (and annoying) construct, so I make effort to code without it. If there is a null somewhere, someone made a mistake, so the best thing is to just barf out instead of pretending everything is ok.
In the end it depends on what you consider to be the semantics of the method. If the method is supposed to accept nulls, then you should pick option number two. If the method is supposed to only accept real arguments (which I prefer), then you should pick option number one.
Always Throw, except in debugging/diagnostic code. It is most embarassing to have a NullPointerException that occurs in production code at a point where only a debugging message should be generated, e.g.
log.debug("id of object is " + obj.getId())
where the logger is turned off, and obj is null.
It is highly subjective, but I always prefer to just ignore non-fatal or recoverable errors. Put them in logs, if you must, but if you know how to continue - please do so.
Note, that when I say fatal, it actually depends on the function itself. Say, there's API function that gets ID and handful of other parameters. Suppose, that this ID also can be guessed from those other stuff that is passed in. API function should guess it if it can but the function somewhere inside that does all the work should get non-null ID and throw otherwise. Because for high level API function it is not fatal, it knows how to guess it, but for low level function it is fatal, it supposed to do something with that ID and with null value it can't continue.
All fatal errors should be noted, of course.
If you api if exposed outside, do always argument checking and throw a argument based exception so the api user can get the result.
Consider using the Null Object pattern is very useful to not clutter your code with try-catch, null checks (or god forbid swallowed errors).
In this particular example, giving nothing to a printer is like saying "print nothing", thus working as it should.
I do know this is an example, but it's just to clarify that this is relative.
If your code displays user-friendly messages on exceptions somehow, what difference does it make ? the first one would be both developer and user friendly.
It really depends on what your invariants are. If the parameter is optiona, then ignoring a null parameter is just fine, but if the parameter is required then that will hide a bug in your application. Also, and depending on the language, if the invariant is bad enough you may consider a third option: abort the application.
All discussions on whether to use or not exceptions can always be mapped to the decision on whether the situation is exceptional or not, and if it is exceptional, throwing or rather aborting the application depends on whether it is recoverable or not.
Id go for
void PrintType(object obj)
{
Console.WriteLine(obj.GetType().Name);
}
Third option, half in pseudocode:
// To Throw A Clean Error
void PrintType(object obj)
{
if(obj == null)
{
throw new ArgumentNullException(STANDARD_ERROR_MESSAGE, obj)
}
Console.WriteLine(obj.GetType().Name);
}
Either catch all errors and wrap them in a single place, so the user sees standard text:
There has been an error. If this error
persists, please contact an
administrator.
Or throw a select few errors, all of which are user-friendly, and display them directly to the user. "A connection error has occurred." "An authentication error has occurred." "A system error has occurred." And so on.
On the backend, have all errors and their stack trace logged, so you can use the debugging information that way.
It really depends on what the function is defined to do. The most important aspect is to have a clearly defined behavior and for the function to implement it correctly.
Now, if the question is whether is better to define the function to accept null and print it out, or to not accept it and throw an exception, I would say the latter, because it's probably less error prone for the user to check for null before calling the function, if that is a possibility.
This is a refactoring question.
try
{
string line = GetFirstLineFromFile(); //Gets first line from a text file, this line would be a number.
int value = ConvertToInteger(line); // Gets the integer value from the string.
int result = DivideByValue(value); // Divides some number with the value retrieved.
}
catch(Exception ex)
{
}
My main concern is, what is the best approach for exception handling in such situations. Certainly wrapping the whole thing in a single try catch is like saying I expect an exception about everything. There must be some place we catch a generic exception right?
Just don't catch a "generic exception".
How can you possibly handle ANY exception and know how to keep your application in a clean state ?
It hides bugs and it's a really bad idea.
Read this serie of posts on catch (Exception).
You need to think about what exceptions can be thrown from the methods in the try block, as well as which ones of those you can deal with at the current level of abstraction.
In your case, I'd expect that the getFirstLineFromFile methods, for example, can definitely throw exceptions you'd want to catch here. Now whether you wrap and rethrow the exception, or take other action, depends on whether you can actually deal with the exception at this level. Consider the case where you have a default file you can fall back to - the approach may just be to log a warning and continue with the default. Or if the whole application is based on reading a file supplied by the user, then this is more likely to be a fatal exception that should be propagated up to the top level and communicated to the user there.
There's no hard-and-fast rule like "always throw" or "never throw"; in general, I consider that one should throw exceptions whenever there's an exceptional-type situation that is not considered a normal result of the method, and consequently cannot be adequately described by the return type of the method. (For example, an isValidDbUser method returning boolean might be able to handle SQLExceptions as just return false; but a getNumProductsRegisteredInDB returning an int should almost certainly propagate an exception).
Don't listen to the (hordes) of people that will tell you that you should never catch multiple exceptions in one big general block. It's a perfectly reasonable way to do general error handling in some cases, which is why the ability to do so exists in the language.
There will be some exceptions that you can do something specific and useful about (i.e. recover from them in the catch block.) These are the kinds of exceptions that you want to catch individually, and as close to the place where they occur as possible.
But the majority of exceptions that you'll face in real life will be completely unexpected, unchecked exceptions. They are the result of programmer error (bugs), failed assertions, failing hardware, dropped network connections, etc.
You should design your software defensively, by designating specific "chokepoints" to handle these unpredictable exceptions with a minimum of disruption to the rest of the application. (Remember, in many cases, "handling" the exception often just means aborting the current operation and logging an error or otherwise telling the user that an unexpected error occurred.)
So for example, if your program saves a file to the disk, you could wrap the entire save operation in a try block to catch things that goes wrong during the save:
try {
// attempt to perform the save operation
doSave();
} catch (Throwable t) {
// tell the user that the save failed for unexpected reasons
// and log the error somewhere
informUser("save failed!");
log("save failed!", t);
} finally {
// last minute cleanup (happens whether save succeeded or failed)
...
}
Notice that we choose a nice chokepoint method here ( doSave() ) and then stop any unexpected errors from bubbling up any further than this point. The chokepoint represents a single, cancellable operation (a save). While that operation is obviously toast if you're getting an unexpected exception, the rest of the application will remain in a good state regardless of what happens on the other side of the chokepoint.
Also notice that this idiom does NOT stop you from handling some of your exceptions further down in doSave() somewhere. So if there are exceptions that might get thrown that you can recover from, or that you want to handle in a special way, go ahead an do so down in doSave(). But for everything else, you have your chokepoint.
You might even want to set up a general uncaught exception handler for your entire program in your main method:
public static void main(String [] args) {
try {
startApplication();
} catch (Throwable t) {
informUser("unexpected error! quitting application");
log("fatal application error", t);
}
But if you've set your other chokepoints up wisely, no exceptions will ever bubble up this far. If you want to make your general error handling complete, you can also create and assign an UncaughtExceptionHandler to important threads, including your main thread or the AWT thread if you are using Swing.
TL;DR; Don't believe the dogma that you should always catch exceptions as specifically as possible. There are times when you want to catch and handle a specific exception, and other times when you want to use chokepoints to catch and deal with "anything else that might go wrong".