Related
Looking over my Raku code, I've realized that I pretty much never use CATCH blocks to actually catch/handle error. Instead, I handle errors with try blocks and testing for undefined values; the only thing I use CATCH blocks for is to log errors differently. I don't seem to be alone in this habit – looking at the CATCH blocks in the Raku docs, pretty much none of them handle the error in any sense beyond printing a message. (The same is true of most of the CATCH blocks in Rakudo.).
Nevertheless, I'd like to better understand how to use CATCH blocks. Let me work through a few example functions, all of which are based on the following basic idea:
sub might-die($n) { $n %% 2 ?? 'lives' !! die 418 }
Now, as I've said, I'd normally use this function with something like
say try { might-die(3) } // 'default';
But I'd like to avoid that here and use CATCH blocks inside the function. My first instinct is to write
sub might-die1($n) {
$n %% 2 ?? 'lives' !! die 418
CATCH { default { 'default' }}
}
But this not only doesn't work, it also (very helpfully!) doesn't even compile. Apparently, the CATCH block is not removed from the control flow (as I would have thought). Thus, that block, rather than the ternary expression, is the last statement in the function. Ok, fair enough. How about this:
sub might-die2($n) {
ln1: CATCH { default { 'default' }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
(those line numbers are Lables. Yes, it's valid Raku and, yes, they're useless here. But SO doesn't give line numbers, and I wanted some.)
This at least compiles, but it doesn't do what I mean.
say might-die2(3); # OUTPUT: «Nil»
To DWIM, I can change this to
sub might-die3($n) {
ln1: CATCH { default { return 'default' }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
say might-die3(3); # OUTPUT: «'default'»
What these two reveal is that the result of the CATCH block is not, as I'd hopped, being inserted into control flow where the exception occurred. Instead, the exception is causing control flow to jump to the CATCH block for the enclosing scope. It's as though we'd written (in an alternate universe where Raku has a GOTO operator [EDIT: or maybe not that alternate of a universe, since we apparently have a NYI goto method. Learn something new every day…]
sub might-die4($n) {
ln0: GOTO ln2;
ln1: return 'default';
ln2: $n %% 2 ?? 'lives' !! GOTO ln1;
}
I realize that some critics of exceptions say that they can reduce to GOTO statements, but this seems to be carrying things a bit far.
I could (mostly) avoid emulating GOTO with the .resume method, but I can't do it the way I'd like to. Specifically, I can't write:
sub might-die5($n) {
ln1: CATCH { default { .resume('default') }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
Because .resume doesn't take an argument. I can write
sub might-die6($n) {
ln1: CATCH { default { .resume }}
ln2: $n %% 2 ?? 'lives' !! do { die 418; 'default' }
}
say might-die6 3; # OUTPUT: «'default'»
This works, at least in this particular example. But I can't help feeling that it's more of a hack than an actual solution and that it wouldn't generalize well. Indeed, I can't help feeling that I'm missing some larger insight behind error handling in Raku that would make all of this fit together better. (Maybe because I've spent too much time programming in languages that handle errors without exceptions?) I would appreciate any insight into how to write the above code in idiomatic Raku. Is one of the approaches above basically correct? Is there a different approach I haven't considered? And is there a larger insight about error handling that I'm missing in all of this?
"Larger insight about error handling"
Is one of the approaches [in my question] basically correct?
Yes. In the general case, use features like try and if, not CATCH.
Is there a different approach I haven't considered?
Here's a brand new one: catch. I invented the first version of it a few weeks ago, and now your question has prompted me to reimagine it. I'm pretty happy with how it's now settled; I'd appreciate readers' feedback about it.
is there a larger insight about error handling that I'm missing in all of this?
I'll discuss some of my thoughts at the end of this answer.
But let's now go through your points in the order you wrote them.
KISS
I pretty much never use CATCH blocks to actually catch/handle error.
Me neither.
Instead, I handle errors with try blocks and testing for undefined values
That's more like it.
Logging errors with a catchall CATCH
the only thing I use CATCH blocks for is to log errors differently.
Right. A judiciously located catchall. This is a use case for which I'd say CATCH is a good fit.
The doc
looking at the CATCH blocks in the Raku docs, pretty much none of them handle the error in any sense beyond printing a message.
If the doc is misleading about:
The limits of the capabilities and applicability of CATCH / CONTROL blocks; and/or
The alternatives; and/or
What's idiomatic (which imo is not use of CATCH for code where try is more appropriate (and now my new catch function too?)).
then that would be unfortunate.
CATCH blocks in the Rakudo compiler source
(The same is true of most of the CATCH blocks in Rakudo.).
At a guess those will be judiciously placed catchalls. Placing one just before the callstack runs out, to specify default exception handling (as either a warning plus .resume, or a die or similar), seems reasonable to me. Is that what they all are?
Why are phasers statements?
sub might-die1($n) {
$n %% 2 ?? 'lives' !! die 418
CATCH { default { 'default' }}
}
this not only doesn't work, it also (very helpfully!) doesn't even compile.
.oO ( Well that's because you forgot a semi-colon at the end of the first statement )
(I would have thought ... the CATCH block [would have been] removed from the control flow)
Join the club. Others have expressed related sentiments in filed bugs, and SO Q's and A's. I used to think the current situation was wrong in the same way you express. I think I could now easily be persuaded by either side of the argument -- but jnthn's view would be decisive for me.
Quoting the doc:
A phaser block is just a trait of the closure containing it, and is automatically called at the appropriate moment.
That suggests that a phaser is not a statement, at least not in an ordinary sense and would, one might presume, be removed from ordinary control flow.
But returning to the doc:
Phasers [may] have a runtime value, and if evaluated [in a] surrounding expression, they simply save their result for use in the expression ... when the rest of the expression is evaluated.
That suggests that they can have a value in an ordinary control flow sense.
Perhaps the rationale for not removing phasers from holding their place in ordinary control flow, and instead evaluating to Nil if they don't otherwise return a value, is something like:
Phasers like INIT do return values. The compiler could insist that one assigns their result to a variable and then explicitly returns that variable. But that would be very un Raku-ish.
Raku philosophy is that, in general, the dev tells the compiler what to do or not do, not the other way around. A phaser is a statement. If you put a statement at the end, then you want it to be the value returned by its enclosing block. (Even if it's Nil.)
Still, overall, I'm with you in the following sense:
It seems natural to think that ordinary control flow does not include phasers that do not return a value. Why should it?
It seems IWBNI the compiler at least warned if it saw a non-value-returning phaser used as the last statement of a block that contains other value-returning statements.
Why don't CATCH blocks return/inject a value?
Ok, fair enough. How about this:
sub might-die2($n) {
ln1: CATCH { default { 'default' }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
say might-die2(3); # OUTPUT: «Nil»
As discussed above, many phasers, including the exception handling ones, are statements that do not return values.
I think one could reasonably have expected that:
CATCH phasers would return a value. But they don't. I vaguely recall jnthn already explaining why here on SO; I'll leave hunting that down as an exercise for readers. Or, conversely:
The compiler would warn that a phaser that did not return a value was placed somewhere a returned value was probably intended.
It's as though we'd written ... a GOTO operator
Raku(do) isn't just doing an unstructured jump.
(Otherwise .resume wouldn't work.)
this seems to be carrying things a bit far
I agree, you are carrying things a bit too far. :P
.resume
Resumable exceptions certainly aren't something I've found myself reaching for in Raku. I don't think I've used them in "userspace" code at all yet.
(from jnthn's answer to When would I want to resume a Raku exception?.)
.resume doesn't take an argument
Right. It just resumes execution at the statement after the one that led to an exception being thrown. .resume does not alter the result of the failed statement.
Even if a CATCH block tries to intervene, it won't be able to do so in a simple, self-contained fashion, by setting the value of a variable whose assignment has thrown an exception, and then .resumeing. cf Should this Raku CATCH block be able to change variables in the lexical scope?.
(I tried several CATCH related approaches before concluding that just using try was the way to go for the body of the catch function I linked at the start. If you haven't already looked at the catch code, I recommend you do.)
Further tidbits about CATCH blocks
They're a bit fraught for a couple reasons. One is what seems to be deliberate limits of their intended capability and applicability. Another is bugs. Consider, for example:
My answer to SO CATCH and throw in custom exception
Rakudo issue: Missing return value from do when calling .resume and CATCH is the last statement in a block
Rakudo issue: return-ing out of a block and LEAVE phaser (“identity”‽)
Larger insight about error handling
is there a larger insight about error handling that I'm missing in all of this?
Perhaps. I think you already know most of it well, but:
KISS #1 You've handled errors without exceptions in other PLs. It worked. You've done it in Raku. It works. Use exceptions only when you need or want to use them. For most code, you won't.
KISS #2 Ignoring some native type use cases, almost all results can be expressed as valid or not valid, without leading to the semi-predicate problem, using simple combinations of the following Raku Truth value that provide ergonomic ways to discern between non-error values and errors:
Conditionals: if, while, try, //, et al
Predicates: .so, .defined, .DEFINITE, et al
Values/types: Nil, Failures, zero length composite data structures, :D vs :U type constraints, et al
Sticking with error exceptions, some points I think worth considering:
One of the use cases for Raku error exceptions is to cover the same ground as exceptions in, say, Haskell. These are scenarios in which handling them as values isn't the right solution (or, in Raku, might not be).
Other PLs support exceptions. One of Raku's superpowers is being able to interoperate with all other PLs. Ergo it supports exceptions if for no other reason than to enable correct interoperation.
Raku includes the notion of a Failure, a delayed exception. The idea is you can get the best of both worlds. Handled with due care, a Failure is just an error value. Handled carelessly, it blows up like a regular exception.
More generally, all of Raku's features are designed to work together to provide convenient but high quality error handling that supports all of the following coding scenarios:
Fast coding. Prototyping, exploratory code, one-offs, etc.
Control of robustness. Gradually narrowing or broadening error handling.
Diverse options. What errors should be signalled? When? By which code? What if consuming code wants to signal that producing code should be more strict? Or more relaxed? What if it's the other way around -- producing code wants to signal that consuming code should be more careful or can relax? What can be done if producing and consuming code have conflicting philosophies? What if producing code cannot be altered (eg it's a library, or written in another language)?
Interoperation between languages / codebases. The only way that can work well is if Raku provides both high levels of control and diverse options.
Convenient refactoring between these scenarios.
All of these factors, and more, underlie Raku's approach to error handling.
CATCH is a really old feature of the language.
It used to only exist inside of a try block.
(Which is not very Rakuish.)
It is also a very rarely used part of Raku.
Which means that not a lot of people have come up with “pain points” of the feature.
So then very rarely has anyone done any work to make it more Rakuish.
Both of those combined make it so that CATCH is a rather featureless part of the language.
If you look at the test file for the feature, you will note that most of it was written in 2009 when the test suite was still a part of the Pugs project.
(And most of the rest are tests for bugs that have been found over the years.)
There is a very good reason that few people have tried to add new behaviours to CATCH, there are plenty of other features that are much nicer to work with.
If you want to replace a result in the event of an exception
sub may-die () {
if Bool.pick {
return 'normal'
} else {
die
}
}
my $result;
{
CATCH { default { $result = 'replacement' }}
$result = may-die();
}
It is much easier to just use try without CATCH, along with defined‑or // to get something that works very similarly.
my $result = try { may-die } // 'replacement';
It is even easier if you are dealing with soft failures instead of hard exceptions, because you can just use defined‑or by itself.
sub may-fail () {
if Bool.pick {
return 'normal'
} else {
fail
}
}
my $result = may-fail() // 'replacement';
In fact the only way to use CATCH with a soft failure is to combine it with try
my $result;
try {
CATCH { default { $result = 'replacement' }}
$result = may-fail();
}
If your soft failure is the base of all failure objects Nil, you can either use // or is default
my $result = may-return-nil // 'replacement';
my $result is default<replacement> = may-return-nil;
But Nil won't just work with CATCH no matter how much you try.
Really the only time I would normally use CATCH is when I want to handle several different errors in different ways.
{
CATCH {
when X::Something { … }
when X::This { … }
when X::That { … }
default { … }
}
# some code that may throw X::This
…
# some code that may throw X::NotSpecified (default)
…
# some code that may throw X::Something
…
# some code that may throw X::This or X::That
…
# some code that may fail instead of throw
# (sunk so that it will throw immediately)
sink may-fail;
}
Or if I wanted to show how you could write this [terrible] Visual Basic line
On Error Resume Next
In Raku
CATCH { default { .resume } }
That of course doesn't really answer your question in the slightest.
You say that you expected CATCH to be removed from the control flow.
The whole point of CATCH is to insert itself into the exceptional control flow.
Actually that's not accurate. It doesn't so much insert itself into the control flow as ending the control flow while doing some processing before moving on to the caller/outside block. Presumably because the data of the current block is in an erroneous state and should no longer be trusted.
That still doesn't explain why your code fails to compile.
You expected CATCH to have its own special syntax rule when it comes to the semicolon ending a statement.
If it worked the way you expected it would fail one of the important [syntax] rules in Raku, “there should be as few special cases as possible”. Its syntax is not special in any way unlike what you seem to expect.
CATCH is just one of many phasers with one important extra bit of functionality, it stops exception propagation down the call stack.
What you seem to be asking for it to instead alter the result of an expression that may throw.
That doesn't seem like a good idea.
$a + may-die() + $b
You want to be able to replace the exception from may-die with a value.
$a + 42 + $b
Basically you are asking for the ability to add action‑at‑a‑distance as a feature.
There is also a problem, what if you actually wanted $a + may‑die to be replaced instead.
42 + $b
There is no way in your idea for you to specify that.
Even worse, there is a way that could accidently happen. What if may‑die started returning a failure instead of exception. Then it would only cause an exception when you tried to use it, for example by adding it to $a.
If some code throws an exception, the block is in an unrecoverable state and it needs to halt execution. This far, no farther.
If an expression throws an exception, the result of executing the statement it is in, is suspect.
Other statements may rely on that broken statement, so then the whole block is also suspect.
I do not think it would be that good of an idea if it instead allowed the code to continue but with a different result for the current expression. Especially if that value can be far removed from the expression somewhere else inside of the block. (action‑at‑a‑distance)
If you could come up with some code that would be vastly improved with .resume(value), then maybe it could be added.
(I personally think that leave(value) would be more useful in such a circumstance.)
I will grant that .resume(value) seems like it may be useful for control exceptions.
(Caught with CONTROL instead of CATCH.)
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 have used try-catch/except-finally variants in many languages for years, today someone asked me what is the point of finally and I couldn't answer.
Basically why would you put a statement in finally instead of just putting it after the whole try-catch block? Or in other words is there a difference between the following blocks of code:
try{ //a}
catch {//b}
finally {//c}
try{//a}
catch{//b}
//c
EDIT:
PEOPLE, I know what finally does, I have been using it for ages, but my question is in the above example putting //c in finally seems redundant, doesn't it?
The purpose of a finally block is to ensure that code gets run in three circumstances which would not very cleanly be handled using "catch" blocks alone:
If code within the try block exits via fallthrough or return
If code within a catch block either rethrows the caught exception, or--accidentally or intentionally--ends up throwing a new one.
If the code within the try block encounters an exception for which the try has no catch.
One could copy the finally code before every return or throw, and wrap catch blocks within their own try/catch to allow for the possibility of an accidental exception occurring, but it's far easier to forgo all that and simply use a finally block.
BTW, one thing I wish language designers would include would be an exception argument to the finally block, to deal with the case where one needs to clean up after an exception but still wants it to percolate up the call stack (e.g. one could wrap the code for a constructor in such a construct, and Dispose the object under construction if the constructor was going to exit with an exception).
Finally block is executed even if an exception thrown in the try block. Therefore, for instance if you opened a stream before, you may want to close that stream either an exception is thrown or not. Finally block is useful for such an issue.
finally is a syntactic sugar to allow DRY principle in try-catch pattern. Exception is usually thrown if the library code has not enough information to handle some state and wants the client code to solve it. If you don't have library-client code separation, you can handle everything by if instead of try.
Let's see a standard situation without finally:
void myFunction() {
var r = allocateResources();
r.doSomething();
if(somethingBadHappens) {
freeResources(r);
throw new Exception(CODE42);
}
r.doSomethingMore();
freeResources(r);
}
In the snippet above, you repeat freeResources(): this can be multiple statements which you need to repeat. This smells and finally block is the solution for clean code:
void myFunction() {
var r = allocateResources();
try {
r.doSomething();
if(somethingBadHappens) throw new Exception(CODE42);
r.doSomethingMore();
}
finally {
freeResources(r);
}
happyFunction();
}
Let's realize three levels of abstraction:
A1 is the library code providing allocateResources() function
A2 is our code providing myFunction, consuming A1
A3 is some client code consuming myFunction in try-catch block:
function A3code() {
try {
myFunction();
doSomething();
}
catch(Exception e) {
// no hanging resources here
Console.WriteLine(e);
}
}
Now let's see what can happen:
if allocateResources() throws in A1, we don't know how to handle it in A2 (A2 code can be run in Console-free environment), so we delagate the situation to A3 without adding any further code. If Exception is thrown here, the finally block is not executed, since finally is bound to try which was not entered.
if somethingBadHappens in try block, the stack unwinds to A3 where the situation is handled BUT before it finally block is executed, so we don't need to repeat it if no exceptions happen.
before finally we can add catch block and try to resolve some potential exceptions from A1 which may appear in calling r.doSomething methods. Usually we want to handle exceptions as soon as possible to make the client code (A3) more comfortable for client coders.
happyFunction() is executed only if nothing throws in myFunction() (inside or outside of try block).
As #supercat points out, the finally block is executed also if try block exits via return. I suggest you avoid this bad habit and have only one return in every function (maybe some early exists at the very beginning of functions). The reasons for single-return functions are:
The code is more readable: you look at the end and see WHAT the function returns. In multiple returns you must find all return occurences, inspect all the ifs, think about when the ifs are satisfied and only then you know what the function returns.
The code can be optimized by compilers, see copy elision.
The reason for multiple returns is avoiding many nested ifs, but there are other techniques to solve it. Exceptions are exception in this rule.
Learn by example
let v = 0;
function f() {
try {
v = 1;
return 2;
} finally {
v = 3;
return 4;
}
v = 5;
return 6;
}
const r = f();
console.log(r, v);
following prints "3, 4"
Finally make sure your code is executed even if you get an exception.
The finally block is useful for cleaning up any resources allocated in the try block as well as running any code that must execute even if there is an exception
http://msdn.microsoft.com/en-us/library/zwc8s4fz(v=vs.80).aspx
This is a general programming question, not pertaining to any specific language.
A new programmer typically will write a method that calculates some value, then returns the value:
public Integer doSomething()
{
// Calculate something
return something;
}
public void main()
{
Integer myValue = doSomething();
}
But when an exception occurs during the calculation of something, what is the best way to handle the exception, especially when giving the user feedback? If you do a try/catch of the calculation of something and if an exception is caught, what do you return? Nothing was calculated, so do you return null? And once you return it (whatever it may be), do you need to do another try/catch in the parent method that checks to see if a valid value was returned? And if not, then make sure the user is given some feedback?
I have heard arguments on both sides of the table about never returning values at all, but instead setting calculated values as pointers or global variables and instead returning only error codes from methods, and then (in the parent method) simply handling the error codes accordingly.
Is there a best practice or approach to this? Are there any good resources that one could access to learn more about the best way to handle this?
UPDATE for Clarification
Consider the following code:
public void main()
{
int myValue = getMyValue();
MyUIObject whatever = new MyUIObject();
whatever.displayValue(myValue); // Display the value in the UI or something
}
public Integer getMyValue()
{
try
{
// Calculate some value
} catch (exception e) {
// ??
}
return value;
}
I call the method to get some int value, then I return it. Back in main(), I do something with the value, like show it in the Log in this case. Usually I would display the value in the UI for the user.
Anyways, if an exception is caught in getMyValue(), so does value get returned but it's null? What happens in main() then? Do I have to test if it's a valid value in main() as well?
I need the program to handle the error accordingly and continue. Someone below suggested displaying the appropriate information in the UI from within the getMyValue() method. I see two potential issues:
It seems like I would be mixing the business logic with (in this case) the logic for the UI.
I would have to pass a reference of the MyUIObject to the getMyValue() or something so I could access it from within the function. In the above simple example that is no big deal, but if there is a BUNCH of UI elements that need to be updated or changed based off of what happens in getMyValue(), passing them all might be a bit much...
I've read a bunch about the fundamentals of all of this but I can't seem to find a straight answer for the above situation. I appreciate any help or insight.
I think you do not quite understand exceptions.
If you throw an exception, you do not return from the function normally:
public Integer doSomething()
{
throw new my_exception();
// The following code does NOT get run
return something;
}
public void main()
{
Integer myValue = doSomething();
}
The main advantages of exceptions are:
You can write your code as though everything is succeeding, which is usually clearer
Exceptions are hard to ignore. If an exception is unhandled, typically an obvious and loud error will be given, with a stack trace. This contrasts with error codes, where it is much easier to ignore the error handling than not.
I recommend this post by Eric Lippert, which discusses exceptions and when it is and is not appropriate to handle them.
UPDATE (in response to comment):
You can absolutely handle an exception and continue, you do this by catching the exception.
eg:
try
{
// Perform calculation
}
catch (ExceptionType ex)
{
// A problem of type 'ExceptionType' occurred - you can do whatever you
// want here.
// You could log it to a list, which will be later shown to the user,
// you could set a flag to pop up a dialog box later, etc
}
// The code here will still get run even if ExceptionType was thrown inside
// the try {} block, because we caught and handled that exception.
The nice thing about this is that you know what kind of thing went wrong (from the exception type), as well as details (by looking into the information in ex), so you
hopefully have the information you need to do the right thing.
UPDATE 2 in response to your edit:
You should handle the exception at the layer where you are able to respond in the way you want. For your example, you are correct, you should not be catching the exception so deep down in the code, since you don't have access to the UI, etc and you thus can't really do anything useful.
How about this version of your example code:
public void main()
{
int myValue = -1; // some default value
String error = null; // or however you do it in Java (:
try
{
getMyValue();
}
catch (exception e)
{
error = "Error calculating value. Check your input or something.";
}
if (error != null)
{
// Display the error message to the user, or maybe add it to a list of many
// errors to be displayed later, etc.
// Note: if we are just adding to a list, we could do that in the catch().
}
// Run this code regardless of error - will display default value
// if there was error.
// Alternatively, we could wrap this in an 'else' if we don't want to
// display anything in the case of an error.
MyUIObject whatever = new MyUIObject();
whatever.displayValue(myValue); // Display the value in the UI or something
}
public Integer getMyValue()
{
// Calculate some value, don't worry about exceptions since we can't
// do anything useful at this level.
return value;
}
Exceptions is a property of object oriented languages (OOL). If you use OOL, you should prefer exceptions. It is much better than to return error codes. You can find nice examples how the error-codes approach generates a vastly longer source code than exceptions based code. For example if you want to read a file and do something with it and save in a different format. You can do it in C without exceptions, but your code will be full of if(error)... statements, maybe you will try to use some goto statements, maybe some macros to make it shorter. But also absolutely nontransparent and hard to understand. Also you can often simply forget to test the return value so you don't see the error and program goes on. That's not good. On the other hand if you write in OOL and use exceptions, your source code focuses on "what to do when there is no error", and error handling is in a different place. Just one single error handling code for many possible file errors. The source code is shorter, more clear etc.
I personally would never try to return error codes in object oriented languages. One exception is C++ where the system of exceptions have some limitations.
You wrote:
I have heard arguments on both sides of the table about never returning values at all, but instead setting calculated values as pointers or global variables and instead returning only error codes from methods, and then (in the parent method) simply handling the error codes accordingly.
[EDIT]
Actually, excetion can be seen as error code, which comes along with the relative message, and you as the programmer should know where your exception must be caught, handled and eventually display to the user. As long as you let the exception to propagate (going down in the stack of called functions) no return values are used, so you don't have to care about handling related missed values. Good exception handling is a quite tough issue.
As jwd replied, I don't see the point to raise an exception in a method and then handle the excpetion in the same method just to return an error value. To clarify:
public Integer doSomething(){
try{
throw new my_exception();}
catch{ return err_value;}
}
is pointless.
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".