A comment by a high rep user on another question I asked earlier today suggested it would be better to swap the order of try/finally and try/except.
So, instead of this:
try
try
//some code
//something that throws an exception, eg: EIndexOutOfRangeException
//more code
except on E : EIndexOutOfRangeException do begin .... end;
finally
// some cleanup code
end;
it would have the try/finally nested inside and the try/except on the outside:
try
try
//some code
//something that throws an exception, eg: EIndexOutOfRangeException
//more code
finally
// some cleanup code
end;
except on E : EIndexOutOfRangeException do begin .... end;
end;
I would like to know when is it appropriate and a good idea to use this idiom, and are there exceptional cases where you shouldn't? Why prefer one over the other? I suppose exceptions being thrown in the cleanup code would be the main consideration, since I imagine it could suppress one of the exceptions if finally throws an exception, but could prevent unexpected bubbling up of errors?
You can use both the ways of writing try,catch and finally and it varies from situation to situation.
Consider the following code listing for try...except inside try...finally.
//You will receive a DataSet is some state.
try
try
//Here you'll change its state and perform operations on it.
//If some exception occurred you will handle it.
except
//Handle exception.
end;
finally
//Put the DataSet again in the same state.
end;
The above code listing shows the uses of try...except inside a try...finally block.
Consider the following code listing for try...finally inside try...except.
try
LObject:= TObject.Create;
//Create an Object. It better idea to create an object outside try..finally block.
//If some exception occured while creating an object an exception will be thrown.
//However its not a good idea to catch such an exception.Let the system handle it.
try
//Use the Object.
finally
//Free Object.
end;
// Returns True
except
// Returns False.
end;
Here the above code listing may be used in such a situation where the function return only true and false. If some exception occurred then simply set the value to false.
Related
How can I avoid getting an error when passing as argument to the function do-http-request an invalid host.
Is there any way that I can catch the error like the Java's exception-handling mechanism ?
Sure, CL has a very nice condition system. One easy option would be wrapping the call to do-http-request in ignore-errors, which returns nil (and the condition as a second value) if an error condition was signalled in the wrapped code. You could then check for nil afterwards.
If you want something more like exception handling in Java, just use handler-case and add an appropriate error clause (I don't have AllegroServe installed, but I suppose you get a socket-error for providing a wrong URL – just change that part if I misread):
(handler-case
(do-http-request …)
(socket-error ()
…))
If you need finally-like functionality, use unwind-protect:
(unwind-protect
(handler-case
(do-http-request …)
(socket-error (condition) ; bind the signalled condition
…) ; code to run when a socket-error was signalled
(:no-error (value) ; bind the returned value
…)) ; code to run when no condition was signalled
…) ; cleanup code (finally)
You can even get more fancy, and e.g. use handler-bind to handle the condition stack upwards by invoking a restart somewhere down the stack, without unwinding it. For example, if do-http-request provided a restart to try again with another URL, you could handle your error condition by invoking that restart with a new URL to retry. I just mention this for the sake of completeness – it would be overkill for your use case, but being able to resume (possibly expensive) computations easily can be a rather convenient feature.
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 the Exception message thrown by Gen_server when its not started.
(ankit#127.0.0.1)32> R11 = system_warning:self_test("SysWarn").
** exception exit: {noproc,
{gen_server,call,
[system_warning_sup,
{start_child,
{system_warning_SysWarn,
{system_warning,start_link,[{system_warning_SysWarn}]},
permanent,10,worker,
[system_warning]}},
infinity]}}
in function gen_server:call/3
in call from system_warning_sup:'-start_child/1-lc$^0/1-0-'/1
in call from system_warning:self_test/1
(ankit#127.0.0.1)33> R11.
* 1: variable 'R11' is unbound
Now, What I want to do is to catch this exception message & put into variable R11 (showed above as unbound). I want to do so because if gen_sever is not started then I want to start after getting this message. I also tried using handle_info but not able to trap the exception or may be not able to implement it correctly. Can any body please help me with this problem providing some code for example.
Both the answers from #W55tKQbuRu28Q4xv and #Zed are correct but a little terse. :-)
There are two ways to locally catch an error: catchand try. Both will also catch non-local returns generated by throw.
catch is the older and simpler of the two and has the syntax catch Expr. If an error occurs in the expression being evaluated then catch returns {'EXIT',ErrorValue}, otherwise it just returns the value of the expression. One problem with it is that there is no way to see how the error return value has been generated so it can easily be faked in the expression. In the same way you can't see if the return value comes from a throw. N.B. this is not a bug but a feature. Also it is a prefix operator with a low priority so you would normally use it like:
R11 = (catch system_warning:self_test (....))
to avoid confusion. This was a mistake, it should have been catch ... end.
throw is more complex and allows you much greater control over over what to catch and how to handle both the normal returns and error/non-local returns. See the manual for a full description. #Zed's example shows the simplest case which catches everything.
> try
> R11 = system_warning:self_test("SysWarn")
> catch
> Ex:Type -> {Ex,Type,erlang:get_stacktrace()}
> end.
Try to use 'catch':
R11 = catch system_warning:self_test (....)
Is using an if coupled with an immediate return like in the example below an acceptable practise instead of having a if with a block of code inside {} ? Are these equivalent in practise or is there a disadvantage to one of the approaches ?
An example in Java:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
ServletContext sc = this.getServletContext();
// Throw exception for fatal error (Servlet not defined in web.xml ?)
if( sc == null )
return; // old-style programming
// Careful with silent bugs ! Correct way of handling this is:
// throw new RuntimeException( "BookDetail: ServletContext is null" );
BookList bookList = WebUtil.getBookList( sc );
Martin Fowler would favour the early return, and calls the idea a Guard Clause.
Personally, I don't like it in Java, as I prefer one return per method. However this is subjective and I may be in the minority.
I've blogged about this for and against.
That is not a return, it's an exception. The code is perfectly ok tho.
Even if you'd replace that throw with a "return something", it would still be ok.
I think it comes down to readability. The code should function the same either way.
I use stuff like this all the time
Function Blah() As Boolean
If expr Then
Return False
End If
Do Other work...
Return result
End Function
For error conditions, generally it's best to throw an exception - exception handling was invented to get rid of the manual return code style error checking in C that comprises about 30% of a C program.
However, early returns are fine - they are far more readable than adding an extra scope with curly braces.
if (!_cache.has_key(key))
return null;
return _cache[key]
Is better than:
if (_cache_has_key(key))
{
return _cache[key]
}
else
return null;
And it only gets more obvious the more early returns that you add, 5 early returns beats the hell out of 5 nested if statements.
Note that I didn't return null on an error condition, it's expected that often the key won't be in the cache - but it still means the caller has to write code to check the result. In .NET there's a better pattern of returning a boolean, and setting the result via an out parameter. The methods beginning with Try usually follow this pattern:
Foo foo;
if (!TryGetCachedFoo("myfoo", foo))
{
foo = new Foo(...);
AddToCache("myfoo", foo);
}
// do something with foo
As long as you're using them for conditional escapes as the first thing in the routine. I think the fact that they are obvious in that location, and avoid at least one level of indentation outweighs the negative of having a multiple returns.
In the example you give, I'd favor throwing an exception because a null ServletContext is usually a sign that something has gone wrong. However, there are times when checking whether a parameter is null and returning immediately from the method is both useful and valid.
For instance, if you are gathering contact information about a user and the user has the option of providing a phone number. In that case, you may have a method that validates that the phone number contains all numbers, has the correct number of digits, etc, but which would immediately return if the phone number was empty or null.
public void validatePhone(String phoneNumber) throws ValidationException {
if (phoneNumber == null || phoneNumber.equals("")) {
return;
}
//do validation stuff, throwing exception if not valid
In your example, there is no return after the if statement; you are throwing an exception. (edit: I see you have changed the code since I posted this answer).
There are purists who think that you should have only one return statement in a method (at the end of the method). There's some merit to that idea - it makes the code more clear, it makes it easier to see what can be returned for the method, and especially when you need to cleanup resources (especially in a language without garbage collection; or in Java where you need to close for example an InputStream) it's more clear and easier if you have just one return at the bottom, and do the cleanup code just before the return.
I would not have any objection against the code in your example, however.
I have a few (subjective, or not) remarks:
I always use accolades with a if even the block contains only one line
I don't like to have many return on one method
I don't think that this null check is necessary. If getServletContext() returns null, then you have a much bigger problem with your webapp that should definitely be fixed. In that case, having a NullPointerException later in the code is an exceptional error so I wouldn't bother handling it.