I want to apply some roles dynamically to a moose class. I am writing that logic inside BUILD function. If there are any failures while applying role, I want to return FALSE. What will be the impact of that return value. How can I handle failure in the code where I am creating the object of this class?
Moose ignores the return value of BUILD; it is called for side-effects only. I'd expect "failures while applying role" to be caused by bugs and those failures to be evidenced by runtime exceptions. If you don't want the program to crash because of runtime exceptions, appropriately wrap your call to new() with eval.
Related
I am using a subroutine (FORM, ENDFORM) where there is a class-method calling that raises an exception. I am catching the exception from the class-method calling, but how can I pass this exception to where I call the subroutine (PERFORM)?
There's a documentation section for Class-Based Exceptions in Procedures .
If a class-based exception is not handled in a procedure, the system attempts to propagate it to the caller of the procedure. The exceptions that can be propagated from a procedure must be declared in its interface. The caller then knows which exceptions to expect from the procedure. Class-based exceptions are divided into three categories, which determine whether the declaration must be explicit and how it is checked.
In methods of local classes and subroutines, the addition RAISING of the statements METHODS and FORM is used for the declaration
Undeclared exceptions cannot leave a procedure and violate the interface if they are not handled within the procedure. A violation of the interface raises an exception of the predefined class CX_SY_NO_HANDLER, whose exception object contains a reference to the original exception in the attribute PREVIOUS.
See also documentation for FORM RAISING.
START-OF-SELECTION.
TRY.
PERFORM test.
CATCH lcx_some_exception INTO DATA(lx_ex).
ENDTRY.
FORM test RAISING lcx_some_exception.
DATA(lo_test) = NEW lcl_test( ).
lo_test->test( ).
ENDFORM.
Both require and assert are used to perform certain checks during runtime to verify certain conditions.
So what is the basic difference between them?
The only one I see is that require throws IllegalArgumentException and assert throws AssertionError.
How do I choose which one to use?
As Kigyo mentioned there is a semantic difference
assert means that your program has reached an inconsistent state this might be a problem with the current method/function (I like to think of it a bit as HTTP 500 InternalServerError)
require means that the caller of the method is at fault and should fix its call (I like to think of it a bit as HTTP 400 BadRequest)
There is also a major technical difference:
assert is annotated with #elidable(ASSERTION)
meaning you can compile your program with -Xelide-below ASSERTION or with -Xdisable-assertions and the compiler will not generate the bytecode for the assertions. This can significantly reduce bytecode size and improve performance if you have a large number of asserts.
Knowing this, you can use an assert to verify all the invariants everywhere in your program (all the preconditions/postconditions for every single method/function calls) and not pay the price in production.
You would usually have the "test" build with all the assertions enabled, it would be slower as it would verify all the assertions at all times, then you could have the "production" build of your product without the assertions, which you would eliminate all the internal state checks done through assertion
require is not elidable, it makes more sense for use in libraries (including internal libraries) to inform the caller of the preconditions to call a given method/function.
This is only my subjective point of view.
I use require whenever I want a constraint on parameters.
As an example we can take the factorial for natural numbers. As we do not want to address negative numbers, we want to throw an IllegalArgumentException.
I would use assert, whenever you want to make sure some conditions (like invariants) are always true during execution. I see it as a way of testing.
Here is an example implementation of factorial with require and assert
def fac(i: Int) = {
require(i >= 0, "i must be non negative") //this is for correct input
#tailrec def loop(k: Int, result: Long = 1): Long = {
assert(result == 1 || result >= k) //this is only for verification
if(k > 0) loop(k - 1, result * k) else result
}
loop(i)
}
When result > 1 is true, then the loop was executed at least once. So the result has to be bigger or equal to k. That would be a loop invariant.
When you are sure that your code is correct, you can remove the assert, but the require would stay.
You can see here for a detailed discussion within Scala language.
I can add that, the key to distinguish between require and assert is to understand these two. These two are both tools of software quality but from different toolboxes of different paradigms. In summary assert is a Software testing tool which takes a corrective approach, whereas require is a design by contract tool which takes a preventive approach.
Both require and assert are means of controlling validity of state. Historically there were 2 distinct paradigms for dealing with invalid states. The first one which is mainstream collectively called software testing discipline methodologies and tools. The other, called design by contract. These are two paradigms which are not comparable.
Software testing ensures a code versatile enough to be capable of error prone actions, were not misused. Design by contract controls code from having such capability. In other words Software testing is corrective, and design by contract is preventive.
assert is used to write unit tests, i.e. if a method passes all tests each written by an assert expression, the code is qualified as error free. So assert seats besides operational code, and is an independent body.
require is embedded within code and part of it to assure nothing harmful can happen.
In very simple language:
Require is used to enforce a precondition on the caller of a function or the creator of an object of some class. Whereas, assert is used to check the code of the function itself.
So, if a precondition fails, then you get an illegal argument exception. Whereas, if an assertion fails and it's not the caller's fault and consequently you get an assertion error.
require, ensure and invariance are concepts in Contract By Design (CBD) development process.
require checks for the pre-conditions that the caller should satisfy to consume the routine.
ensure checks for the correctness in the return value (and to also verify only the desired change has happened and nothing more)
invariance checks for the validness of the class at all critical times.
CBD is a development methodology to build correct/robust software. For more details on CBD Google and you should hit a link from Eiffel Software. Hope this helps.
Scaladocs/javadocs are pretty good as well:
assert()
Tests an expression, throwing an AssertionError if false. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.
require()
Tests an expression, throwing an IllegalArgumentException if false. This method is similar to assert, but blames the caller of the method for violating the condition.
Beginning in Scala and reading about Either I naturally comparing new concepts to something I know (in this case from Java). Are there any differences from the concept of checked exceptions and Either?
In both cases
the possibility of failure is explicitly annotated in the method (throws or returning Either)
the programmer can handle the error case directly when it occurs or move it up (returning again an Either)
there is a way to inform the caller about the reason of the error
I suppose one uses for-comprehensions on Either to write code as there would be no error similar to checked exceptions.
I wonder if I am the only beginner who has problems to see the difference.
Thanks
Either can be used for more than just exceptions. For example, if you were to have a user either type input for you or specify a file containing that input, you could represent that as Either[String, File].
Either is very often used for exception handling. The main difference between Either and checked exceptions is that control flow with Either is always explicit. The compiler really won't let you forget that you are dealing with an Either; it won't collect Eithers from multiple places without you being aware of it, everything that is returned must be an Either, etc.. Because of this, you use Either not when maybe something extraordinary will go wrong, but as a normal part of controlling program execution. Also, Either does not capture a stack trace, making it much more efficient than a typical exception.
One other difference is that exceptions can be used for control flow. Need to jump out of three nested loops? No problem--throw an exception (without a stack trace) and catch it on the outside. Need to jump out of five nested method calls? No problem! Either doesn't supply anything like this.
That said, as you've pointed out there are a number of similarities. You can pass back information (though Either makes that trivial, while checked exceptions make you write your own class to store any extra information you want); you can pass the Either on or you can fold it into something else, etc..
So, in summary: although you can accomplish the same things with Either and checked exceptions with regards to explicit error handling, they are relatively different in practice. In particular, Either makes creating and passing back different states really easy, while checked exceptions are good at bypassing all your normal control flow to get back, hopefully, to somewhere that an extraordinary condition can be sensibly dealt with.
Either is equivalent to a checked exception in terms of the return signature forming an exclusive disjunction. The result can be a thrown exception X or an A. However, throwing an exception isn't equivalent to returning one – the first is not referentially transparent.
Where Scala's Either is not (as of 2.9) equivalent is that a return type is positively biased, and requires effort to extract/deconstruct the Exception, Either is unbiased; you need to explicitly ask for the left or right value. This is a topic of some discussion, and in practice a bit of pain – consider the following three calls to Either producing methods
for {
a <- eitherA("input").right
b <- eitherB(a).right
c <- eitherC(b).right
} yield c // Either[Exception, C]
you need to manually thread through the RHS. This may not seem that onerous, but in practice is a pain and somewhat surprising to new-comers.
Yes, Either is a way to embed exceptions in a language; where a set of operations that can fail can throw an error value to some non-local site.
In addition to the practical issues Rex mentioned, there's some extra things you get from the simple semantics of an Either:
Either forms a monad; so you can use monadic operations over sets of expressions that evaluate to Either. E.g. for short circuiting evaluation without having to test the result
Either is in the type -- so the type checker alone is sufficient to track incorrect handling of the value
Once you have the ability to return either an error message (Left s) or a successful value Right v, you can layer exceptions on top, as just Either plus an error handler, as is done for MonadError in Haskell.
Suppose you have a property with copy semantics. What should you do in the setter if the copy method fails? (I presume this is a possibility, since a copy usually starts with an alloc/init combo, which can fail and return nil.) Apple recommends returning error codes rather than using exceptions, but a setter generally has a void return type. What is the recommended approach? How do you signal that an error has occurred?
The Apple recommendation is really that exceptions should be reserved for exceptional situations. This is sometimes a recommended programming practice anyway, but in the case of Objective-C is reinforced due to the higher cost of exception handling.
So you can throw an exception if you wish and it is appropriate, e.g. running out of memory (copy failed) is (hopefully!) exceptional.
That said, some programming practices also recommend that properties should not throw exceptions; usually on the basis that something that looks like assignment obj.property = value; would be confusing if exceptions were thrown (unlike [obj setProperty:value]).
So that get us to setting the property to the "zero" for the type (nil, 0, 0.0, NO etc.).
To return more details of the error record details of the error which can be queried after the "zero" has been detected. This is essentially the approach used by the underlying ("Unix") syscalls, and many library functions, were errno is set before a "zero" is returned.
There is no way to signal an error, other than that the property whose setter you called would be nil. You can check for nil after executing the setter, just as you would to confirm success after alloc/init'ing a new instance.
Question:
What is considered to be "Best practice" - and why - of handling errors in a constructor?.
"Best Practice" can be a quote from Schwartz, or 50% of CPAN modules use it, etc...; but I'm happy with well reasoned opinion from anyone even if it explains why the common best practice is not really the best approach.
As far as my own view of the topic (informed by software development in Perl for many years), I have seen three main approaches to error handling in a perl module (listed from best to worst in my opinion):
Construct an object, set an invalid flag (usually "is_valid" method). Often coupled with setting error message via your class's error handling.
Pros:
Allows for standard (compared to other method calls) error handling as it allows to use $obj->errors() type calls after a bad constructor just like after any other method call.
Allows for additional info to be passed (e.g. >1 error, warnings, etc...)
Allows for lightweight "redo"/"fixme" functionality, In other words, if the object that is constructed is very heavy, with many complex attributes that are 100% always OK, and the only reason it is not valid is because someone entered an incorrect date, you can simply do "$obj->setDate()" instead of the overhead of re-executing entire constructor again. This pattern is not always needed, but can be enormously useful in the right design.
Cons: None that I'm aware of.
Return "undef".
Cons: Can not achieve any of the Pros of the first solution (per-object error messages outside of global variables and lightweight "fixme" capability for heavy objects).
Die inside the constructor. Outside of some very narrow edge cases, I personally consider this an awful choice for too many reasons to list on the margins of this question.
UPDATE: Just to be clear, I consider the (otherwise very worthy and a great design) solution of having very simple constructor that can't fail at all and a heavy initializer method where all the error checking occurs to be merely a subset of either case #1 (if initializer sets error flags) or case #3 (if initializer dies) for the purposes of this question. Obviously, choosing such a design, you automatically reject option #2.
It depends on how you want your constructors to behave.
The rest of this response goes into my personal observations, but as with most things Perl, Best Practices really boils down to "Here's one way to do it, which you can take or leave depending on your needs." Your preferences as you described them are totally valid and consistent, and nobody should tell you otherwise.
I actually prefer to die if construction fails, because we set it up so that the only types of errors that can occur during object construction really are big, obvious errors that should halt execution.
On the other hand, if you prefer that doesn't happen, I think I'd prefer 2 over 1, because it's just as easy to check for an undefined object as it is to check for some flag variable. This isn't C, so we don't have a strong typing constraint telling us that our constructor MUST return an object of this type. So returning undef, and checking for that to establish success or failure, is a great choice.
The 'overhead' of construction failure is a consideration in certain edge cases (where you can't quickly fail before incurring overhead), so for those you might prefer method 1. So again, it depends on what semantics you've defined for object construction. For example, I prefer to do heavyweight initialization outside of construction. As to standardization, I think that checking whether a constructor returns a defined object is as good a standard as checking a flag variable.
EDIT: In response to your edit about initializers rejecting case #2, I don't see why an initializer can't simply return a value that indicates success or failure rather than setting a flag variable. Actually, you may want to use both, depending on how much detail you want about the error that occurred. But it would be perfectly valid for an initializer to return true on success and undef on failure.
I prefer:
Do as little initialization as possible in the constructor.
croak with an informative message when something goes wrong.
Use appropriate initialization methods to provide per object error messages etc
In addition, returning undef (instead of croaking) is fine in case the users of the class may not care why exactly the failure occurred, only if they got a valid object or not.
I despise easy to forget is_valid methods or adding extra checks to ensure methods are not called when the internal state of the object is not well defined.
I say these from a very subjective perspective without making any statements about best practices.
I would recommend against #1 simply because it leads to more error handling code which will not be written. For example, if you just return false then this works fine.
my $obj = Class->new or die "Construction failed...";
But if you return an object which is invalid...
my $obj = Class->new;
die "Construction failed #{[ $obj->error_message ]}" if $obj->is_valid;
And as the quantity of error handling code increases the probability of it being written decreases. And its not linear. By increasing the complexity of your error handling system you actually decrease the amount of errors it will catch in practical use.
You also have to be careful that your invalid object in question dies when any method is called (aside from is_valid and error_message) leading to yet more code and opportunities for mistakes.
But I agree there is value in being able to get information about the failure, which makes returning false (just return not return undef) inferior. Traditionally this is done by calling a class method or global variable as in DBI.
my $dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
But it suffers from A) you still have to write error handling code and B) its only valid for the last operation.
The best thing to do, in general, is throw an exception with croak. Now in the normal case the user writes no special code, the error occurs at the point of the problem, and they get a good error message by default.
my $obj = Class->new;
Perl's traditional recommendations against throwing exceptions in library code as being impolite is outdated. Perl programmers are (finally) embracing exceptions. Rather than writing error handling code ever and over again, badly and often forgetting, exceptions DWIM. If you're not convinced just start using autodie (watch pjf's video about it) and you'll never go back.
Exceptions align Huffman encoding with actual use. The common case of expecting the constructor to just work and wanting an error if it doesn't is now the least code. The uncommon case of wanting to handle that error requires writing special code. And the special code is pretty small.
my $obj = eval { Class->new } or do { something else };
If you find yourself wrapping every call in an eval you are doing it wrong. Exceptions are called that because they are exceptional. If, as in your comment above, you want graceful error handling for the user's sake, then take advantage of the fact that errors bubble up the stack. For example, if you want to provide a nice user error page and also log the error you can do this:
eval {
run_the_main_web_code();
} or do {
log_the_error($#);
print_the_pretty_error_page;
};
You only need it in one place, at top of your call stack, rather than scattered everywhere. You can take advantage of this at smaller increments, for example...
my $users = eval { Users->search({ name => $name }) } or do {
...handle an error while finding a user...
};
There's two things going on. 1) Users->search always returns a true value, in this case an array ref. That makes the simple my $obj = eval { Class->method } or do work. That's optional. But more importantly 2) you only need to put special error handling around Users->search. All the methods called inside Users->search and all the methods they call... they just throw exceptions. And they're all caught at one point and handled the same. Handling the exception at the point which cares about it makes for much neater, compact and flexible error handling code.
You can pack more information into the exception by croaking with a string overloaded object rather than just a string.
my $obj = eval { Class->new }
or die "Construction failed: $# and there were #{[ $#->num_frobnitz ]} frobnitzes";
Exceptions:
Do the right thing without any thought by the caller
Require the least code for the most common case
Provide the most flexibility and information about the failure to the caller
Modules such as Try::Tiny fix most of the hanging issues surrounding using eval as an exception handler.
As for your use case where you might have a very expensive object and want to try and continue with it partially build... smells like YAGNI to me. Do you really need it? Or you have a bloated object design which is doing too much work too early. IF you do need it, you can put the information necessary to continue the construction in the exception object.
First the pompous general observations:
A constructor's job should be: Given valid construction parameters, return a valid object.
A constructor that does not construct a valid object cannot perform its job and is therefore a perfect candidate for exception generation.
Making sure the constructed object is valid is part of the constructor's job. Handing out a known-to-be-bad object and relying on the client to check that the object is valid is a surefire way to wind up with invalid objects that explode in remote places for non-obvious reasons.
Checking that all the correct arguments are in place before the constructor call is the client's job.
Exceptions provide a fine-grained way of propagating the particular error that occurred without needing to have a broken object in hand.
return undef; is always bad[1]
bIlujDI' yIchegh()Qo'; yIHegh()!
Now to the actual question, which I will construe to mean "what do you, darch, consider the best practice and why". First, I'll note that returning a false value on failure has a long Perl history (most of the core works that way, for example), and a lot of modules follow this convention. However, it turns out this convention produces inferior client code and newer modules are moving away from it.[2]
[The supporting argument and code samples for this turn out to be the more general case for exceptions that prompted the creation of autodie, and so I will resist the temptation to make that case here. Instead:]
Having to check for successful creation is actually more onerous than checking for an exception at an appropriate exception-handling level. The other solutions require the immediate client to do more work than it should have to just to obtain an object, work that is not required when the constructor fails by throwing an exception.[3] An exception is vastly more expressive than undef and equally expressive as passing back a broken object for purposes of documenting errors and annotating them at various levels in the call stack.
You can even get the partially-constructed object if you pass it back in the exception. I think this is a bad practice per my belief about what a constructor's contract with its clients ought to be, but the behavior is supported. Awkwardly.
So: A constructor that cannot create a valid object should throw an exception as early as possible. The exceptions a constructor can throw should be documented parts of its interface. Only the calling levels that can meaningfully act on the exception should even look for it; very often, the behavior of "if this construction fails, don't do anything" is exactly correct.
[1]: By which I mean, I am not aware of any use cases where return; is not strictly superior. If someone calls me on this I might have to actually open a question. So please don't. ;)
[2]: Per my extremely unscientific recollection of the module interfaces I've read in the last two years, subject to both selection and confirmation biases.
[3]: Note that throwing an exception does still require error-handling, as would the other proposed solutions. This does not mean wrapping every instantiation in an eval unless you actually want to do complex error-handling around every construction (and if you think you do, you're probably wrong). It means wrapping the call which is able to meaningfully act on the exception in an eval.