In vimscript, function definitions can take an abort argument. To quote the docs,
When the [abort] argument is added, the function will
abort as soon as an error is detected
This leads me to seriously question what exactly functions normally do when they encounter errors. Stumble blindly forth into the darkness?
What does abort actually do? Does it break all of the try...endtry blocks? When do you want to use it, and when do you want to avoid it?
As glts mentioned, all the complex details are documented at :help except-compat, and the answer basically boils down to backwards compatibility and the inherent flexibility of Vimscript.
There's a natural progression from recorded macros to mappings to custom functions. With that in mind, it may make sense that when a command in a function causes an error (e.g. a %s/foo/bar/ that is not matching and missing the e flag), processing should continue.
On the other hand, when you write "industrial-grade" mappings, you'll almost always use a try..catch block inside your function call hierarchy, anyway (to avoid any multiline-errors Error detected while processing function: ..., and instead show a nice error message to the user).
So in practice, most published plugins do not use abort, but try..catch, and for quick, off-the-cuff stuff, you typically don't care too much about error handling, anyway.
Related
What's the syntax for accessing a subroutine Capture once it's called? self only works for objects, and &?ROUTINE refers to the static routine, not its state once called. So first, is it possible to access the routine's Capture from inside? If so, what's the syntax for accessing it? I've looked at the related Synopse but I can't find a way, if there's one.
There's no way to do exactly what you're asking for. While conceptually arguments are passed by forming a Capture object holding them, which is then unpacked by a signature, for most calls no Capture ever really exists. With every operator in Perl 6 being a multi-dispatch subroutine call, the performance of calling is important, and the language design is such that there's plenty of room for implementations to cheat in order to achieve acceptable performance.
It is possible to explicitly ask for a Capture, however:
sub foo(|c ($a, $b)) { say c.perl; }
foo(1, 2);
This will capture the arguments into c and then unpack them also into $a and $b, enforcing that inner signature.
One might realize that things like callsame do indeed find a way to access the arguments to pass them on, even though no Capture appears in the signature. Their need to do so causes the compiler to opt any routine containing a callsame out of various optimizations, which would otherwise discard information needed to discover the arguments. This isn't ideal, and it's probable it will change in the future - most likely by finding a way to sneak a |SECRET-CAPTURE into the signature or similar.
Let's say I would generally like to be warned about incomplete patterns in my code, but sometimes I know about a pattern incompleteness for a certain function and I know it's fine.
Is it still true that GHC's warning granularity is per-module, and there's no way to change warnings regarding a particular function or definition?
Yes, still true, but you can work around this by using error.
f (Just a) = show a
without a case for Nothing gives warnings but adding
f Nothing = error "f: Nothing supplied as an argument. This shouldn't have happened. Oops."
gets rid of the warning.
A per-function solution of your problem is to give Haskell some code you think will never be run, to keep it quiet.
Please note: I think your code should be robust and cover every eventuality unless you can prove it will never happen.
Working around this restriction isn't very good practice, I think.
(You might think that is a wide-open back door to hack away a useful compile-time check and should be stopped by -Wall, but I can obfuscate my round any simple restriction you'd choose and I think a complete solution to that problem would essentially solve the halting problem, so let's not blame the compiler.)
I'm new to programming and I have a conceptual question.
That is, can "exception" be perfectly replaced by "if.. else" ?
I know "exception" is to handling some exceptional conditions that might cause error or crash.
But we also use "if.. else" to ensure the correctness of value of variables, don't we?
Or "exception" can really be replaced by "if.. else", but using "exception" has other benefits(like convenience?)
Thank you, and sorry for my poor English.
The biggest difference between exceptions and "if..else" is that exceptions pass up the call stack: an exception raised in one function can be caught in a caller any number of frames up the stack. Using "if" statements doesn't let you transfer control in this way, everything has to be handled in the same function that detected the condition.
Most of your questions relate to Python, so here is an answer based on that fact.
In Python, it is idiomatic (or "pythonic") to use try-except blocks. We call this "EAFP": Easier to ask for forgiveness than permission.
In C, where there were no exceptions, it was usual to "LBYL": Look before you leap, resulting in lots of if (...) statements.
So, while you can LBYL, you should follow the idioms of the language in which you are programming: using exceptions for handling exceptional cases and if-statements for conditionals.
Technically, the answer is yes, exceptions can be perfectly replaced by if-else. Many languages, C for example, have no native notion of exceptions that can be thrown and caught.
The primary advantage of exceptions is code readability and maintainability. They serve a different purpose than if-else. Exceptions are for exceptional conditions, while if-else is for program flow.
See this excellent article explaining the difference.
That's a lot of branch conditions to manage. In theory, exceptions aren't necessary for perfect code, but perfect code does not exist in real life. Exceptions are a well-established mechanism for dealing with problems in a controlled manner.
The old way for handling an error from a function looks something like this:
int result = function_returns_error_code();
if (result != GOOD)
{
/* handle problem */
}
else
{
/* keep going */
}
The problem with this solution (and others like it - using if-else) is that if there is a real problem, and the programmer does not properly handle it with an if...else (if the function returns an error code indicating major problems, but the programmer forgets about it), it is left ignored. With an exception, it goes further and further up the call stack ) until it is either handled or the program quits.
Further, it is tedious to check for error codes in functions, or pass a parameter into which to put an error code. It is simpler, cleaner, and better to use exceptions, for maintainability and abstraction.
In most high-level languages working with exceptions is often more efficient than if-else because you avoid multiple validation. eg:
if value is not 0 then print 10 / value
In most interpreters 10 / value will internally test whether value is a valid divider before using it so you've actually tested for the same problem twice. In some cases the exception may come all the way up from hardware so no software validation is happening at all.
On the other hand:
try print 10 / value ... catch exception
Will only test whether value is valid once. Furthermore there's a good chance the test will be better optimised than your own code and more capable of handling truly unexpected conditions (like out of memory errors).
So, I was just coding a bit today, and I realized that I don't have much consistency when it comes to a coding style when programming functions. One of my main concerns is whether or not its proper to code it so that you check that the input of the user is valid OUTSIDE of the function, or just throw the values passed by the user into the function and check if the values are valid in there. Let me sketch an example:
I have a function that lists hosts based on an environment, and I want to be able to split the environment into chunks of hosts. So an example of the usage is this:
listhosts -e testenv -s 2 1
This will get all the hosts from the "testenv", split it up into two parts, and it is displaying part one.
In my code, I have a function that you pass it in a list, and it returns a list of lists based on you parameters for splitting. BUT, before I pass it a list, I first verify the parameters in my MAIN during the getops process, so in the main I check to make sure there are no negatives passed by the user, I make sure the user didnt request to split into say, 4 parts, but asking to display part 5 (which would not be valid), etc.
tl;dr: Would you check the validity of a users input the flow of you're MAIN class, or would you do a check in your function itself, and either return a valid response in the case of valid input, or return NULL in the case of invalid input?
Obviously both methods work, I'm just interested to hear from experts as to which approach is better :) Thanks for any comments and suggestions you guys have! FYI, my example is coded in Python, but I'm still more interested in a general programming answer as opposed to a language-specific one!
Good question! My main advice is that you approach the problem systematically. If you are designing a function f, here is how I think about its specification:
What are the absolute requirements that a caller of f must meet? Those requirements are f's precondition.
What does f do for its caller? When f returns, what is the return value and what is the state of the machine? Under what circumstances does f throw an exception, and what exception is thrown? The answers to all these questions constitute f's postcondition.
The precondition and postcondition together constitute f's contract with callers.
Only a caller meeting the precondition gets to rely on the postcondition.
Finally, bearing directly on your question, what happens if f's caller doesn't meet the precondition? You have two choices:
You guarantee to halt the program, one hopes with an informative message. This is a checked run-time error.
Anything goes. Maybe there's a segfault, maybe memory is corrupted, maybe f silently returns a wrong answer. This is an unchecked run-time error.
Notice some items not on this list: raising an exception or returning an error code. If these behaviors are to be relied upon, they become part of f's contract.
Now I can rephrase your question:
What should a function do when its caller violates its contract?
In most kinds of applications, the function should halt the program with a checked run-time error. If the program is part of an application that needs to be reliable, either the application should provide an external mechanism for restarting an application that halts with a checked run-time error (common in Erlang code), or if restarting is difficult, all functions' contracts should be made very permissive so that "bad input" still meets the contract but promises always to raise an exception.
In every program, unchecked run-time errors should be rare. An unchecked run-time error is typically justified only on performance grounds, and even then only when code is performance-critical. Another source of unchecked run-time errors is programming in unsafe languages; for example, in C, there's no way to check whether memory pointed to has actually been initialized.
Another aspect of your question is
What kinds of contracts make the best designs?
The answer to this question varies more depending on the problem domain.
Because none of the work I do has to be high-availability or safety-critical, I use restrictive contracts and lots of checked run-time errors (typically assertion failures). When you are designing the interfaces and contracts of a big system, it is much easier if you keep the contracts simple, you keep the preconditions restrictive (tight), and you rely on checked run-time errors when arguments are "bad".
I have a function that you pass it in a list, and it returns a list of lists based on you parameters for splitting. BUT, before I pass it a list, I first verify the parameters in my MAIN during the getops process, so in the main I check to make sure there are no negatives passed by the user, I make sure the user didnt request to split into say, 4 parts, but asking to display part 5.
I think this is exactly the right way to solve this particular problem:
Your contract with the user is that the user can say anything, and if the user utters a nonsensical request, your program won't fall over— it will issue a sensible error message and then continue.
Your internal contract with your request-processing function is that you will pass it only sensible requests.
You therefore have a third function, outside the second, whose job it is to distinguish sense from nonsense and act accordingly—your request-processing function gets "sense", the user is told about "nonsense", and all contracts are met.
One of my main concerns is whether or not its proper to code it so that you check that the input of the user is valid OUTSIDE of the function.
Yes. Almost always this is the best design. In fact, there's probably a design pattern somewhere with a fancy name. But if not, experienced programmers have seen this over and over again. One of two things happens:
parse / validate / reject with error message
parse / validate / process
This kind of design has one data type (request) and four functions. Since I'm writing tons of Haskell code this week, I'll give an example in Haskell:
data Request -- type of a request
parse :: UserInput -> Request -- has a somewhat permissive precondition
validate :: Request -> Maybe ErrorMessage -- has a very permissive precondition
process :: Request -> Result -- has a very restrictive precondition
Of course there are many other ways to do it. Failures could be detected at the parsing stage as well as the validation stage. "Valid request" could actually be represented by a different type than "unvalidated request". And so on.
I'd do the check inside the function itself to make sure that the parameters I was expecting were indeed what I got.
Call it "defensive programming" or "programming by contract" or "assert checking parameters" or "encapsulation", but the idea is that the function should be responsible for checking its own pre- and post-conditions and making sure that no invariants are violated.
If you do it outside the function, you leave yourself open to the possibility that a client won't perform the checks. A method should not rely on others knowing how to use it properly.
If the contract fails you either throw an exception, if your language supports them, or return an error code of some kind.
Checking within the function adds complexity, so my personal policy is to do sanity checking as far up the stack as possible, and catch exceptions as they arise. I also make sure that my functions are documented so that other programmers know what the function expects of them. They may not always follow such expectations, but to be blunt, it is not my job to make their programs work.
It often makes sense to check the input in both places.
In the function you should validate the inputs and throw an exception if they are incorrect. This prevents invalid inputs causing the function to get halfway through and then throw an unexpected exception like "array index out of bounds" or similar. This will make debugging errors much simpler.
However throwing exceptions shouldn't be used as flow control and you wouldn't want to throw the raw exception straight to the user, so I would also add logic in the user interface to make sure I never call the function with invalid inputs. In your case this would be displaying a message on the console, but in other cases it might be showing a validation error in a GUI, possibly as you are typing.
"Code Complete" suggests an isolation strategy where one could draw a line between classes that validate all input and classes that treat their input as already validated. Anything allowed to pass the validation line is considered safe and can be passed to functions that don't do validation (they use asserts instead, so that errors in the external validation code can manifest themselves).
How to handle errors depends on the programming language; however, when writing a commandline application, the commandline really should validate that the input is reasonable. If the input is not reasonable, the appropriate behavior is to print a "Usage" message with an explanation of the requirements as well as to exit with a non-zero status code so that other programs know it failed (by testing the exit code).
Silent failure is the worst kind of failure, and that is what happens if you simply return incorrect results when given invalid arguments. If the failure is ever caught, then it will most likely be discovered very far away from the true point of failure (passing the invalid argument). Therefore, it is best, IMHO to throw an exception (or, where not possible, to return an error status code) when an argument is invalid, since it flags the error as soon as it occurs, making it much easier to identify and correct the true cause of failure.
I should also add that it is very important to be consistent in how you handle invalid inputs; you should either check and throw an exception on invalid input for all functions or do that for none of them, since if users of your interface discover that some functions throw on invalid input, they will begin to rely on this behavior and will be incredibly surprised when other function simply return invalid results rather than complaining.
I recently took in a small MCF C++ application, which is obviously in a working state. To get started I'm running PC-Lint over the code, and lint is complaining that CStringT's are being passed to Format. Opinion on the internet seems to be divided. Some say that CSting is designed to handle this use case without error, but others (and an MSDN article) say that it should always be cast when passed to a variable argument function. Can Stackoverflow come to any consensus on the issue?
CString has been carefully designed to be passed as part of a variable argument list, so it is safe to use it that way. And you can be fairly sure that Microsoft will take care not to break this particular behavior. So I'd say you are safe to continue using it that way, if you want to.
That said, personally I'd prefer the cast. It is not common behavior that string classes behave that way (e.g. std::string does not) and for mental consistency it may be better to just do it the "safe" way.
P.S.: See this thread for implementation details and further notes on how to cast.