When calling a Go function that returns an error, I wonder how to deal with a non-nil error value. I can just abort, log it, or pass it to the caller. Or a combination thereof. But it would be better if I found out what went wrong and reacted in a more fine-grained manner.
Thus, how can I find out about possible error values and their meanings? For example, I want to use the http.NewRequest function. Look it up in the docs. There, it only says that there are possible error conditions but not which ones. How can I find out about these?
First off, if you haven't read it yet, I recommend reading this blog article about error handling in Go, it might clarify some things for you.
As for finding out the possible error values of Go stdlib functions, if it's not explicitly documented, one option is to go read the code. For example, on the http.NewRequest function, you can click on the header, which takes you to the source code. There you can see that right now, it only returns an error in one case, when the URL parsing fails. Exploring the url.Parse function, you'll see that it may return url.Error, which you can check for with a type assertion:
package main
import (
"log"
"net/http"
"net/url"
)
func main() {
r, err := http.NewRequest("GET", "http://[fe80::%31%25en0]:8080/", nil)
if perr, ok := err.(*url.Error); ok && perr != nil {
log.Fatal("Parse error, check URL formatting: ", perr)
} else if err != nil {
log.Fatal("Error creating HTTP request")
}
// ... success case
log.Println(r)
}
Running this gives you the parse error, which you may or may not want to handle differently in your application:
2009/11/10 23:00:00 Parse error, check URL formatting: parse http://[fe80::%31%25en0]:8080/: percent-encoded characters in host
I agree with JimB's comment however, specifically checking the type of error in this case is probably not a useful thing to do. In 99.99% of cases all you need to know is that creating the request failed (err != nil) and handle that eventuality in an appropriate way (logging, returning, etc).
I've made a large program that opens and closes files and databases, perform writes and reads on them etc among other things. Since there no such thing as "exception handling in go", and since I didn't really know about "defer" statement and "recover()" function, I applied error checking after every file-open, read-write, database entry etc. E.g.
_,insert_err := stmt.Run(query)
if insert_err != nil{
mylogs.Error(insert_err.Error())
return db_updation_status
}
For this, I define db_updation_status at the beginning as "false" and do not make it "true" until everything in the program goes right.
I've done this in every function, after every operation which I believe could go wrong.
Do you think there's a better way to do this using defer-panic-recover? I read about these here http://golang.org/doc/articles/defer_panic_recover.html, but can't clearly get how to use them. Do these constructs offer something similar to exception-handling? Am I better off without these constructs?
I would really appreciate if someone could explain this to me in a simple language, and/or provide a use case for these constructs and compare them to the type of error handling I've used above.
It's more handy to return error values - they can carry more information (advantage to the client/user) than a two valued bool.
What concerns panic/recover: There are scenarios where their use is completely sane. For example, in a hand written recursive descent parser, it's quite a PITA to "bubble" up an error condition through all the invocation levels. In this example, it's a welcome simplification if there's a deferred recover at the top most (API) level and one can report any kind of error at any invocation level using, for example
panic(fmt.Errorf("Cannot %v in %v", foo, bar))
If an operation can fail and returns an error, than checking this error immediately and handling it properly is idiomatic in go, simple and nice to check if anything gets handled properly.
Don't use defer/recover for such things: Needed cleanup actions are hard to code, especially if stuff gets nested.
The usual way to report an error to a caller is to return an error as an extra return value. The canonical Read method is a well-known instance; it returns a byte count and an error.
But what if the error is unrecoverable? Sometimes the program simply cannot continue.
For this purpose, there is a built-in function panic that in effect creates a run-time error that will stop the program (but see the next section). The function takes a single argument of arbitrary type—often a string—to be printed as the program dies. It's also a way to indicate that something impossible has happened, such as exiting an infinite loop.
http://golang.org/doc/effective_go.html#errors
assume you have a function that polls some kind of queue and blocks for a certain amount of time. If this time has passed without something showing up on the queue, some indication of the timeout should be delivered to the caller, otherwise the something that showed up should be returned.
Now you could write something like:
class Queue
{
Thing GetThing();
}
and throw an exception in case of a timeout. Or you
write
class Queue
{
int GetThing(Thing& t);
}
and return an error code for success and timeout.
However, drawback of solution 1 is that the on a not so busy queue timeout is not an exceptional case, but rather common. And solution 2 uses return values for errors and ugly syntax, since you can end up with a Thing that contains nothing.
Is there another (smart) solution for that problem? What is the preferred solution in an object oriented environment?
I would use exceptions only when the error is serious enough to stop the execution of the application, or of any big-enough application's component. But I wouldn't use exceptions for common cases, after which we continue the normal execution or execute the same function again. This would be just using exceptions for flow control, which is wrong.
So, I suggest you to either use the second solution that you proposed, or to do the following:
class Queue
{
bool GetThing(Thing& t); // true on success, false on failure
string GetLastError();
};
Of course you can stick with an int for an error code, instead of a string for the full error message. Or even better, just define class Error and have GetLastError() return it.
Why not just return null from GetThing in your first solution, changing it to return a Thing *? It seems to fit the bill, at least from the information you've given so far.
In the first, and second case, you can't do anything but throw an exception. When you return a Thing, or a Thing&, you don't have the option of not returning a Thing.
If you want to fail without using an exception then you need:
class Queue
{
// Either something like this. GetThing retuns NULL on an error,
// GetError returns a specific error code
Thing* GetThing();
int GetError();
// This kind of pattern is common. Return a result code
// and set ppOut to a valid thing or NULL.
int GetThing(Thing** ppOut);
};
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I've heard some voices saying that checking for a returned null value from methods is bad design. I would like to hear some reasons for this.
pseudocode:
variable x = object.method()
if (x is null) do something
The rationale behind not returning null is that you do not have to check for it and hence your code does not need to follow a different path based on the return value. You might want to check out the Null Object Pattern which provides more information on this.
For example, if I were to define a method in Java that returned a Collection I would typically prefer to return an empty collection (i.e. Collections.emptyList()) rather than null as it means my client code is cleaner; e.g.
Collection<? extends Item> c = getItems(); // Will never return null.
for (Item item : c) { // Will not enter the loop if c is empty.
// Process item.
}
... which is cleaner than:
Collection<? extends Item> c = getItems(); // Could potentially return null.
// Two possible code paths now so harder to test.
if (c != null) {
for (Item item : c) {
// Process item.
}
}
Here's the reason.
In Clean Code by Robert Martin he writes that returning null is bad design when you can instead return, say, empty array. Since expected result is an array, why not? It'll enable you to iterate over result without any extra conditions. If it's an integer, maybe 0 will suffice, if it's a hash, empty hash. etc.
The premise is to not force calling code to immediately handle issues. Calling code may not want to concern itself with them. That's also why in many cases exceptions is better than nil.
Good uses of returning null:
If null is a valid functional result, for example: FindFirstObjectThatNeedsProcessing() can return null if not found and the caller should check accordingly.
Bad uses: Trying to replace or hide exceptional situations such as:
catch(...) and return null
API dependency initialization failed
Out of disk space
Invalid input parameters (programming error, inputs must be sanitized by the caller)
etc
In those cases throwing an exception is more adequate since:
A null return value provides no meaningful error info
The immediate caller most likely cannot handle the error condition
There is no guarantee that the caller is checking for null results
However, Exceptions should not be used to handle normal program operation conditions such as:
Invalid username/password (or any user-provided inputs)
Breaking loops or as non-local gotos
Yes, returning NULL is a terrible design, in object-oriented world. In a nutshell, NULL usage leads to:
ad-hoc error handling (instead of exceptions)
ambiguous semantic
slow instead of fast failing
computer thinking instead of object thinking
mutable and incomplete objects
Check this blog post for a detailed explanation: http://www.yegor256.com/2014/05/13/why-null-is-bad.html. More in my book Elegant Objects, Section 4.1.
Who says this is bad design?
Checking for nulls is a common practice, even encouraged, otherwise you run the risk of NullReferenceExceptions everywhere. Its better to handle the error gracefully than throw exceptions when you don't need to.
Based on what you've said so far, I think there's not enough information.
Returning null from a CreateWidget()method seems bad.
Returning null from a FindFooInBar() method seems fine.
Its inventor says it is a billion dollar mistake!
It depends on the language you're using. If you're in a language like C# where the idiomatic way of indicating the lack of a value is to return null, then returning null is a good design if you don't have a value. Alternatively, in languages such as Haskell which idiomatically use the Maybe monad for this case, then returning null would be a bad design (if it were even possible).
If you read all the answers it becomes clear the answer to this question depends on the kind of method.
Firstly, when something exceptional happens (IOproblem etc), logically exceptions are thrown. When exactly something is exceptional is probably something for a different topic..
Whenever a method is expected to possibly have no results there are two categories:
If it is possible to return a neutral value, do so.
Empty enumrables, strings etc are good examples
If such a neutral value does not exist, null should be returned.
As mentioned, the method is assumed to possibly have no result, so it is not exceptional, hence should not throw an exception. A neutral value is not possible (for example: 0 is not especially a neutral result, depending on the program)
Untill we have an official way to denote that a function can or cannot return null, I try to have a naming convention to denote so.
Just like you have the TrySomething() convention for methods that are expected to fail, I often name my methods SafeSomething() when the method returns a neutral result instead of null.
I'm not fully ok with the name yet, but couldn't come up with anything better. So I'm running with that for now.
I have a convention in this area that served me well
For single item queries:
Create... returns a new instance, or throws
Get... returns an expected existing instance, or throws
GetOrCreate... returns an existing instance, or new instance if none exists, or throws
Find... returns an existing instance, if it exists, or null
For collection queries:
Get... always returns a collection, which is empty if no matching[1] items are found
[1] given some criteria, explicit or implicit, given in the function name or as parameters.
Exceptions are for exceptional circumstances.
If your function is intended to find an attribute associated with a given object, and that object does has no such attribute, it may be appropriate to return null. If the object does not exist, throwing an exception may be more appropriate. If the function is meant to return a list of attributes, and there are none to return, returning an empty list makes sense - you're returning all zero attributes.
It's not necessarily a bad design - as with so many design decisions, it depends.
If the result of the method is something that would not have a good result in normal use, returning null is fine:
object x = GetObjectFromCache(); // return null if it's not in the cache
If there really should always be a non-null result, then it might be better to throw an exception:
try {
Controller c = GetController(); // the controller object is central to
// the application. If we don't get one,
// we're fubar
// it's likely that it's OK to not have the try/catch since you won't
// be able to really handle the problem here
}
catch /* ... */ {
}
It's fine to return null if doing so is meaningful in some way:
public String getEmployeeName(int id){ ..}
In a case like this it's meaningful to return null if the id doesn't correspond to an existing entity, as it allows you to distinguish the case where no match was found from a legitimate error.
People may think this is bad because it can be abused as a "special" return value that indicates an error condition, which is not so good, a bit like returning error codes from a function but confusing because the user has to check the return for null, instead of catching the appropriate exceptions, e.g.
public Integer getId(...){
try{ ... ; return id; }
catch(Exception e){ return null;}
}
For certain scenarios, you want to notice a failure as soon as it happens.
Checking against NULL and not asserting (for programmer errors) or throwing (for user or caller errors) in the failure case can mean that later crashes are harder to track down, because the original odd case wasn't found.
Moreover, ignoring errors can lead to security exploits. Perhaps the null-ness came from the fact that a buffer was overwritten or the like. Now, you are not crashing, which means the exploiter has a chance to execute in your code.
What alternatives do you see to returning null?
I see two cases:
findAnItem( id ). What should this do if the item is not found
In this case we could: Return Null or throw a (checked) exception (or maybe create an item and return it)
listItemsMatching (criteria) what should this return if nothing is found?
In this case we could return Null, return an empty list or throw an Exception.
I believe that return null may be less good than the alternatives becasue it requires the client to remember to check for null, programmers forget and code
x = find();
x.getField(); // bang null pointer exception
In Java, throwing a checked exception, RecordNotFoundException, allows the compiler to remind the client to deal with case.
I find that searches returning empty lists can be quite convenient - just populate the display with all the contents of the list, oh it's empty, the code "just works".
Make them call another method after the fact to figure out if the previous call was null. ;-) Hey, it was good enough for JDBC
Well, it sure depends of the purpose of the method ... Sometimes, a better choice would be to throw an exception. It all depends from case to case.
Sometimes, returning NULL is the right thing to do, but specifically when you're dealing with sequences of different sorts (arrays, lists, strings, what-have-you) it is probably better to return a zero-length sequence, as it leads to shorter and hopefully more understandable code, while not taking much more writing on API implementer's part.
The base idea behind this thread is to program defensively. That is, code against the unexpected.
There is an array of different replies:
Adamski suggests looking at Null Object Pattern, with that reply being up voted for that suggestion.
Michael Valenty also suggests a naming convention to tell the developer what may be expected.
ZeroConcept suggests a proper use of Exception, if that is the reason for the NULL.
And others.
If we make the "rule" that we always want to do defensive programming then we can see that these suggestions are valid.
But we have 2 development scenarios.
Classes "authored" by a developer: The Author
Classes "consumed" by another(maybe) developer: the Developer
Regardless of whether a class returns NULL for methods with a return value or not,
the Developer will need to test if the object is valid.
If the developer cannot do this, then that Class/method is not deterministic.
That is, if the "method call" to get the object does not do what it "advertises" (eg getEmployee) it has broken the contract.
As an author of a class, I always want to be as kind and defensive ( and deterministic) when creating a method.
So given that either NULL or the NULL OBJECT (eg if(employee as NullEmployee.ISVALID)) needs to be checked
and that may need to happen with a collection of Employees, then the null object approach is the better approach.
But I also like Michael Valenty's suggestion of naming the method that MUST return null eg getEmployeeOrNull.
An Author who throws an exception is removing the choice for the developer to test the object's validity,
which is very bad on a collection of objects, and forces the developer into exception handling
when branching their consuming code.
As a developer consuming the class, I hope the author gives me the ability to avoid or program for the null situation
that their class/methods may be faced with.
So as a developer I would program defensively against NULL from a method.
If the author has given me a contract that always returns a object (NULL OBJECT always does)
and that object has a method/property by which to test the validity of the object,
then I would use that method/property to continue using the object, else the object is not valid
and I cannot use it.
Bottom line is that the Author of the Class/Methods must provide mechanisms
that a Developer can use in their defensive programming. That is, a clearer intention of the method.
The Developer should always use defensive programming to test the validity of the objects returned
from another class/method.
regards
GregJF
Other options to this, are:
returning some value that indicates success or not (or type of an error), but if you just need boolean value that will indicate success / fail, returning null for failure, and an object for success wouldn't be less correct, then returning true/false and getting the object through parameter.
Other approach would to to use exception to indicates failures, but here - there are actually many more voices, that say this is a BAD practice (as using exceptions may be convenient but has many disadvantages).
So I personally don't see anything bad in returning null as indication that something went wrong, and checking it later (to actually know if you have succeeded or not). Also, blindly thinking that your method will not return NULL, and then base your code on it, may lead to other, sometimes hard to find, errors (although in most cases it will just crash your system :), as you will reference to 0x00000000 sooner or later).
Unintended null functions can arise during the development of a complex programs, and like dead code, such occurrences indicate serious flaws in program structures.
A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.
Null_function #wikipedia
If the code is something like:
command = get_something_to_do()
if command: # if not Null
command.execute()
If you have a dummy object whose execute() method does nothing, and you return that instead of Null in the appropriate cases, you don't have to check for the Null case and can instead just do:
get_something_to_do().execute()
So, here the issue is not between checking for NULL vs. an exception, but is instead between the caller having to handle special non-cases differently (in whatever way) or not.
For my use case I needed to return a Map from method and then looking for a specific key. But if I return an empty Map, then it will lead to NullPointerException and then it wont be much different returning null instead of an empty Map.
But from Java8 onward we could use Optional. The above is the very reason Optional concept was introduced.
G'day,
Returning NULL when you are unable to create a new object is standard practise for many APIs.
Why the hell it's bad design I have no idea.
Edit: This is true of languages where you don't have exceptions such as C where it has been the convention for many years.
HTH
'Avahappy,