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when i search this on google then it show that error means compile time error and exception is runtime error? but i think that it is not that so....
That is generally correct. Although the terms are colloquially interchangeable in many domains.
An Error, also known as a compile-time error, is a statement of fact. (Or NOT fact) The compiler is unable to compile the output.
Technically an error is a state in code that has raised an exception in the compiler's runtime :)
An Exception is raised at runtime and is the result of an exceptional combination of values.
Because an Exception is raised at runtime, we can generally write code to catch and handle or workaround an exception within your script or code. An Error prevents the code from being compiled and thus executed at all, so our only option is to modify the code to resolve an Error.
A compiler may perform syntax and sometimes type checking to ensure that the code follows a set of pre-determined rules and can be compiled into executable statements, but it is not until invalid values are passed into those statements that an Exception can occur, that is harder for a compiler to do and so is generally only detected at Runtime.
Some advanced or specialised compilers may perform checks against common values and as a programer you can write unit tests to try and pre-emptively detect exceptions before releasing your code.
Exceptions and Errors are the same things. Somewhere in software history, someone came up with the phrase “Exceptions are exceptional”. That sounds catchy but it doesn’t translate to Exceptions and Errors. If you go that route then who decides what is Exceptional? It is very subjective an error for one context could be exceptional to one and not to another. Exceptional and Exception are very different words. Someone apparently thought the catchy phrase enforces the idea that there is a difference.
They are the same thing. I’m not a Java developer and I realize it has different classes for each but in the .NET world they are the same thing. The preferred error handling framework in .NET is the Exception class. It doesn’t care what you want to call it, an error or an exception. Nor does it care if it is exceptional or not. It is just a vehicle for communicating error information. You can communicate the severity of an error by creating and using classes that inherit from Exception.
The Source of my information is here:
Cwalina, K., Abrams, B. F., Barton, J., Icaza, M. de, & Hejlsberg, A. (2020). Framework design guidelines: Conventions, idioms, and patterns for reusable .net libraries. Addison-Wesley.
I tried to find resources on what is the standardised and accepted idiomatic D-way of handling errors, but I couldn't find any. If one is reading the official documentation on error handling, then one finds the following very important statements there:
Errors are not part of the normal flow of a program. Errors are exceptional, unusual, and unexpected.
Because errors are unusual, execution of error handling code is not performance critical.
The normal flow of program logic is performance critical.
I call them important, because the reasoning on using exceptions for such, well, exceptional cases is what leads the article towards the conclusion, that errors are special cases after all, and exceptions are the way to go, no matter what the costs are. Again from the same article:
Because errors are unusual, execution of error handling code is not performance critical. Exception handling stack unwinding is a relatively slow process.
In some special cases, where the exceptions may not be handled explicitly, but their presence should effect the state of things anyway, one should use the exception safe scope guards.
My main problem is, the above mentioned solutions and their examples in the documentations are indeed exceptional cases, which are pretty useful when we are hitting for example memory-related problems, but we don't want our program to fail, we want to maintain integrity and recover from those scenarios if possible, but what about other cases?
As we all know errors are not only used for exceptional cases and unexpected scenarios, but they are ways of communicating between callers and callees. For example errors can be used in a sanitiser. Let's say we want to implement a schema validation for an associative array. The type system alone is not capable of defining the constraints of the keys and values, so we create a function to check such objects during run time. So what should happen if the schema fails? Since we are interested on how it fails, the error that happened in it (that is, invalid data found) should also contain the information on what went wrong, therefore the caller would know how to act upon on it. Using exceptions is an expensive abstraction according to the author of the first article. Using the C-style function conventions, where the return values are all used for error states is the wrong way according to the same author in the same article.
So, what is the proper and idiomatic way of handling errors that are not exceptions in D?
Well, the TLDR version is that using exceptions is the idiomatic way to handle error conditions in D, but of course, the details get a bit more complicated than that.
Part of the question is what constitutes an error. The term error is used for many things, and as a result, talking about errors can be quite confusing. Some classes of errors are programmatic errors (and thus the result of bugs in the program), others aren't programmatic errors but are so catastrophic that the program can't continue, and others depend on stuff like user input and can often be recovered from.
For programmatic errors and catastrophic errors, D has the Error class, which derives from Throwable. Two of the commonly used subclasses of Error are AssertError and RangeError - AssertError being the result of a failed assertion, and RangeError being what you get when you try to index an array with an index that's out-of-bounds. Both of those are programmatic errors; they're the result of bugs in your program, and recovering from them makes no sense, because by definition, your program is in an invalid state at that point. An example of an error that's not a bug but which is generally catastrophic enough that your program should be terminated is MemoryError, which is thrown when new fails to allocate memory.
When an Error is thrown, there is no guarantee that any clean-up code will be run (e.g. destructors and scope statements may be skipped, because the assumption is that because your code is in an invalid state, the clean-up code could actually make things worse). The program simply unwinds the stack, prints out the Error's message and stack trace, and terminates your program. So, attempting to catch an Error and have the program continue is almost always a terrible idea, because the program is in an unknown and invalid state. If something is considered an Error, then it is the sort of condition where the error condition is considered unrecoverable, and programs should not be attempting to recover from it.
For the most part, you probably won't do anything explicit with Errors. You'll put assertions in your code to catch bugs when not compiling with -release, but you probably won't be throwing any Errors explicitly. They're mostly the result of D's runtime or assertions in code you're running catching bugs in your program.
The other class that derives from Throwable is Exception. It's used for cases where the problem is not a bug in your program but rather a problem due to user input or the environment (e.g. the XML that the user provided is invalid, or a file that your program attempts to open does not exist). Exceptions provide a way for a function to report that its input was invalid or that it's unable to complete its task due to issues outside its control. The program can then choose to catch that Exception and try to recover from it, or it can let it bubble up to the top and kill the program (though typically, it's more user-friendly to catch them and print out something more user-friendly than a message with a stack trace). Unlike Errors, Exceptions do result in all clean-up code being run. So, it's completely safe to catch them and continue executing.
However, what the program can do in response to the exception and whether it can do more than report to the user that an error occurred and terminate depends on what the exception was and what the program is doing (which is part of why some code subclasses Exception - it provides a way to report what went wrong beyond just an error message and allows the program to respond to it programmatically based on the type of thing that went wrong rather than simply responding to the fact that "something" went wrong). By using exceptions to report when something goes wrong, it allows for code to not directly handle errors unless it's the place in the code that you want to be handling errors, resulting in much cleaner code overall but with the downside that you can sometimes get exceptions being thrown that you weren't expecting if you weren't familiar enough with what could be thrown when. But that also means that errors that are reported don't get missed like they can be with error codes. If you forget to handle an exception, you'll know it when it happens, whereas with something like an error code, it's easy to forget to check it or not realize that you need to, and errors can be missed. So, while unexpected exceptions can be annoying, they help ensure that you catch problems in your program when they occur.
Now, the best time to use assertions vs exceptions can be a bit of an art. For instance, with Design by Contract, you use assertions to check the input to a function, because any code that calls that function with invalid arguments is in violation of the contract and therefore considered buggy, whereas in defensive programming, you don't assume that the input is valid, so the function always checks its input (not just when not compiling with -release), and it throws an Exception on failure. Which approach makes more sense depends on what you're doing and where the input for the function is likely to come from. But it's never appropriate to use assertions to check user input or anything that is outside of the program's control, because bad input is not a bug in the program.
However, while in general, the idiomatic way to handle error cases in D may be to throw an exception, there are times where that really doesn't make sense. For instance, if the error condition is actually extremely likely to occur, throwing an exception is an awfully expensive way to handle it. Exceptions are generally fast enough for cases that aren't happening all the time, but for something that happens frequently - especially in performance-critical code - they can be too expensive. And in such cases, doing something like an error code can make more sense - or doing something like returning a Nullable and having it be null when the function failed to get a result.
In general, exceptions make the most sense when it's reasonable to assume that the function will succeed and/or when it streamlines the code to make it so that it doesn't have to worry about the error condition.
For instance, imagine writing an XML parser that used error codes instead of exceptions. Each function in its implementation would have to be checking whether any function it called succeeded and return whether it itself succeeded, which would not only be error-prone, but it would mean that you'd essentially have error-handling code everywhere throughout the parser. On the other hand, if you use exceptions, then most of the parser doesn't have to care about errors in the XML. Instead of code that encounters invalid XML having to return an error code that the function calling it has to deal with, it can just throw an exception, and whatever code in the call chain is actually a good place to handle the error (probably the code that called the parser in the first place) is then the only code that has to deal with the error. The only error handling code in your program is then code that needs to deal with errors rather than most of your program. The code is much cleaner that way.
Another example where exceptions really clean up code would be a function like std.file.isDir. It returns whether the file name it's given corresponds to a directory and throws a FileException when something goes wrong (e.g. the file doesn't exist, or the user doesn't have permission to access it). For that to work with an error code, you'd be stuck doing something like
int isDir(string filename, ref bool result);
which means that you can't simply put it in a condition like
if(file.isDir)
{
...
}
You'd be stuck with something ugly like
bool result;
immutable error = file.isDir(result);
if(error != 0)
{
...
}
else if(result)
{
...
}
It's true that in many cases, there's a high risk of the file not existing, which would be an argument for using error codes, but std.file.exists makes it possible to easily check for that condition before calling isDir and thus ensure that isDir failing is the uncommon case - or if the code in question is written in a way that it's highly likely that the file exists (e.g. it was gotten from dirEntries), then you don't have to bother checking whether the file exists. Either way, the result is much cleaner and less error-prone than dealing with error codes.
In any case, the most appropriate solution depends on what your code is doing, and there are cases where exceptions really don't make sense, but in general, they are the idiomatic way to deal with errors that are not bugs in your program or catastrophic errors like running out of memory, and Error's are normally the best way to deal with encountering bugs in your program or catastrophic errors. Ultimately though, it is a bit of art to know when and how to use exceptions vs other techniques, and it generally takes experience to have a good feel for it, which is part of why questions about when to use exceptions, assertions, and error codes pop up from time to time.
In programming, are exceptions always errors (divide by zero, access violation, ...)?
If not, can you provide examples of exceptions which are not errors?
Thanks.
Exceptions are often used to manage errors, they make error handling easier but they aren't always errors.
Every unordinary situation that require a separate code path could be a candidate for an exception.
Although the use of exceptions for control flow can be confusing (it depends largely on the language), they can be used to break out of a loop.
Sometimes you can use an exception to check if a string contains a value or if a file exists.
You can use exceptions to terminate threads collaboratively.
You should also consider that different languages have different conventions about when exceptions should be thrown (e.g. Python > C++ > Objective C).
Objective C is an extreme:
When you’re writing code with Objective-C, exceptions are used solely for programmer errors
(iOS Developer Library - Dealing with Errors)
but this isn't the norm.
An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.
check here
I have come across exceptions many times especially checked exceptions.When the syntax of the code is correct why should we have to put in try-catch block.
if we don't put into try-catch block it will give error.
please explain me about checked exceptions.why would some code will throw exceptions even the syntax is correct.
Expections have nothing to do with illegal syntax. Excpetions are used in cases, where an error happens that can't be known about when the code is written or compiled, one example would be that there is no more memory available.
For languages which are compiled (e.g. Java) Expections are thrown while the program runs. On the contrary, syntax errors are handled by the compiler, at compile-time.
Syntax isn't the only resource for Exceptions. Invalid indexing, Mathematical errors, Connection issues, Db structure incompatibilities do cause Exception too!
Unless you are not handling a spesific type of Exception, the purpose of catching general Exceptions is to manage them. If you are not intented to manage them, you may not catch them at all. But whatever the situation is DO NOT SUPPRESS Exceptions. If you have to handle some erronous situation use logical locks.
Exceptions are for exceptional situations only, and should not be used for data validty. Exceptions are way too much costly for that.
Exceptions has nothing to do with syntax errors. An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons.
For example, you can see the exceptions in following reasons:
Invalid entry data.
File or DB that needs to be opened cannot be found.
A network connection has been lost in the middle of communications or the VM has run out of memory.
These exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.
To make the program work smoother, the program should handle the exceptions properly with logic.
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It's my understanding that common wisdom says to only use exceptions for truly exceptional conditions (In fact, I've seen that statement here at SO several times).
However, Krzysztof Cwalina says:
One of the biggest misconceptions about exceptions is that they are for “exceptional conditions.” The reality is that they are for communicating error conditions. From a framework design perspective, there is no such thing as an “exceptional condition”. Whether a condition is exceptional or not depends on the context of usage, --- but reusable libraries rarely know how they will be used. For example, OutOfMemoryException might be exceptional for a simple data entry application; it’s not so exceptional for applications doing their own memory management (e.g. SQL server). In other words, one man’s exceptional condition is another man’s chronic condition.
He then also goes on to say that exceptions should be used for:
Usage errors
Program errors
System failures
Considering Krzysztof Cwalina is the PM for the CLR team at MS I ask: What do you think of his statement?
This sounds over-simplistic, but I think it makes sense to simply use exceptions where they are appropriate. In languages like Java and Python, exceptions are very common, especially in certain situations. Exceptions are appropriate for the type of error you want to bubble up through a code path and force the developer to explicitly catch. In my own coding, I consider the right time to add an exception when the error either can't be ignored, or it's simply more elegant to throw an exception instead of returning an error value to a function call etc.
Some of the most appropriate places for exceptions that I can think of offhand:
NotImplementedException - very appropriate way of designating that a particular
method or function isn't available, rather than simply returning without doing
anything.
OutOfMemory exceptions - it's difficult to imagine a better way of handling this
type of error, since it represents a process-wide or OS-wide memory allocation
failure. This is essential to deal with, of course!
NullPointerException - Accessing a null variable is a programmer mistake, and IMO
this is another good place to force an error to bubble to the surface
ArrayIndexException - In an unforgiving language like C, buffer overflows
are disastrous. Nicer languages might return a null value of some type, or in
some implementations, even wrap around the array. In my opinion, throwing an
exception is a much more elegant response.
This is by no means a comprehensive list, but hopefully it illustrates the point. Use exceptions where they are elegant and logical. As always with programming, the right tool for the right job is good advice. There's no point going exception-crazy for nothing, but it's equally unwise to completely ignore a powerful and elegant tool at your disposal.
For people who write frameworks, perhaps it's interesting.
For the rest of us, it's confusing (and possibly useless.) For ordinary applications, exceptions have to be set aside as "exceptional" situations. Exceptions interrupt the ordinary sequential presentation of your program.
You should be circumspect about breaking the ordinary top-to-bottom sequential processing of your program. The exception handling is -- intentionally -- hard to read. Therefore, reserve exceptions for things that are outside the standard scenarios.
Example: Don't use exceptions to validate user input. People make input mistakes all the time. That's not exceptional, that's why we write software. That's what if-statements are for.
When your application gets an OutOfMemory exception, there's no point in catching it. That's exceptional. The "sequential execution" assumption is out the window. Your application is doomed, just crash and hope that your RDBMS transaction finishes before you crash.
It is indeed difficult to know what exactly construes an "exceptional condition" which warrants the use of an exception in a program.
One instance that is very helpful for using communicating the cause of errors. As the quote from Krzysztof Cwalina mentions:
One of the biggest misconceptions
about exceptions is that they are for
“exceptional conditions.” The reality
is that they are for communicating
error conditions.
To give a concrete example, say we have a getHeader(File f) method that is reading some header from a file and returns a FileHeader object.
There can be several problems which can arise from trying to read data from a disk. Perhaps the file specified doesn't exist, file contains data that can't be read, unexpected disk access errors, running out of memory, etc. Having multiple means of failure means that there should be multiple ways to report what went wrong.
If exceptions weren't used, but there was a need to communicate the kind of error that occurred, with the current method signature, the best we can do is to return a null. Since getting a null isn't very informative, the best communication we get from that result is that "some kind of error happened, so we couldn't continue, sorry." -- It doesn't communicate the cause of the error.
(Or alternatively, we may have class constants for FileHeader objects which indicate FileNotFound conditions and such, emulating error codes, but that really reeks of having a boolean type with TRUE, FALSE, FILE_NOT_FOUND.)
If we had gotten a FileNotFound or DeviceNotReady exception (hypothetical), at least we know what the source of the error was, and if this was an end user application, we could handle the error in ways to solve the problem.
Using the exception mechanism gives a means of communication that doesn't require a fallback to using error codes for notification of conditions that aren't within the normal flow of execution.
However, that doesn't mean that everything should be handled by exceptions. As pointed out by S.Lott:
Don't use exceptions to validate user
input, for example. People make
mistakes all the time. That's what
if-statements are for.
That's one thing that can't be stressed enough. One of the dangers of not knowing when exactly to use exceptions is the tendency to go exception-happy; using exceptions where input validation would suffice.
There's really no point in defining and throwing a InvalidUserInput exception when all that is required to deal in such a situation is to notify the user of what is expected as input.
Also, it should be noted that user input is expected to have faulty input at some point. It's a defensive measure to validate input before handing off input from the outside world to the internals of the program.
It's a little bit difficult to decide what is exceptional and what is not.
Since I usually program in Python, and in that language exceptions are everywhere, to me an exception may represent anything from a system error to a completely legitimate condition.
For example, the "pythonic" way to check if a string contains an integer is to try int(theString) and see if it raises an exception. Is that an "exceptional error"?
Again, in Python the for loop is always thought of as acting on an iterator, and an iterator must raise a 'StopIteration' exception when it finishes its job (the for loop catches that exception). Is that "exceptional" by any means?
I think the closer to the ground are you are the less appropriate exceptions as a means of error communication become. At a higher abstraction such as in Java or .net, an exception may make for an elegant way to pass error messages to your callers. This however is not the case in C. This is also a framework vs api design decision.
If you practice "tell, don't ask" then an exception is just the way a program says "I can't do that". It is "exceptional" in that you say "do X" and it cannot do X. A simple error-handling situation. In some languages it is quite common to work this way, in Java and C++ people have other opinions because exceptions become quite costly.
General: exception just means "I can't"
Pragmatic: ... if you can afford to work that way in your language.
Citizenship: ... and your team allows it.
Here is the definition for exception: An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
Therefore, to answer your question, no. Exceptions are for disruptive events, which may or may not be exceptional. I love this definition, it's simple and works every time - if you buy into exceptions like I do. E.g., a user submits an incorrect un/pw, or you have an illegal argument/bad user input. Throwing an exception here is the most straightforward way of solving these problems, which are disruptive, but not exceptional, nor even unanticipated.
They probably should have been called disruptions, but that boat has sailed.
I think there are a couple of good reasons why exceptions should be used to catch unexpected problems.
Firstly, they create an object to encapsulate the exception, which by definition must make it a lot more expensive than processing a simple if-statement. As a Java example, you should call File.exists() rather than routinely expecting and handling a FileNotFoundException.
Secondly, exceptions that are caught outside the current method (or maybe even class) make the code much harder to read than if the handling is all there in in the one method.
Having said that, I personally love exceptions. They relieve you of the need of explicitly handling all of those may-happen-but-probably-never-will type errors, which cause you to repetitively write print-an-error-and-abort-on-non-zero-return-code handling of every method call.
My bottom line is... if you can reasonably expect it to happen then it's part of your application and you should code for it. Anything else is an exception.
I've been wondering about this myself. What do we mean by "exceptional"? Maybe there's no strict definition, but are there any rules of thumb that we can use to decide what's exceptional, in a given context?
For example, would it be fair to say that an "exceptional" condition is one that violates the contract of a function?
KCwalina has a point.
It will be good to identify cases where the code will fail (upto a limit)
I agree with S.Lott that sometimes validating is better than to throw Exception.
Having said that, OutOfMemory is not what you might expect in your application (unless it is allocating a large memory & needs memory to go ahead).
I think, it depends on the domain of the application.
The statement from Krzysztof Cwalina is a little misleading. The original statement refers 'exceptional conditions', for me it is natural that I am the one who defines what's exceptional or not. Nevertheless, I think the message passed through OK, since I think we are all talking about 'developer' exceptions.
Exceptions are great for communication, but with a little hierarchy design they are also great for some separation of concerns, specially between layers (DAO, Business, etc). Of course, this is only useful if you treat these exceptions differently.
A nice example of hierarchy is spring's data access exception hierarchy.
I think he is right. Take a look at number parsing in java. You cant even check input string before parsing. You are forced to parse and retrieve NFE if something went wrong. Is parse failure something exceptional? I think no.
I certainly believe exceptions should be used only if you have an exceptional condition.
The trouble is in the definition of "exceptional". Here is mine:
A condition is exceptional if it is outside the assumed normal
behaviour of the part of the system that raises the exception.
This has some implications:
Exceptional depends on your assumptions. If a function assumes that it is passed valid parameters, then throwing an IllegalArgumentException is OK. However if a function's contract says that it will correct input errors in input in some way, then this usage is "normal" and it shouldn't throw an exception on an input error.
Exceptional depends on sub-system layering. A network IO function could certainly raise an exception if the network is discommented, as it assumes a valid connection. A ESB-based message broker however would be expected to handle dropped connections, so if it used such a network IO function internally then it would need to catch and handle the error appropriately. In case it isn't obvious, try/catch is effectively equivalent to a subsystem saying "a condition which is exceptional for one of my components is actually considered normal by me, so I need to handle it".
The saying that exceptions should be used for exceptional circumstances is used in "Effective Java Second Edition": one of the best java books.
The trouble is that this is taken out of context. When the author states that exceptions should be exceptional, he had just shown an example of using exceptions to terminate a while loop - a bad exception use. To quote:
exceptions are, as their name implies, to
be used only for exceptional conditions; they should never be used for ordinary
control flow.
So it all depends on your definition of "exception condition". Taken out of context you can imply that it should very rarely be used.
Using exceptions in place of returning error codes is good, while using them in order to implement a "clever" or "faster" technique is not good. That's usually what is meant by "exceptional condition".
Checked exception - minor errors that aren't bugs and shouldn't halt execution. ex. IO or file parsing
Unchecked exception - programming "bug" that disobeys a method contract - ex. OutOfBoundsException. OR a error that makes continuing of execution a very bad idea - ex IO or file parsing of a very important file. Perhaps a config file.
What it comes down to is what tool is needed to do the job.
Exceptions are a very powerful tool. Before using them ask if you need this power and the complexity that comes with it.
Exceptions may appear simple, because you know that when the line with the exception is hit everything comes to a halt. What happens from here though?
Will an uncaught exception occur?
Will the exception be caught by global error handling?
Will the exception be handled by more nested and detailed error handling?
You have to know everything up the stack to know what that exception will do. This violates the concept of independence. That method now is dependent on error handling to do what you expect it to.
If I have a method I shouldn't care what is outside of that method. I should only care what the input is, how to process it, and how to return the response.
When you use an exception you are essentially saying, I don't care what happens from here, something went wrong and I don't want it getting any worse, do whatever needs to be done to mitigate the issue.
Now if you care about how to handle the error, you will do some more thinking and build that into the interface of the method e.g. if you are attempting to find some object possibly return the default of that object if one can't be found rather than throwing some exception like "Object not found".
When you build error handling into your methods interface, not only is that method's signature more descriptive of what it can do, but it places the responsibility of how to handle the error on the caller of the method. The caller method may be able to work through it or not, and it would report again up the chain if not. Eventually you will reach the application's entry point. Now it would be appropriate to throw an exception, since you better have a good understanding of how exceptions will be handled if you're working with the applications public interface.
Let me give you an example of my error handling for a web service.
Level 1. Global error handling in global.asax - That's the safety net to prevent uncaught exceptions. This should never intentionally be reached.
Level 2. Web service method - Wrapped in a try/catch to guarantee it will always comply with its json interface.
Level 3. Worker methods - These get data, process it, and return it raw to the web service method.
In the worker methods it's not right to throw an exception. Yes I have nested web service method error handling, but that method can be used in other places where this may not exist.
Instead if a worker method is used to get a record and the record can't be found, it just returns null. The web service method checks the response and when it finds null it knows it can't continue. The web service method knows it has error handling to return json so throwing an exception will just return the details in json of what happened. From a client's perspective it's great that it got packaged into json that can be easily parsed.
You see each piece just knows what it needs to do and does it. When you throw an exception in the mix you hijack the applications flow. Not only does this lead to hard to follow code, but the response to abusing exceptions is the try/catch. Now you are more likely to abuse another very powerful tool.
All too often I see a try/catch catching everything in the middle of an a application, because the developer got scared a method they use is more complex than it appears.