5 ms·

My philosophy of exceptions: they're always ambiguous (2021)

24 points by zbentley 2y ago

wvenable 2y ago
> So you think you know what an “exception” means? An exception is when code can no longer do any meaningful work. This could be because of a programming bug, unexpected input, network issue, hardware issue, etc. The cause doesn't really matter. What matters is that there is no way forward for the code. And this is a decision that is made (or not made) by the programmer. In this author's Python example, he chooses accept the input as correct in the rest of the code. If a valid URL is not provided then the code cannot continue because it is assumed correct. One could easily change this code to have it continue; have it check for a invalid URL and reprint the prompt. However, this would be a different program with different requirements.
- hn_throwaway_99 2y ago
  FWIW I didn't particularly like the article, but I really liked your comment. I think "an exception is when code can no longer do any meaningful work" is a great definition, and also explains why flow control with exception handling works the way it does. Obviously there are gray areas re what it is to mean "code can no longer do any meaningful work", which is why there are decisions to be made about when to throw an exception or when to do something else. Usually, though, that "something else" is to return an error sentinel value, most often null or similar (e.g. "undefined"). And then I think the reason it's sometimes a tough call is that since null/undefined essentially means "empty", the APIs that most often return null do so in cases where you're asking for something and it's not there, so returning "empty" feels reasonable.
  - wvenable 2y ago
    In .NET, they have 2 sets of parsing functions, "Parse" and "TryParse" for parsing strings into other types. The first throws an exception if it can't parse and the second returns a Boolean indicating success or failure. Which function of these you use as a programmer is dependent on the context and it indicates your expectation. You might use "Parse" if you reading a well-defined file format where the string will always contain an integer and if it doesn't then something has gone horribly wrong. But you might use "TryParse" on user input where getting a non-integer is expected and you'll have to something about that other than error out.
    - erik_seaberg 2y ago
      Was this a workaround for frequent exception handling being too expensive on some platform? I like how Java lets some exception instances be reused without stack frames, so performance issues are less likely to compromise API design.
      - wvenable 2y ago
        Semantically I think it's great. .NET has a bunch of these especially in LINQ. A parsing failure isn't necessary an error. Sometimes it is, sometimes it's not. Even if exceptions weren't expensive, catching exceptions on all parsing (as you would in Java) is just ugly and, I would argue, not semantically correct. In my opinion, one should throw often but catch infrequently. Most of my large applications have a very small number of catches. It's also perfectly reasonable to move this down a layer and have Parse return an Option type and choose to handle did-not-parse case or don't and let that trigger an exception.
      - jkrejcha 2y ago
        It, according to Eric Lippert, was at least partially to fix the problem of vexing exceptions[1] that plagued Int32.Parse. Parse was first and it was kind of an unfortunate design decision to make a almost expected result (parsing to fail) throw an exception. It's worth noting that nowadays Parse is just a wrapper around TryParse that throws (although this is an implementation detail) [1]: https://ericlippert.com/2008/09/10/vexing-exceptions/ https://ericlippert.com/2008/09/10/vexing-exceptions/
      - winstonewert 2y ago
        I think the causation runs in the opposite direction. These exceptions for these platforms were implemented in an inefficient fashion BECAUSE they thought that exceptions should almost never be thrown.
  - lelanthran 2y ago
    > I think "an exception is when code can no longer do any meaningful work" is a great definition, I'm not so sure: that definition glibly turns every error or failure into an exception. Is "Failure to Open File" really an exceptional condition? This is a common condition that is basically a business rule[1]. Think about the "saving a file" action. There's errors that you expect to happen, and have to provide code to handle (like, permission denied, path exists and is a directory, etc), and exceptional conditions that should never have arisen in a program which no sane person would try to handle (Out of memory, disk write errors, etc). In both cases, no progress can be made and the callee has to return to the caller. But, in the error cases a well-written program will defend against the errors in a meaningful manner, whereas in the exceptional cases there is literally no point in putting in code to handle those cases. Just some food for thought - I'm not particularly married to these definitions. [1] Here are some hypothetical options for failing to open a file: 1. If the file doesn't exist, prompt the user asking if it must be created, and proceed only if "Yes" is received, return to caller if "No" is received. 2. If the file is a directory, display a fatal error message and return to caller. 3. If permission is denied, display an ACL error message and return to caller.
    - wvenable 2y ago
      I have a desktop application with a single error handler at its event loop. All that handler does is show the exception message in a messagebox and then the event loop resumes. This application has a "save file" option. In this application it doesn't matter if the permission is denied, the path exists and is a directory, it was a network share that went away, or a disk write error, or out of memory, or a bug. If I press "Save" and the exception occurs, I can read the message and correct the problem if possible, and then just press Save again. It is incredibly robust and has effectively no error handling. Of course, that doesn't handle all the cases that you describe. For example, for your case #1 I think that isn't a case that no more meaningful work can be done and isn't an error. Instead, you're just asking a question and then proceeding or not based on the answer.
- kelnos 2y ago
  > An exception is when code can no longer do any meaningful work. That is quite obviously not true, not even on the surface. I have written tons of code where I've caught exceptions and then continued to do meaningful work. The author's Python example even contradicts your statement. Sure, maybe they should validate the URL first. But even if they did that, the server could be down, the user's internet could be down, or there could be a typo in the URL that makes it unreachable, even if the URL itself is still valid. And that doesn't have to mean the code can no longer do meaningful work. Maybe the exception handler could consult a local disk cache, and read the contents of the cache and display that instead. I think exceptions could (and perhaps should) only be used when code can no longer do any meaningful work, but in practice that is very much not how real-world software works. Ultimately, though, I prefer languages that don't have exceptions. Errors should be passed as return values, preferably using language features that make it impossible to forget to handle them. If they truly get into a state where they can't (or it's unsafe to) do more meaningful work, then they should abort() and the program should be terminated.
  - wvenable 2y ago
    > But even if they did that, the server could be down, the user's internet could be down, or there could be a typo in the URL that makes it unreachable, even if the URL itself is still valid. That doesn't contradict my statement. In all those cases, the code cannot continue to do anymore meaningful work. Handling the parsing error just handles that one situation. > I have written tons of code where I've caught exceptions and then continued to do meaningful work. But the code that threw the exception could not continue to do meaningful work. You can the catch the exception and do something else or maybe try the same operation again. But either way, the original code could not and did not continue. In general, you have to reach higher up in the call stack before you find a spot where you can something different or retry. This is why exceptions are useful.
    - kelnos 2y ago
      > That doesn't contradict my statement. In all those cases, the code cannot continue to do meaningful work. Just fixing the parsing error just takes one such condition out. I never claimed otherwise. If you continued reading to the next paragraph, I presented an option for how the code could continue doing more work. > But the code that threw the exception could not continue to do meaningful work. You can the catch the exception and do something else or maybe try the same operation again. But either way, the original code could not continue. But that's not what you said! Or at least what you said was ambiguous. "The code can no longer do any meaningful work." What code? I interpreted that as "all the following code", which feels like an entirely reasonable interpretation to me. I feel like this just proves the author's point -- it's ambiguous! Ok, so then what you really mean is: "the code that threw the exception can no longer do any meaningful work". Ok, sure, that's true, but I don't think that's a useful definition. I suppose perhaps it's useful if you're explaining it to a non-programmer who is unlikely to need any more information (arguably, you shouldn't bring up exceptions to a non-programmer; just tell them "the code encountered an error and couldn't continue anymore"). But if you're talking to a programmer, or, rather, teaching a new programmer, that definition is entirely insufficient. Because from there, you need to talk about why the way to signal that inability is to use an exception. You need to talk about other ways to signal that, and what the trade offs are. You need to dig deeper. At best your definition is only the very very very beginning of a longer conversation; it doesn't give enough information on its own.
winstonewert 2y ago
My wish: people stop using the dumb line that exceptions are gotos.
- wvenable 2y ago
  The pattern for returning errors is something like the following pseudocode: main() { (result or error) = functionA(); if (error) die("There was an error: " + error); print result } functionA() { (result1 or error1) = functionB() if (error1) return error1; (result2 or error2) = functionC(result1); if (error2) return error2; (result3 or error3) = functionD(result2); return result3; } functionB() { (result or error) = readFileAndGetResult() if (error) return error; return result; } Exceptions simply automate the error returns; the same code with exceptions is merely: main() { try { result = functionA(); print result; } catch (error) { die("There was an error: " + error); } } functionA() { result1 = functionB() result2 = functionC(result1); result3 = functionD(result2); return result3; } functionB() { return readFileAndGetResult(); } The most naive implementation of exceptions would do exactly what the top code does. Check for an error result from the function and return it to it's caller until it finds a catch block. The advantage of exceptions is that you don't get the bug that exists in functionA where it fails to propagate the error from functionD. It also makes the logic of the code much more readable and writable because you don't need to put an error guard around the call to every single function. But what is telling is that, in both examples, the error is handled in exactly the same place in the code. How can one reasonably say the second case is like a goto but the first isn't?
  - deleted 2y ago
    [deleted]
Mathnerd314 2y ago
I will paraphrase the Wikipedia definition (itself from Flaviu Cristian and John B. Goodenough): Each procedure has a precondition, a set of circumstances for which it will terminate "normally". The set of "normal" circumstances is subjective and defined entirely by the programmer. An exception handling mechanism allows the procedure to "raise an exception" if this precondition is violated. The exception handling mechanism then specifies how the exception is handled.
_ZeD_ 2y ago
>>> When I shifted from Rust to Python, I tried to write matcher-style code. It wasn’t legible. I became irate. so, in other words... rust developer tried to write rust in python, and it didn't work. wasn't that the same story for each combination of two languages? I remember people criticize other people for "always writing java in any language they work with"...
- wiseowise 2y ago
  > I remember people criticize other people for "always writing java in any language they work with"... Apples to oranges. When people say “writing Java in any language…” they mean it usually in a derogatory term, as in overly verbose variable names, AbstractSingletonProxyFactoryBean, etc. When people talk about Rust they usually mean functional style, exhaustive checks, safe constructs, etc.
lloeki 2y ago
> Python, C++, Ruby and Java all use exceptions. To them, “exception” means, “nifty piece of syntax kinda like goto.” Not Ruby. In idiomatic Ruby^ exceptions are for exceptional cases, not code flow. "nifty piece of syntax kinda like goto." is the much underused but very powerful catch+throw. Or rather, it's a stack unwinding mechanism: think baseball, the catcher expects a ball, with the ball being a message to be passed. Upon that you could implement a specialized and more costly case of stack unwinding which does what fail/raise+rescue does, but catch+throw is more general than exceptions. Take Python's StopIteration as an example: there's absolutely no need for this to be a full exception, instead it's like `break`ing a loop but across method calls. ^ Nevermind Rails practices. Rails is not Ruby.
- LtWorf 2y ago
  [1]: isinstance(StopIteration, Exception) [1]: False
  - _ZeD_ 2y ago
    yeah.. StopIteration is not an instance of Exception, is a subclass of it :^) no, really: help(StopIteration) class StopIteration(Exception) | Signal the end from iterator.__next__(). | | Method resolution order: | StopIteration | Exception | BaseException | object ...
kaba0 2y ago
I think this topic has been discussed countless times, and this post fails to add anything worthy to it. My two cents: use result types, or their analog in your language for expected error conditions, and use exceptions for exceptional situations. Example for the former is parsing, which has an expected failure mode - something couldn’t be parsed as intended. Example for the latter is a network issue during an API call/writing a file/etc. Nonetheless, they are a homologous structure to a Result type in more FP languages, so it’s purely which is a better fit for the given circumstance. I would also like to defend exceptions, as they are a very good construction with sane and safe defaults. They let people concentrate on the actual business logic, not sprinkling little bit of business and error handling logic all over the place; they do the sane thing by default: auto-unwrap the correct result, auto-bubble up if not handled at a given scope; and often unappreciated: they allow as fine- or course-grained error handling as needed, while it is only possible with some less than readable Monad magic with Result types. And most importantly, they never let an erroneous condition silently go unnoticed, you have to deliberately ignore them for that. With errno checking (c, go), and a bit less so with result types you can fall victim to that. Edit: Oh, and they add stacktraces! That’s also a very important and useful feature of them, much more so than “grepping for this hopefully unique error code in the codebase to find where it might come from with zero further detail”.
- MaxBarraclough 2y ago
  > They let people concentrate on the actual business logic, not sprinkling little bit of business and error handling logic all over the place This can lead to trouble if the programmer doesn't keep in mind that various (but perhaps not all) operations can throw. In particular, proper behaviour in the exceptional case may require a more specific order of operations than is needed for proper behaviour in the normal case. Raymond Chen did a great blog post on this in 2005, Cleaner, more elegant, and harder to recognize. [0][1] [0] https://devblogs.microsoft.com/oldnewthing/20050114-00/?p=36693 https://devblogs.microsoft.com/oldnewthing/20050114-00/?p=36... [1] https://news.ycombinator.com/item?id=406391 https://news.ycombinator.com/item?id=406391
- taberiand 2y ago
  The main problem I have with exceptions is that libraries can't be trusted to only throw exceptions in exceptional situations, or even properly document the situations in which they throw exceptions, so my code must be constantly paranoid about every call into library code. Contrast this with languages that provide result types and the certainty of either handling or passing up the error, particularly when the language provides a clean construct like ? in Rust - I just really like the feeling of certainty I get when writing that code over code that can throw exceptions.
TheTrueScotsman 2y ago
[flagged]
- wiseowise 2y ago
  Braindead take.
Kwpolska 2y ago
> Here’s the big lie: in Python, this snippet will almost always raise an Exception. That’s because most users, faced with an input prompt, will press Ctrl+C or enter a non-URL. And even if they enter a URL, it probably has a typo. [citation needed] > In English, “exception” means, “something unusual.” But if this code snipet raises ValueError 60 per cent of the time and prints the contents of a web page 10 per cent of the time … well, surely we need a new term? The purpose of this code snippet is to download a web page and show it to the user. The code considers situations that lead to it not fulfilling its purpose “exceptional”. If someone exits when they see a prompt for a URL, or type gibberish, why did they run a program to download a web page in the first place?
xg15 2y ago
You can have the same philosophical distinction over the term "error". Is it really an error if your computer did not literally catch fire or otherwise took physical damage? And yet this hasn't stopped even nontechnical people from using the term without any significant communication problems. I think a more interesting discussion might be what exactly the difference between "errors" and "exceptions" is in computing, though I'm not sure there is one in practice. (I think in theory, "errors" are supposed to be caused by factors external to your program, like hitting memory constraints, while "exceptions" are internal to your program? I'm not sure if this is universally true though or just a java-ism - and even in java it's not consistently followed through everywhere). So until this is decided, I think the most useful practical definition of exceptions is "it's a special language construct that makes it easier to manage errors".
- Mikhail_Edoshin 2y ago
  There was a book “C Interfaces and Implementations” by David R. Hanson and he put it so: there are user errors (input/data errors), program bugs (things we 'assert') and everything else are exceptions. So a non-existent file is a user error, uninitialized memory is a program bug, and an arithmetic overflow is an exception. Not sure if this is useful. I myself think errors and exceptions are misleading terms. There is an instruction; it does this and that and can produce the following results. We call some of there results errors because usually we imply a goal. But instructions do not really have goals. We search for a key in a dictionary and either find it or not. Either result can be an error or not an error depending on whether we expect the key to be there or want to make sure it does not exist. Yet the searching instruction is same. There is no reason to prefer one result or another. It is more important to make all the necessary distinctions.
  - 7bit 2y ago
    > There was a book “C Interfaces and Implementations” by David R. Hanson and he put it so: there are user errors (input/data errors), program bugs (things we 'assert') and everything else are exceptions. So a non-existent file is a user error, uninitialized memory is a program bug, and an arithmetic overflow is an exception. I don't agree with this definition in the same way I don't agree with many others not found in books. These definitions always reflect the personal view of their authors, but are fundamentally just an interpretation of their own thoughts. There is nothing objective about it. It's just a philosophical or semantic debate. For example, why is uninitialized memory a bug, but an arithmetic overflow is not? In both cases it would have been the developers responsibility to initialize the memory and prevent an arithmetic operation that overflows.
mayoff 2y ago
> If you’re using Python, you’re stuck with the endless debates. For instance, you’ll never know why there’s a dict.get() to avoid KeyError but no list.get() to avoid IndexError. Probably for the same reason that, in Swift, dictionary lookup returns an Optional but Array traps on out-of-bounds: dictionary keys tend to come from outside the dictionary, but array indices tend to be derived from the array bounds. This means a dictionary lookup failure tends to be expected, and an out-of-bounds array index tends to be a logic error. More details here: https://stackoverflow.com/a/75780575/77567 https://stackoverflow.com/a/75780575/77567
wruza 2y ago
I’ve been programming for many years and never thought of or met any issues with exceptions. It’s a useful mechanism to stop execution and send a structured signal up the call stack to where you expect it. If you don’t have issues (e.g. data, syntax, performance related) with the way you use exceptions, it’s safe to ignore all philosophical distresses. What does it actually mean? When should I use it? Is this correct? Is my usage aligned with the idea behind it? These are all questions of a restless mind. The one that stands in front of a toolbox and cannot choose a type of a hammer for a trivial job where even a rock will do.
TheTrueScotsman 2y ago
[flagged]

24 points by zbentley 2y ago

> So you think you know what an “exception” means? An exception is when code can no longer do any meaningful work. This could be because of a programming bug, unexpected input, network issue, hardware issue, etc. The cause doesn't really matter. What matters is that there is no way forward for the code. And this is a decision that is made (or not made) by the programmer. In this author's Python example, he chooses accept the input as correct in the rest of the code. If a valid URL is not provided then the code cannot continue because it is assumed correct. One could easily change this code to have it continue; have it check for a invalid URL and reprint the prompt. However, this would be a different program with different requirements.

hn_throwaway_99 2y ago
FWIW I didn't particularly like the article, but I really liked your comment. I think "an exception is when code can no longer do any meaningful work" is a great definition, and also explains why flow control with exception handling works the way it does. Obviously there are gray areas re what it is to mean "code can no longer do any meaningful work", which is why there are decisions to be made about when to throw an exception or when to do something else. Usually, though, that "something else" is to return an error sentinel value, most often null or similar (e.g. "undefined"). And then I think the reason it's sometimes a tough call is that since null/undefined essentially means "empty", the APIs that most often return null do so in cases where you're asking for something and it's not there, so returning "empty" feels reasonable.
- wvenable 2y ago
  In .NET, they have 2 sets of parsing functions, "Parse" and "TryParse" for parsing strings into other types. The first throws an exception if it can't parse and the second returns a Boolean indicating success or failure. Which function of these you use as a programmer is dependent on the context and it indicates your expectation. You might use "Parse" if you reading a well-defined file format where the string will always contain an integer and if it doesn't then something has gone horribly wrong. But you might use "TryParse" on user input where getting a non-integer is expected and you'll have to something about that other than error out.
  - erik_seaberg 2y ago
    Was this a workaround for frequent exception handling being too expensive on some platform? I like how Java lets some exception instances be reused without stack frames, so performance issues are less likely to compromise API design.
    - wvenable 2y ago
      Semantically I think it's great. .NET has a bunch of these especially in LINQ. A parsing failure isn't necessary an error. Sometimes it is, sometimes it's not. Even if exceptions weren't expensive, catching exceptions on all parsing (as you would in Java) is just ugly and, I would argue, not semantically correct. In my opinion, one should throw often but catch infrequently. Most of my large applications have a very small number of catches. It's also perfectly reasonable to move this down a layer and have Parse return an Option type and choose to handle did-not-parse case or don't and let that trigger an exception.
    - jkrejcha 2y ago
      It, according to Eric Lippert, was at least partially to fix the problem of vexing exceptions[1] that plagued Int32.Parse. Parse was first and it was kind of an unfortunate design decision to make a almost expected result (parsing to fail) throw an exception. It's worth noting that nowadays Parse is just a wrapper around TryParse that throws (although this is an implementation detail) [1]: https://ericlippert.com/2008/09/10/vexing-exceptions/ https://ericlippert.com/2008/09/10/vexing-exceptions/
    - winstonewert 2y ago
      I think the causation runs in the opposite direction. These exceptions for these platforms were implemented in an inefficient fashion BECAUSE they thought that exceptions should almost never be thrown.
- lelanthran 2y ago
  > I think "an exception is when code can no longer do any meaningful work" is a great definition, I'm not so sure: that definition glibly turns every error or failure into an exception. Is "Failure to Open File" really an exceptional condition? This is a common condition that is basically a business rule[1]. Think about the "saving a file" action. There's errors that you expect to happen, and have to provide code to handle (like, permission denied, path exists and is a directory, etc), and exceptional conditions that should never have arisen in a program which no sane person would try to handle (Out of memory, disk write errors, etc). In both cases, no progress can be made and the callee has to return to the caller. But, in the error cases a well-written program will defend against the errors in a meaningful manner, whereas in the exceptional cases there is literally no point in putting in code to handle those cases. Just some food for thought - I'm not particularly married to these definitions. [1] Here are some hypothetical options for failing to open a file: 1. If the file doesn't exist, prompt the user asking if it must be created, and proceed only if "Yes" is received, return to caller if "No" is received. 2. If the file is a directory, display a fatal error message and return to caller. 3. If permission is denied, display an ACL error message and return to caller.
  - wvenable 2y ago
    I have a desktop application with a single error handler at its event loop. All that handler does is show the exception message in a messagebox and then the event loop resumes. This application has a "save file" option. In this application it doesn't matter if the permission is denied, the path exists and is a directory, it was a network share that went away, or a disk write error, or out of memory, or a bug. If I press "Save" and the exception occurs, I can read the message and correct the problem if possible, and then just press Save again. It is incredibly robust and has effectively no error handling. Of course, that doesn't handle all the cases that you describe. For example, for your case #1 I think that isn't a case that no more meaningful work can be done and isn't an error. Instead, you're just asking a question and then proceeding or not based on the answer.
kelnos 2y ago
> An exception is when code can no longer do any meaningful work. That is quite obviously not true, not even on the surface. I have written tons of code where I've caught exceptions and then continued to do meaningful work. The author's Python example even contradicts your statement. Sure, maybe they should validate the URL first. But even if they did that, the server could be down, the user's internet could be down, or there could be a typo in the URL that makes it unreachable, even if the URL itself is still valid. And that doesn't have to mean the code can no longer do meaningful work. Maybe the exception handler could consult a local disk cache, and read the contents of the cache and display that instead. I think exceptions could (and perhaps should) only be used when code can no longer do any meaningful work, but in practice that is very much not how real-world software works. Ultimately, though, I prefer languages that don't have exceptions. Errors should be passed as return values, preferably using language features that make it impossible to forget to handle them. If they truly get into a state where they can't (or it's unsafe to) do more meaningful work, then they should abort() and the program should be terminated.
- wvenable 2y ago
  > But even if they did that, the server could be down, the user's internet could be down, or there could be a typo in the URL that makes it unreachable, even if the URL itself is still valid. That doesn't contradict my statement. In all those cases, the code cannot continue to do anymore meaningful work. Handling the parsing error just handles that one situation. > I have written tons of code where I've caught exceptions and then continued to do meaningful work. But the code that threw the exception could not continue to do meaningful work. You can the catch the exception and do something else or maybe try the same operation again. But either way, the original code could not and did not continue. In general, you have to reach higher up in the call stack before you find a spot where you can something different or retry. This is why exceptions are useful.
  - kelnos 2y ago
    > That doesn't contradict my statement. In all those cases, the code cannot continue to do meaningful work. Just fixing the parsing error just takes one such condition out. I never claimed otherwise. If you continued reading to the next paragraph, I presented an option for how the code could continue doing more work. > But the code that threw the exception could not continue to do meaningful work. You can the catch the exception and do something else or maybe try the same operation again. But either way, the original code could not continue. But that's not what you said! Or at least what you said was ambiguous. "The code can no longer do any meaningful work." What code? I interpreted that as "all the following code", which feels like an entirely reasonable interpretation to me. I feel like this just proves the author's point -- it's ambiguous! Ok, so then what you really mean is: "the code that threw the exception can no longer do any meaningful work". Ok, sure, that's true, but I don't think that's a useful definition. I suppose perhaps it's useful if you're explaining it to a non-programmer who is unlikely to need any more information (arguably, you shouldn't bring up exceptions to a non-programmer; just tell them "the code encountered an error and couldn't continue anymore"). But if you're talking to a programmer, or, rather, teaching a new programmer, that definition is entirely insufficient. Because from there, you need to talk about why the way to signal that inability is to use an exception. You need to talk about other ways to signal that, and what the trade offs are. You need to dig deeper. At best your definition is only the very very very beginning of a longer conversation; it doesn't give enough information on its own.

winstonewert 2y ago

My wish: people stop using the dumb line that exceptions are gotos.

wvenable 2y ago
The pattern for returning errors is something like the following pseudocode: main() { (result or error) = functionA(); if (error) die("There was an error: " + error); print result } functionA() { (result1 or error1) = functionB() if (error1) return error1; (result2 or error2) = functionC(result1); if (error2) return error2; (result3 or error3) = functionD(result2); return result3; } functionB() { (result or error) = readFileAndGetResult() if (error) return error; return result; } Exceptions simply automate the error returns; the same code with exceptions is merely: main() { try { result = functionA(); print result; } catch (error) { die("There was an error: " + error); } } functionA() { result1 = functionB() result2 = functionC(result1); result3 = functionD(result2); return result3; } functionB() { return readFileAndGetResult(); } The most naive implementation of exceptions would do exactly what the top code does. Check for an error result from the function and return it to it's caller until it finds a catch block. The advantage of exceptions is that you don't get the bug that exists in functionA where it fails to propagate the error from functionD. It also makes the logic of the code much more readable and writable because you don't need to put an error guard around the call to every single function. But what is telling is that, in both examples, the error is handled in exactly the same place in the code. How can one reasonably say the second case is like a goto but the first isn't?
- deleted 2y ago
  [deleted]

Mathnerd314 2y ago

I will paraphrase the Wikipedia definition (itself from Flaviu Cristian and John B. Goodenough): Each procedure has a precondition, a set of circumstances for which it will terminate "normally". The set of "normal" circumstances is subjective and defined entirely by the programmer. An exception handling mechanism allows the procedure to "raise an exception" if this precondition is violated. The exception handling mechanism then specifies how the exception is handled.

_ZeD_ 2y ago

>>> When I shifted from Rust to Python, I tried to write matcher-style code. It wasn’t legible. I became irate. so, in other words... rust developer tried to write rust in python, and it didn't work. wasn't that the same story for each combination of two languages? I remember people criticize other people for "always writing java in any language they work with"...

wiseowise 2y ago
> I remember people criticize other people for "always writing java in any language they work with"... Apples to oranges. When people say “writing Java in any language…” they mean it usually in a derogatory term, as in overly verbose variable names, AbstractSingletonProxyFactoryBean, etc. When people talk about Rust they usually mean functional style, exhaustive checks, safe constructs, etc.

lloeki 2y ago

> Python, C++, Ruby and Java all use exceptions. To them, “exception” means, “nifty piece of syntax kinda like goto.” Not Ruby. In idiomatic Ruby^ exceptions are for exceptional cases, not code flow. "nifty piece of syntax kinda like goto." is the much underused but very powerful catch+throw. Or rather, it's a stack unwinding mechanism: think baseball, the catcher expects a ball, with the ball being a message to be passed. Upon that you could implement a specialized and more costly case of stack unwinding which does what fail/raise+rescue does, but catch+throw is more general than exceptions. Take Python's StopIteration as an example: there's absolutely no need for this to be a full exception, instead it's like `break`ing a loop but across method calls. ^ Nevermind Rails practices. Rails is not Ruby.

LtWorf 2y ago
[1]: isinstance(StopIteration, Exception) [1]: False
- _ZeD_ 2y ago
  yeah.. StopIteration is not an instance of Exception, is a subclass of it :^) no, really: help(StopIteration) class StopIteration(Exception) | Signal the end from iterator.__next__(). | | Method resolution order: | StopIteration | Exception | BaseException | object ...

kaba0 2y ago

I think this topic has been discussed countless times, and this post fails to add anything worthy to it. My two cents: use result types, or their analog in your language for expected error conditions, and use exceptions for exceptional situations. Example for the former is parsing, which has an expected failure mode - something couldn’t be parsed as intended. Example for the latter is a network issue during an API call/writing a file/etc. Nonetheless, they are a homologous structure to a Result type in more FP languages, so it’s purely which is a better fit for the given circumstance. I would also like to defend exceptions, as they are a very good construction with sane and safe defaults. They let people concentrate on the actual business logic, not sprinkling little bit of business and error handling logic all over the place; they do the sane thing by default: auto-unwrap the correct result, auto-bubble up if not handled at a given scope; and often unappreciated: they allow as fine- or course-grained error handling as needed, while it is only possible with some less than readable Monad magic with Result types. And most importantly, they never let an erroneous condition silently go unnoticed, you have to deliberately ignore them for that. With errno checking (c, go), and a bit less so with result types you can fall victim to that. Edit: Oh, and they add stacktraces! That’s also a very important and useful feature of them, much more so than “grepping for this hopefully unique error code in the codebase to find where it might come from with zero further detail”.

MaxBarraclough 2y ago
> They let people concentrate on the actual business logic, not sprinkling little bit of business and error handling logic all over the place This can lead to trouble if the programmer doesn't keep in mind that various (but perhaps not all) operations can throw. In particular, proper behaviour in the exceptional case may require a more specific order of operations than is needed for proper behaviour in the normal case. Raymond Chen did a great blog post on this in 2005, Cleaner, more elegant, and harder to recognize. [0][1] [0] https://devblogs.microsoft.com/oldnewthing/20050114-00/?p=36693 https://devblogs.microsoft.com/oldnewthing/20050114-00/?p=36... [1] https://news.ycombinator.com/item?id=406391 https://news.ycombinator.com/item?id=406391
taberiand 2y ago
The main problem I have with exceptions is that libraries can't be trusted to only throw exceptions in exceptional situations, or even properly document the situations in which they throw exceptions, so my code must be constantly paranoid about every call into library code. Contrast this with languages that provide result types and the certainty of either handling or passing up the error, particularly when the language provides a clean construct like ? in Rust - I just really like the feeling of certainty I get when writing that code over code that can throw exceptions.

TheTrueScotsman 2y ago

[flagged]

wiseowise 2y ago
Braindead take.

Kwpolska 2y ago

> Here’s the big lie: in Python, this snippet will almost always raise an Exception. That’s because most users, faced with an input prompt, will press Ctrl+C or enter a non-URL. And even if they enter a URL, it probably has a typo. [citation needed] > In English, “exception” means, “something unusual.” But if this code snipet raises ValueError 60 per cent of the time and prints the contents of a web page 10 per cent of the time … well, surely we need a new term? The purpose of this code snippet is to download a web page and show it to the user. The code considers situations that lead to it not fulfilling its purpose “exceptional”. If someone exits when they see a prompt for a URL, or type gibberish, why did they run a program to download a web page in the first place?

xg15 2y ago

You can have the same philosophical distinction over the term "error". Is it really an error if your computer did not literally catch fire or otherwise took physical damage? And yet this hasn't stopped even nontechnical people from using the term without any significant communication problems. I think a more interesting discussion might be what exactly the difference between "errors" and "exceptions" is in computing, though I'm not sure there is one in practice. (I think in theory, "errors" are supposed to be caused by factors external to your program, like hitting memory constraints, while "exceptions" are internal to your program? I'm not sure if this is universally true though or just a java-ism - and even in java it's not consistently followed through everywhere). So until this is decided, I think the most useful practical definition of exceptions is "it's a special language construct that makes it easier to manage errors".

Mikhail_Edoshin 2y ago
There was a book “C Interfaces and Implementations” by David R. Hanson and he put it so: there are user errors (input/data errors), program bugs (things we 'assert') and everything else are exceptions. So a non-existent file is a user error, uninitialized memory is a program bug, and an arithmetic overflow is an exception. Not sure if this is useful. I myself think errors and exceptions are misleading terms. There is an instruction; it does this and that and can produce the following results. We call some of there results errors because usually we imply a goal. But instructions do not really have goals. We search for a key in a dictionary and either find it or not. Either result can be an error or not an error depending on whether we expect the key to be there or want to make sure it does not exist. Yet the searching instruction is same. There is no reason to prefer one result or another. It is more important to make all the necessary distinctions.
- 7bit 2y ago
  > There was a book “C Interfaces and Implementations” by David R. Hanson and he put it so: there are user errors (input/data errors), program bugs (things we 'assert') and everything else are exceptions. So a non-existent file is a user error, uninitialized memory is a program bug, and an arithmetic overflow is an exception. I don't agree with this definition in the same way I don't agree with many others not found in books. These definitions always reflect the personal view of their authors, but are fundamentally just an interpretation of their own thoughts. There is nothing objective about it. It's just a philosophical or semantic debate. For example, why is uninitialized memory a bug, but an arithmetic overflow is not? In both cases it would have been the developers responsibility to initialize the memory and prevent an arithmetic operation that overflows.

mayoff 2y ago

> If you’re using Python, you’re stuck with the endless debates. For instance, you’ll never know why there’s a dict.get() to avoid KeyError but no list.get() to avoid IndexError. Probably for the same reason that, in Swift, dictionary lookup returns an Optional but Array traps on out-of-bounds: dictionary keys tend to come from outside the dictionary, but array indices tend to be derived from the array bounds. This means a dictionary lookup failure tends to be expected, and an out-of-bounds array index tends to be a logic error. More details here: https://stackoverflow.com/a/75780575/77567 https://stackoverflow.com/a/75780575/77567

wruza 2y ago

I’ve been programming for many years and never thought of or met any issues with exceptions. It’s a useful mechanism to stop execution and send a structured signal up the call stack to where you expect it. If you don’t have issues (e.g. data, syntax, performance related) with the way you use exceptions, it’s safe to ignore all philosophical distresses. What does it actually mean? When should I use it? Is this correct? Is my usage aligned with the idea behind it? These are all questions of a restless mind. The one that stands in front of a toolbox and cannot choose a type of a hammer for a trivial job where even a rock will do.

TheTrueScotsman 2y ago

[flagged]