Re: [Haskell-cafe] Decorating exceptions with backtrace information

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Carter Schonwald

8 May 2020 8 May '20

1:33 p.m.

On Fri, May 8, 2020 at 9:32 AM Carter Schonwald wrote:

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ben, could you please email the libraries list with this too? This seems like a core libraries / base change rather than a ghc-the-compiler change

On Thu, May 7, 2020 at 6:57 PM Michael Sloan wrote:

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Thanks so much for making a proposal for this, Ben!! It's great to see progress here.

I'm also glad that there is now a proposal process. I made a fairly similar proposal almost exactly 5 years ago to the libraries list - https://mail.haskell.org/pipermail/libraries/2015-April/025471.html - but without the subtlety of particular backtrace representations. Skimming the ensuing thread may still be informative.

In particular, there is one thing I would like to highlight from that old proposal. I think it'd be good to have a standard way to represent a chain of exceptions, and build this into `catch` and `finally`. Python and Java both have a mechanism for this, and both refer to it as a "cause" exception. When an exception is thrown during exception handling, the exception being handled is preserved as its "cause". I find this mechanism to be incredibly useful in Java, it has made the underlying issue much clearer in many cases, and in other cases at least provides helpful context. I have no doubt such a mechanism would have saved me many hours of debugging exceptions in Haskell systems I've worked on in the past.

I considered commenting about that directly on the proposal, but I figure this is a better place to suggest expanding the scope of the change :) . Totally understandable if you want to keep this proposal focused on stacktraces, but I think it'd be good to consider this as a potential future improvement.

-Michael

On Thu, May 7, 2020 at 3:55 PM Ben Gamari wrote:

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Hi everyone,

After a nice discussion on IRC about the unfortunate state of error reporting in Haskell, I felt compelled to write down some long-lingering thoughts regarding backtraces on exceptions. The result is GHC proposal #330 [1]. I think the approach is viable and perhaps even straightforward. I have the sketch of an implementation here [2].

Please have a look at the proposal and leave your comments. If there is consensus it is possible that we could have this done for 8.12.

Cheers,

- Ben

[1] https://github.com/ghc-proposals/ghc-proposals/pull/330 [2] https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3236 _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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Henning Thielemann

8 May 8 May

3:37 p.m.

New subject: Decorating exceptions with backtrace information

There seem to be multiple beginnings of the discussion. What is currently discussed? If someone says "exceptions" and "backtrace" in one sentence, I suspect like many times before, that again confusion of the concepts of exceptions and errors is ahead. Errors already support call stacks. Why should exceptions get them, too? Exceptions should carry information that is useful for a user, but a callstack is not useful for a user. I can imagine that it would be helpful for the user to get a stacked exception information like: Parse error on line 42, column 23 while reading file "foo/bar" while traversing directory "blabla" But since you refer to the CallStack feature of GHC, this seems not to be addressed in the proposals. On Fri, 8 May 2020, Carter Schonwald wrote:

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I have no doubt such a mechanism would have saved me many hours of debugging exceptions in Haskell systems I've worked on in the past.

If you must debug exceptions, then this sounds like exceptions were abused for programming errors. Ben writes in: http://www.well-typed.com/blog/2020/04/dwarf-3/ "Unfortunately, the untyped nature of Haskell exceptions complicates the migration path for existing code." Actually, it only proves again, that it was wrong from the beginning to hide information about potential exceptions in the IO monad instead of making them explicit via ExceptionalT, ExceptT or the like.

Niklas Hambüchen

5:27 p.m.

New subject: Decorating exceptions with backtrace information

On 5/8/20 5:37 PM, Henning Thielemann wrote:

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I can imagine that it would be helpful for the user to get a stacked exception information like: Parse error on line 42, column 23 while reading file "foo/bar" while traversing directory "blabla"

That seems to be rather specific use case. It'd be a cool feature but I'm not aware of any programming language following that interpretation so far. I personally would be happy to be able to get the same type of stack trace for exceptions as in other programming langues (and as the proposal suggests).

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If you must debug exceptions, then this sounds like exceptions were abused for programming errors.

I'd be pretty happy to be able to debug them better; no matter if they were "abused" for anything or not, I must still debug them in practice. Given that they traverse program flow invisibly (e.g. not lexically, like return values) and can become visible in different places than they arose, having a call stack to debug their creation would be useful.

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a callstack is not useful for a user.

Call stacks have been very useful to me as a user of non-Haskell tools so far, because they are excellent for attaching to bug reports and usually led to developers fixing my problems faster.

Henning Thielemann

5:32 p.m.

New subject: Decorating exceptions with backtrace information

On Fri, 8 May 2020, Niklas Hambüchen wrote:

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On 5/8/20 5:37 PM, Henning Thielemann wrote:

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a callstack is not useful for a user.

Call stacks have been very useful to me as a user of non-Haskell tools so far, because they are excellent for attaching to bug reports and usually led to developers fixing my problems faster.

This confirms that they are not for you, but you only forward them to the developer. Can someone please give me examples where current state lacks and how they are addressed by the proposal(s)?

Niklas Hambüchen

5:47 p.m.

New subject: Decorating exceptions with backtrace information

On 5/8/20 7:32 PM, Henning Thielemann wrote:

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This confirms that they are not for you, but you only forward them to the developer.

Yes, stack traces are in general for developers.

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Can someone please give me examples where current state lacks

* Currently stack traces are not printed, so users cannot forward them to the developer, even if both the users and the developers would like that. * Developers cannot easily produce stack traces do debug unintended exceptions.

Henning Thielemann

5:52 p.m.

New subject: Decorating exceptions with backtrace information

On Fri, 8 May 2020, Niklas Hambüchen wrote:

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On 5/8/20 7:32 PM, Henning Thielemann wrote:

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Can someone please give me examples where current state lacks

* Currently stack traces are not printed, so users cannot forward them to the developer, even if both the users and the developers would like that.

We are talking about the HasCallStack stack traces, yes? How is their emission addressed by extending exceptions with stack traces?

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* Developers cannot easily produce stack traces do debug unintended exceptions.

What are "unintended exceptions"? What is an example of an "unintended exception"?

Niklas Hambüchen

6 p.m.

New subject: Decorating exceptions with backtrace information

On 5/8/20 7:52 PM, Henning Thielemann wrote:

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We are talking about the HasCallStack stack traces, yes? How is their emission addressed by extending exceptions with stack traces?

The way I understand the proposal, we may be equally talking about DWARF or profiling cost-center based stack traces.

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From a debugging perspective, I guess the developer does not care so much about which implementation is used, as long as the trace points out the code path that led to the creation of the exception.

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What are "unintended exceptions"? What is an example of an "unintended exception"?

A recent example from my production server: hPutBuf: resource vanished (Broken pipe)

Henning Thielemann

12 May 12 May

8:55 p.m.

New subject: Decorating exceptions with backtrace information

On Fri, 8 May 2020, Niklas Hambüchen wrote:

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What are "unintended exceptions"? What is an example of an "unintended exception"?

A recent example from my production server:

hPutBuf: resource vanished (Broken pipe)

Ok, I lookup the Haddock comment of hPutBuf and it says: "This operation may fail with: * ResourceVanished if the handle is a pipe or socket, and the reading end is closed." That is, ResourceVanished is part of the public interface and in no way unexpected (or what "unintended" may be). I would prefer to make this explicit in the type of hPutBuf: hPutBuf :: (ResourceVanishedException e) => Handle -> Ptr a -> Int -> ExceptT e IO () Now, what do you intend to do with the call-stack? Isn't it something you can attach to the e value?

Niklas Hambüchen

10:14 p.m.

New subject: Decorating exceptions with backtrace information

On 5/12/20 10:55 PM, Henning Thielemann wrote:

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"This operation may fail with:

* ResourceVanished if the handle is a pipe or socket, and the reading end is closed."

That is, ResourceVanished is part of the public interface and in no way unexpected (or what "unintended" may be). I would prefer to make this explicit in the type of hPutBuf:

hPutBuf :: (ResourceVanishedException e) => Handle -> Ptr a -> Int -> ExceptT e IO ()

Now, what do you intend to do with the call-stack? Isn't it something you can attach to the e value?

Why is this relevant? The point of debugging is to find programming errors. It does not matter what the Haddocks say; if a programmer uses the function wrong, the exception will occur. hPutBuf does not currently have that type, nor can anybody rewrite all the existing libraries easily. The point of the proposal is to make the RTS help us debug problems in code as it exists today.

Ben Gamari

8 May 8 May

6:18 p.m.

New subject: Decorating exceptions with backtrace information

Henning Thielemann writes:

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On Fri, 8 May 2020, Niklas Hambüchen wrote:

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On 5/8/20 7:32 PM, Henning Thielemann wrote:

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Can someone please give me examples where current state lacks

* Currently stack traces are not printed, so users cannot forward them to the developer, even if both the users and the developers would like that.

We are talking about the HasCallStack stack traces, yes? How is their emission addressed by extending exceptions with stack traces?

HasCallStack stack traces are one type of backtrace that the proposal supports. However, it's not the only (nor is it even the most useful sort, in my opinion). Other mechanisms include cost center stacks from the cost-center profiler and native stack unwinding.

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* Developers cannot easily produce stack traces do debug unintended exceptions.

What are "unintended exceptions"? What is an example of an "unintended exception"?

For instance, * Somewhere deep in my code a colleague used `fromJust` due to a miscommunicated invariant * Somewhere in my system a `writeFile "tmp" $ repeat 'a'` failed due to filling the disk * Somewhere in my system I have a partial pattern match in a module which was compiled without -Wall * Somewhere in my system I `div` by zero due to lack of input validation * I use a record selector on a sum. * A logic error results in an assertion failure deep in my program, but it's unclear which path my program took to arrive at the assertion This list could go on and on... Currently the proposal does not cover asynchronous exceptions but it wouldn't be particularly hard to extend it in this direction. This would allow far better reporting of heap/stack overflows and MVar deadlocks (which are particularly hard to debug at the moment). Cheers, - Ben

Compl Yue

9 May 9 May

6:35 a.m.

New subject: Decorating exceptions with backtrace information

This reminds me a joke to put it in a humorous way:

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A software QA engineer walks into a bar. He orders a beer. Orders 0 beers. Orders 99999999999 beers. Orders a lizard. Orders -1 beers. First real customer walks in and asks where the bathroom is. The bar bursts into flames, killing everyone.

LOL, Compl

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On 2020-05-09, at 02:18, Ben Gamari wrote:

Henning Thielemann writes:

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On Fri, 8 May 2020, Niklas Hambüchen wrote:

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On 5/8/20 7:32 PM, Henning Thielemann wrote:

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Can someone please give me examples where current state lacks

* Currently stack traces are not printed, so users cannot forward them to the developer, even if both the users and the developers would like that.

We are talking about the HasCallStack stack traces, yes? How is their emission addressed by extending exceptions with stack traces?

HasCallStack stack traces are one type of backtrace that the proposal supports. However, it's not the only (nor is it even the most useful sort, in my opinion).

Other mechanisms include cost center stacks from the cost-center profiler and native stack unwinding.

...
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* Developers cannot easily produce stack traces do debug unintended exceptions.

What are "unintended exceptions"? What is an example of an "unintended exception"?

For instance,

* Somewhere deep in my code a colleague used `fromJust` due to a miscommunicated invariant

* Somewhere in my system a `writeFile "tmp" $ repeat 'a'` failed due to filling the disk

* Somewhere in my system I have a partial pattern match in a module which was compiled without -Wall

* Somewhere in my system I `div` by zero due to lack of input validation

* I use a record selector on a sum.

* A logic error results in an assertion failure deep in my program, but it's unclear which path my program took to arrive at the assertion

This list could go on and on...

Currently the proposal does not cover asynchronous exceptions but it wouldn't be particularly hard to extend it in this direction. This would allow far better reporting of heap/stack overflows and MVar deadlocks (which are particularly hard to debug at the moment).

Cheers,

- Ben _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

Henning Thielemann

12 May 12 May

9:29 p.m.

New subject: Decorating exceptions with backtrace information

On Fri, 8 May 2020, Ben Gamari wrote:

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Henning Thielemann writes:

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We are talking about the HasCallStack stack traces, yes? How is their emission addressed by extending exceptions with stack traces?

HasCallStack stack traces are one type of backtrace that the proposal supports. However, it's not the only (nor is it even the most useful sort, in my opinion).

Other mechanisms include cost center stacks from the cost-center profiler and native stack unwinding.

Interesting. That's a completely new thing.

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* Developers cannot easily produce stack traces do debug unintended exceptions.

What are "unintended exceptions"? What is an example of an "unintended exception"?

For instance,

* Somewhere deep in my code a colleague used `fromJust` due to a miscommunicated invariant

That's a programming error.

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* Somewhere in my system a `writeFile "tmp" $ repeat 'a'` failed due to filling the disk

Hm, that's also a programming error, but it ends in an IO exception. If it would not end in an IO exception (e.g. writing to /dev/null) it would go to an infinite loop. Anyway, it is a programming error. However it is an unchecked one. That is, there is no warranty that you can catch it by a debugger. So I do not think you can achieve much with callstacks here.

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* Somewhere in my system I have a partial pattern match in a module which was compiled without -Wall

Programming error and btw. before thinking about a GHC extension I would enable -Wall ...

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* Somewhere in my system I `div` by zero due to lack of input validation

Programming error

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* I use a record selector on a sum.

Programming error

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* A logic error results in an assertion failure deep in my program, but it's unclear which path my program took to arrive at the assertion

Sounds like Programming error

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This list could go on and on...

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From your list of examples I deduce that the proposal is about programming errors. But we have HasCallStack for that one. How does the proposal improve or alter the HasCallStack solution? And how does it relate to the IO exception system with hierarchical exceptions and SomeException and so on?

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Currently the proposal does not cover asynchronous exceptions but it wouldn't be particularly hard to extend it in this direction. This would allow far better reporting of heap/stack overflows and MVar deadlocks (which are particularly hard to debug at the moment).

Hm, what kind of heap or stack overflow are you thinking of? A stack overflow sounds like unlimited recursion and thus like a programming error. In contrast to that, a program must be prepared for a failure of "malloc". Memory exhaustion is an IO exception, it should be explicit in the type. Are MVar deadlocks always detected by the runtime system?

Tyson Whitehead

14 May 14 May

4:06 p.m.

New subject: Decorating exceptions with backtrace information

On Tue, 12 May 2020 at 17:30, Henning Thielemann < lemming@henning-thielemann.de> wrote:

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From your list of examples I deduce that the proposal is about programming errors. But we have HasCallStack for that one. How does the proposal improve or alter the HasCallStack solution? And how does it relate to the IO exception system with hierarchical exceptions and SomeException and so on?

As a parallel item, maybe it would be good if incomplete patterns could have a HasCallStack constraint so the current "Non-exaustive patterns in function foo" message could be extended with a helpful backtrace? If a programmer doesn't want this, then they could use complete matches?

Ben Gamari

8 May 8 May

6:03 p.m.

New subject: Decorating exceptions with backtrace information

Henning Thielemann writes:

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On Fri, 8 May 2020, Niklas Hambüchen wrote:

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On 5/8/20 5:37 PM, Henning Thielemann wrote:

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a callstack is not useful for a user.

Call stacks have been very useful to me as a user of non-Haskell tools so far, because they are excellent for attaching to bug reports and usually led to developers fixing my problems faster.

This confirms that they are not for you, but you only forward them to the developer.

Can someone please give me examples where current state lacks and how they are addressed by the proposal(s)?

We can debate whether partial functions like `fromJust` should exist; however, the fact of the matter is that they do exist and they are used. Furthermore, even `base`'s own IO library (e.g. `openFile`) uses synchronous exceptions to report errors. This becomes particularly painful when building large systems: Even if I am careful to avoid such functions in my own code, as my dependency footprint grows it becomes more likely that some transitive dependency will expose a partial interface (perhaps even without my knowledge). This is a problem that industrial users are all too familiar with. Perhaps this helps to shed some light on the motivation? Cheers, - Ben

Henning Thielemann

12 May 12 May

9:06 p.m.

New subject: Decorating exceptions with backtrace information

On Fri, 8 May 2020, Ben Gamari wrote:

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We can debate whether partial functions like `fromJust` should exist; however, the fact of the matter is that they do exist and they are used.

That's not my point. I say: fromJust on Nothing is a programming error, ok. We must debug this. HasCallStack helps here. However, it does not have to do with exceptions or with the proposals as I understand them.

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Furthermore, even `base`'s own IO library (e.g. `openFile`) uses synchronous exceptions to report errors.

Right. I say: Such exceptions are part of the public interface and should be expressed in types. If you encounter any problems when not doing this, I would first try to solve the problem with exceptions explicit in the type. E.g. Haddock for openFile says: This operation may fail with: * isAlreadyInUseError ... * isDoesNotExistError ... * isPermissionError ... Thus the type should be: openFile :: (AlreadyInUseException e, DoesNotExistException e, PermissionException e) => FilePath -> IOMode -> ExceptT e IO Handle

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Perhaps this helps to shed some light on the motivation?

Unfortunately no. I only see the immortal confusion about (programming) errors vs. (IO) exceptions. And I think that part of this confusion is that IO exceptions in 'base' are hidden in the IO type and that there are hybrid functions like 'throw' that can be called like 'error' but they cause IO exceptions that can be caught by 'catch'.

Ben Franksen

13 May 13 May

1:52 p.m.

New subject: Decorating exceptions with backtrace information

Am 12.05.20 um 23:06 schrieb Henning Thielemann:

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Thus the type should be:

openFile :: (AlreadyInUseException e, DoesNotExistException e, PermissionException e) => FilePath -> IOMode -> ExceptT e IO Handle

Yes, but then these errors would no longer be exceptions but normal return values (if you remove the ExceptT veneer). So what you say amounts to "exceptions should be removed from Haskell except for programming errors". I agree that communicating IO errors (and similar things) explicitly ie. via return value is the better choice. But the proposal is about Haskell (the language and standard libraries) as it presently is i.e. the one that uses exceptions for IO errors, not the one you want to have and in which IO errors are communicated as return values and openFile would have the type above. And that means your objection to the proposal at hand is irrelevant and beside the point.

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Ben Franksen
Ben Gamari
Carter Schonwald
Compl Yue
Henning Thielemann
Niklas Hambüchen
Tyson Whitehead

Re: [Haskell-cafe] Decorating exceptions with backtrace information

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