[Haskell-cafe] Decorating exceptions with backtrace information

Compl Yue compl.yue at gmail.com
Sat May 9 06:35:17 UTC 2020

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

> 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.


> On 2020-05-09, at 02:18, Ben Gamari <ben at well-typed.com> wrote:
> Henning Thielemann <lemming at henning-thielemann.de> writes:
>> On Fri, 8 May 2020, Niklas Hamb├╝chen wrote:
>>> On 5/8/20 7:32 PM, Henning Thielemann wrote:
>>>> 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.
>>> * 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
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