[Haskell-cafe] How to diagnose a loop in a large Haskell program?

[Dominik Schrempf]

I just noticed, maybe you are interested in the `-xc` runtime system
option? This option can be activated when compiling with profiling
support. See here:

https://downloads.haskell.org/ghc/latest/docs/users_guide/profiling.html#rts-flag--xc

Dominik

Dominik Schrempf <dominik.schrempf at gmail.com> writes:

> Han Joosten <han.joosten.han at gmail.com> writes:
>
>> Hi,
>>
>> I am struggling to debug a loop (some nontermination behaviour) of a huge program. Normally, I use `stack build` to generate it. I have tried to use
>> `trace` at lots of places in the code, recompile and see what happens. However, it isn't always clear in what order the trace statements are fired,
>> because of the lazy evaluation of Haskell.
>>
>> So I tried another approach. I have a function
>>
>> fatal :: HasCallStack => Text -> a
>> fatal msg =
>>   exitWith . Fatal . T.lines $
>>     ( msg <> "\n"
>>         <> (utf8BuilderToText . displayCallStackFull $ callStack)
>>     )
>>
>> displayCallStackFull :: CallStack -> Utf8Builder
>> displayCallStackFull cs =
>>   case reverse $ getCallStack cs of
>>     [] -> "<no call stack found>"
>>     xs -> mconcat $ fmap showCall xs
>>   where
>>     showCall :: (String, SrcLoc) -> Utf8Builder
>>     showCall (desc, loc) =
>>       let file = srcLocFile loc
>>        in "\n"
>>             <> fromString file
>>             <> ":"
>>             <> displayShow (srcLocStartLine loc)
>>             <> ":"
>>             <> displayShow (srcLocStartCol loc)
>>             <> " "
>>             <> fromString desc
>>
>> (The function to show the stack was inspired from https://hackage.haskell.org/package/rio-0.1.22.0/docs/RIO.html#v:displayCallStack).
>>
>> I would expect that at any place where I force a call to `fatal`, I would get the full stacktrace. Unfortunately, I only get the place where the call to
>> fatal is in the code.
>
> Hi.
>
> Do the calling functions have call stack constraints?
>
> Quoting from the GHC manual: https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/callstack.html
>
> "The call stack will only extend as far as the types allow it."
>
> I think compilation with `--profile` (stack) or `--enable-profiling` (cabal-install) activates "simulated call-stacks" which seem to differ from the call stacks obtained by `HasCallStack`. Is this correct?
>
> On a side note: I usually get a complete call stacks when compiling with `cabal build --enable-profiling`.
>
> Dominik
>
>>
>> I have been looking at the documentation of both stack and ghc, but I don't understand how to compile my code in a way that the full stacktrace is
>> shown.
>>
>> Until now the best guess I have is
>>
>> stack install --profile --no-strip && ampersand -- check testing/Sentinel/Tests/ShouldSucceed/OnlyValidation/Issue280.adl
>>
>> The ultimate goal is to get rid of this ugly bug regarding the loop. Any help/suggestions is really appreciated!
>>
>> Thanks for reading.
>> Han Joosten
>>
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