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

22 Sep 2022

      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:displayCall...
).

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.

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