--no-time flag seems to change format of exceptions

[ezyang]

I wrote the following code to handle unwrapping Shake's exceptions thrown by the Shake runner:

-- | When we raise a 'GhcException' or a 'SourceError', try to give
-- @ghc --make@ compatible output (without the extra Shake wrapping.)
-- This is way better for users, since Shake does not print line numbers
-- for SourceErrors.
handleGhcErrors :: IO a -> IO a
handleGhcErrors m =
    handle (\(e :: ShakeException) ->
                -- TODO: there should be a better way of doing this
                case fromException (shakeExceptionInner e) of
                    Just (e' :: GhcException) -> throwIO e'
                    Nothing -> case fromException (shakeExceptionInner e) of
                        Just (e' :: SourceError) -> throwIO e'
                        Nothing -> throwIO e
                ) m

This code only works when --no-time is passed; when it is not passed this code doesn't seem to work. I haven't investigated any further.

[ndmitchell]

Thanks for the info, just pass --no-time for now and I'll look at this with #380, which I suspect is related.

[ezyang]

Actually, the answer is very simple, you need --exception. When you pass --no-time, Shake implicitly assumes exception.

[ndmitchell]

I wonder if there is a tension between having a good API (which would do --exception) and a good main :: function (which should raise ExitFailure).

[ezyang]

(Not hot path, in case it wasn't obvious.)

[ndmitchell]

See also #425 - this is a nasty corner case that I expect a lot of people will trip over.