Shrinking hits "empty generator in function"

[robinp]

Sometimes when a property fails and hedgehog shrinks, then hedgehog-fn errors out with the above message, so the failing test value can't be observed.

This seems to be called from apply, in

-- | Extract the root value from a 'TreeT'. Unsafe.
unsafeFromTree :: Functor m => TreeT (MaybeT m) a -> m a
unsafeFromTree =
  fmap (maybe (error "empty generator in function") nodeValue) .
  runMaybeT .
  runTreeT

Is there any hint how to avoid generating functions that fail during shrinking? I would prefer no shrinking rather than an erroring out shrink.

Note that there's some workaround where I just try to re-run and hope that eventually a non-failing shrink path would be hit.

Also note that I tried to limit shrinks using withShrinks, but for some reason that seems to get caught in some endless loop (so it never generates an output). I only guess it is an endless loop, since I recall that when I try to print some generated functions, it would loop writing the function table endlessly. I only assume this is similar, hedgehog trying to print and format a not sufficiently shrunk function, and getting stuck in the loop. (Maybe this is worth a different issue?)

[LightAndLight]

Do you have a small property test that (probabilistically) replicates the issue? I haven't thought about this project for a long time, so I'd need to play around to debug things.

[robinp]

@LightAndLight yes, I managed to bare it down. Please give me some time to remove some more cruft so it can be ran independently, will share sometime around next week.