With MonadFailDesugaring as default landing in GHC 8.6, the lack of a MonadFail instance for HeistT may become more acute, and will probably cause code breakage. It's easy and I think most sensible to add a simple pass-through instance:
instance MonadFail m => MonadFail (HeistT n m) where fail = lift . fail
However, this would differ from the legacy fail, which is not defined for HeistT and thus falls to the default implementation, which is just a regular error. So for pure compatibility that could be done instead, although my feeling is that pass-through is a better design.
With
MonadFailDesugaringas default landing in GHC 8.6, the lack of aMonadFailinstance forHeistTmay become more acute, and will probably cause code breakage. It's easy and I think most sensible to add a simple pass-through instance:However, this would differ from the legacy
fail, which is not defined forHeistTand thus falls to the default implementation, which is just a regular error. So for pure compatibility that could be done instead, although my feeling is that pass-through is a better design.