Heist won't compile against the latest version of the errors package. If there is a chance, could you please release a quick update that remedies this issue? The error message I get trying to compile Heist is:
Not in scope: type constructor or class `EitherT'
Perhaps you meant one of these:
`Either' (imported from Prelude),
`EitherR' (imported from Control.Error),
`EitherRT' (imported from Control.Error)
This is due to a bug in the errors package no longer exporting EitherT. It should automatically resolve itself with the next release of errors.
You were right -- the issue has been fixed by the latest update of errors. Thanks!