fail is moving out of Monad and into it's own MonadFail class.
This will be enforced in GHC 8.8 (I think).
base-compat/base-compat-batteries 0.11.0 have adapted to this,
and are approaching stackage nightly
(https://github.com/commercialhaskell/stackage/issues/4802).
hledger is now ready to build with base-compat-batteries 0.11.0, once
all of our deps do (eg aeson). We are still compatible with the older
0.10.x and GHC 7.10.3 as well.
For now we are using both fails:
- new fail (from Control.Monad.Fail), used in our parsers, imported
via base-compat-batteries Control.Monad.Fail.Compat to work with
older GHC versions.
- old fail (from GHC.Base, exported by Prelude, Control.Monad,
Control.Monad.State.Strict, Prelude.Compat, ...), used in easytest's
Test, since I couldn't find their existing fail implementation to update.
To reduce (my) confusion, these are imported carefully, consistently,
and qualified everywhere as Fail.fail and Prelude.fail, with clashing
re-exports suppressed, like so:
import Prelude hiding (fail)
import qualified Prelude (fail)
import Control.Monad.State.Strict hiding (fail)
import "base-compat-batteries" Prelude.Compat hiding (fail)
import qualified "base-compat-batteries" Control.Monad.Fail.Compat as Fail
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