6.2.9. New monadic failure desugaring mechanism


Use the MonadFail.fail instead of the legacy Monad.fail function when desugaring refutable patterns in do blocks.

The -XMonadFailDesugaring extension switches the desugaring of do-blocks to use MonadFail.fail instead of Monad.fail.

This extension is enabled by default since GHC 8.6.1, under the MonadFail Proposal (MFP).

This extension is temporary, and will be deprecated in a future release.