6.2.9. New monadic failure desugaring mechanism — Glasgow Haskell Compiler 9.0.0.20201227 User's Guide

# 6.2.9. New monadic failure desugaring mechanism

`MonadFailDesugaring`

  Since: | 8.0.1

  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.