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Herbert Valerio Riedel authored
The MonadFail proposal implemented so far via #10751 only warns about missing `MonadFail` instances based on existence of failible pattern matches in `do`-blocks. However, based on the noncanonical Monad warnings implemented via #11150 we can provide a different mechanism for detecting missing `MonadFail` instances quite cheaply. That is, by checking for canonical `fail` definitions. In the case of `Monad`/`MonadFail`, we define the canonical implementation of `fail` to be such that the soft-deprecated method shall (iff overridden) be defined in terms of the non-deprecated method. Consequently, in case of `MonadFail`, the `Monad(fail)` method shall be defined as alias of the `MonadFail(fail)` method. This allows us at some distant point in the future to remove `fail` from the `Monad` class, while having GHC ignore/tolerate such literal canonical method definitions. Reviewed By: bgamari, RyanGlScott Differential Revision: https://phabricator.haskell.org/D1838
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