MonadFail instance for Either

Daniel Bergey bergey at
Thu Oct 25 02:43:43 UTC 2018

Is there still consensus in favor of adding this instance?

 instance IsString str => MonadFail (Either str) where
    fail = Left . fromString

In 2016 there was some discussion, and my reading is that there was consensus in favor at the time:
libaries mailing list:

Does anyone know of a later decision not to add it, or was it simply no one's top priority?

What is the next step to move this proposal forward?  Is more discussion in order?  Should I just submit a patch?


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