As suggested in the GHC Trac [1], it would be nice to have a MonadFail instance for Either. The original suggestion by lexi.lambda was instance MonadFail (Either String) where fail = Left However, it was rightfully pointed out that this instance is overly specific. The discussion arrived at the following alternative proposed by Iceland_jack: instance IsString str => MonadFail (Either str) where fail = Left . fromString For previous discussion on this proposal, please take a look at the Trac thread. I am looking forward to your input! Tobias [1] GHC Trac Ticket #12160, https://ghc.haskell.org/trac/ghc/ticket/12160 -- Tobias Markus "They who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." -- Benjamin Franklin (1706-1790)