Re: [Haskell-cafe] regex problem with ghc-8.6 and older

The `fail` method was part of the `Monad` type class from GHC 8.0 (`base` `4.9.0.0`) until GHC 8.6 (`base` `4.12.0.0`). https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Monad.html#v:... It was removed from GHC 8.8 (`base` `4.13.0.0`). https://hackage.haskell.org/package/base-4.13.0.0/docs/Control-Monad.html#t:... There is information about this in the MonadFail Proposal (MFP): https://gitlab.haskell.org/haskell/prime/-/wikis/libraries/proposals/monad-f... Cheers, Travis