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

[Travis Cardwell]

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:fail>

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:Monad>

There is information about this in the MonadFail Proposal (MFP):

<https://gitlab.haskell.org/haskell/prime/-/wikis/libraries/proposals/monad-fail>

Cheers,

Travis