(un-)deprecation of Control.Monad.Error

[Bertram Felgenhauer]

Paterson, Ross wrote:
> I think that fail on Either or EitherT has always been the default (error).

By "old" I meant "really old". The instance was changed 8 years ago,
when ghc-7.0.1 was released. You can find traces of it in
Control.Monad.Error in mtl-1.*. mtl-2.* removed those traces. The last
appearance was in mtl-1.1.1.1, conditional on the version of base:

  https://hackage.haskell.org/package/mtl-1.1.1.1/docs/src/Control-Monad-Error.html#line-82

Further back in history, mtl-1.0.0 was released with ghc-6.4.1, and
contained the above instance unconditionally; before that, the instance
was in base. I stopped looking at ghc-6.0.1.

Cheers,

Bertram