Warn against using 'fail' directly in user-code

[Niklas Hambüchen]

> (Fwiw, I've tested attaching such a warning to `fail` in GHC HEAD's
> libraries/base/GHC/Base.lhs and it seems to work just fine so far... so
> if there's consensus, we could add that for GHC 7.8)

That sounds great!

And as far as I remember, we will have MonadFail, as Malcolm mentioned,
soon.

On 16/12/13 13:54, Andreas Abel wrote
>   fail :: String -> m a
>
>   Fail with a message. This operation is not part of the mathematical
>  definition of a monad, but is invoked on pattern-match failure in a do
> expression.

This seems to go well with your change.