[Haskell-cafe] [Haskell] Functor => Applicative => Monad

[Jonathan Geddes]

Fail can't just be removed. That would just break too much code. For
example, I find myself writing code like the following:

>[a,b,c] <- Just someList

in place of

>let [a,b,c] = someList

so that pattern match failure is "lifted" into the maybe monad (as
long as I'm already in the maybe monad).

I would like to see a MonadFail class, with one method 'fail', such
that it is a compile-time error to try 'failable' pattern matches in a
monad that is not an instance of MonadFail.

--Jonathan