Why is join standalone, instead of part of the minimal implementation of the Monad typeclass?

Question

I would prefer defining an instance of Monad via the join function instead of >>= ... Starting from that equivalence:

x >>= f = join (fmap f x)

Could you define a Monad instance only with join?

I would have expected join in Monad with: {-# MINIMAL (>>=)| join #-}

Why is join at the top level and not even in the Monad typeclass?

Answer 1

Sadly, join is not a part of the Monad typeclass in GHC’s standard library because of technical restrictions related to generalized newtype deriving and the roles system. Long story short, given some newtype newtype T m a = MkT (m a), GHC is not smart enough to figure out how to prove representational equality between m (m a) and m (T m a), which is necessary for proving representational equality for the first argument of join (which has type m (m a) -> m a).

Fortunately, a recent extension to GHC Haskell, QuantifiedConstraints, might make it possible to make the roles system smart enough to support this. For a more detailed treatment of both the problem and its potential solution, see Ryan Scott’s blog post, How QuantifiedConstraints can let us put join back in Monad.

Answer 2

join is not in Monad because it would break GeneralizedNewtypeDeriving in a subtle way for newtype-wrapped monad transformers, a very common use case. See the GHC wiki for details.