join-coma-wtf.hs

Prelude> :m Control.Monad
Prelude Control.Monad> join (,) 1
(1,1)

-- So it produces pairs somehow. Let's look at type signatures

Prelude Control.Monad> :t join
join :: Monad m => m (m a) -> m a

Prelude Control.Monad> :t (,)
(,) :: a -> b -> (a, b)

Prelude Control.Monad> :t join (,)
join (,) :: a -> (a, a)

-- Does not make any sense by looking at this.
-- Explanation of how type checker deduces what to do by Edward Kmett is here:
--     http://www.reddit.com/r/haskell/comments/1olk04/the_magic_of_join/cct4kaz