:: Monad m => (m (m a)) -> m a -package:relude

join :: Monad m => m (m a) -> m a
base Control.Monad GHC.Base, ghc-internal GHC.Internal.Base GHC.Internal.Control.Monad, verset Verset
The join function is the conventional monad join operator. It is used to remove one level of monadic structure, projecting its bound argument into the outer level. 'join bss' can be understood as the do expression 
do bs <- bss
bs

Examples

>>> join [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
[1,2,3,4,5,6,7,8,9]

>>> join (Just (Just 3))
Just 3
A common use of join is to run an IO computation returned from an STM transaction, since STM transactions can't perform IO directly. Recall that 
atomically :: STM a -> IO a
is used to run STM transactions atomically. So, by specializing the types of atomically and join to 
atomically :: STM (IO b) -> IO (IO b)
join       :: IO (IO b)  -> IO b
we can compose them as 
join . atomically :: STM (IO b) -> IO b
to run an STM transaction and the IO action it returns.
join :: Monad m => m (m a) -> m a
base-compat Control.Monad.Compat, protolude Protolude.Monad, streaming Streaming, rio RIO.Prelude, base-prelude BasePrelude, turtle Turtle, classy-prelude ClassyPrelude, Cabal-syntax Distribution.Compat.Prelude, universum Universum.Monad.Reexport, ihaskell IHaskellPrelude, base-compat-batteries Control.Monad.Compat, Agda Agda.Utils.Monad, rebase Rebase.Prelude, massiv-test Test.Massiv.Utils, cabal-install-solver Distribution.Solver.Compat.Prelude, incipit-base Incipit.Base, LambdaHack Game.LambdaHack.Core.Prelude, faktory Faktory.Prelude, can-i-haz Control.Monad.Except.CoHas Control.Monad.Reader.Has, yesod-paginator Yesod.Paginator.Prelude, distribution-opensuse OpenSuse.Prelude
The join function is the conventional monad join operator. It is used to remove one level of monadic structure, projecting its bound argument into the outer level. 'join bss' can be understood as the do expression 
do bs <- bss
bs

Examples

A common use of join is to run an IO computation returned from an STM transaction, since STM transactions can't perform IO directly. Recall that 
atomically :: STM a -> IO a
is used to run STM transactions atomically. So, by specializing the types of atomically and join to 
atomically :: STM (IO b) -> IO (IO b)
join       :: IO (IO b)  -> IO b
we can compose them as 
join . atomically :: STM (IO b) -> IO b
to run an STM transaction and the IO action it returns.
(.$) :: Monad m => m (m a) -> m a
composition-prelude Control.Composition
Infix version of join As an example, one could use this to rewrite 
between (char '"') (char '"')
to 
between .$ (char '"')
Or 
fromEither :: Either a a -> a
fromEither = either id id
to 
fromEither :: Either a a -> a
fromEither = either .$ id