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.
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.