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DimaD/join-coma-wtf.hs

Last active August 29, 2015 14:02
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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
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