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Restrict type of monadic binding

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This page addresses an aspect of Haskell style, which is to some extent a matter of taste. Just pick what you find appropriate for you and ignore the rest.

According to Do notation considered harmful#Safety the type of the monadic binding combinator (>>) should be restricted to

(>>) :: m () -> m a -> m a
This way, you can omit
 _ <-
only if the monadic return value has type
 ()
.

Types of some other monadic operations should be restricted, too.

mapM_ :: Monad m => (a -> m ()) -> [a] -> m ()
replicateM_ :: Monad m => Int -> m () -> m ()
sequence_ :: Monad m => [m ()] -> m ()
zipWithM_ :: Monad m => (a -> b -> m ()) -> [a] -> [b] -> m ()