mapM package:universum

Map each element of a structure to a monadic action, evaluate these actions from left to right, and collect the results. For a version that ignores the results see mapM_.

Examples

mapM is literally a traverse with a type signature restricted to Monad. Its implementation may be more efficient due to additional power of Monad.

Constrained to Container version of mapM_.

>>> mapM_ print [True, False]
True
False

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.

Examples

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import Text.Read ( readMaybe )

>>> let readMaybeInt = readMaybe :: String -> Maybe Int

>>> mapMaybe readMaybeInt ["1", "Foo", "3"]
[1,3]

>>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]
[1,3]

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3]
[1,2,3]

Lifting bind into a monad. Generalized version of concatMap that works with a monadic predicate. Old and simpler specialized to list version had next type:

concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b]

Side note: previously it had type

concatMapM :: (Applicative q, Monad m, Traversable m)
=> (a -> q (m b)) -> m a -> q (m b)

Such signature didn't allow to use this function when traversed container type and type of returned by function-argument differed. Now you can use it like e.g.

concatMapM readFile files >>= putTextLn