同时过滤和映射

Posted 2021-02-24

The `filtermap` functional below allows to perform the standard `map` and `filter` operations in one go. The mapping function has to return a `Maybe` monad, and values of `Nothing` are filtered out.

-- Prepend an element to a list if available.  Leave the list as it is if the
-- first argument is Nothing.
maybecons :: Maybe t -> [t] -> [t]
maybecons Nothing l = l
maybecons (Just e) l = e : l
 
-- Variant of map which deletes elements if the map function returns Nothing.
filtermap :: (a -> Maybe b) -> [a] -> [b]
filtermap _ [] = []
filtermap f (a:as) = maybecons (f a) $ filtermap f as