maybe package:streamly-core

Consume a single input and transform it using the supplied Maybe returning function. Pre-release

Map a Maybe returning function on the next element in the stream. The parser fails if the function returns Nothing otherwise returns the Just value.

>>> toEither = Maybe.maybe (Left "maybe: predicate failed") Right

>>> maybe f = Parser.either (toEither . f)

>>> maybe f = Parser.fromFoldMaybe "maybe: predicate failed" (Fold.maybe f)

Pre-release

A strict Maybe

Modify a fold to receive a Maybe input, the Just values are unwrapped and sent to the original fold, Nothing values are discarded.

>>> catMaybes = Fold.mapMaybe id

>>> catMaybes = Fold.filter isJust . Fold.lmap fromJust

mapMaybe f fold maps a Maybe returning function f on the input of the fold, filters out Nothing elements, and return the values extracted from Just.

>>> mapMaybe f = Fold.lmap f . Fold.catMaybes

>>> mapMaybe f = Fold.mapMaybeM (return . f)

>>> f x = if even x then Just x else Nothing

>>> fld = Fold.mapMaybe f Fold.toList

>>> Stream.fold fld (Stream.enumerateFromTo 1 10)
[2,4,6,8,10]

Use a Maybe returning fold as a filtering scan.

>>> scanMaybe p f = Fold.postscan p (Fold.catMaybes f)

Pre-release

In a stream of Maybes, discard Nothings and unwrap Justs.

>>> catMaybes = Stream.mapMaybe id

>>> catMaybes = fmap fromJust . Stream.filter isJust

Pre-release

Map a Maybe returning function to a stream, filter out the Nothing elements, and return a stream of values extracted from Just. Equivalent to:

>>> mapMaybe f = Stream.catMaybes . fmap f

Like mapMaybe but maps a monadic function. Equivalent to:

>>> mapMaybeM f = Stream.catMaybes . Stream.mapM f

>>> mapM f = Stream.mapMaybeM (\x -> Just <$> f x)

Use a filtering fold on a stream.

>>> scanMaybe f = Stream.catMaybes . Stream.postscan f

>>> mapMaybeM f = Fold.lmapM f . Fold.catMaybes

Convert strict Maybe' to lazy Maybe

Convert a Maybe returning fold to an error returning parser. The first argument is the error message that the parser would return when the fold returns Nothing. Pre-release

Write a stream of Maybe values. Keep buffering the just values in an array until a Nothing is encountered or the buffer size exceeds the specified limit, at that point flush the buffer to the handle. Pre-release