Agda.Utils.Maybe

Extend Maybe by common operations for the Maybe type.

Note: since this module is usually imported unqualified, we do not use short names, but all names contain Maybe, Just, or 'Nothing.

Collection operations.

unionMaybeWith :: (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a

unionWith for collections of size <= 1.

unzipMaybe :: Maybe (a, b) -> (Maybe a, Maybe b)

Unzipping a list of length <= 1.

filterMaybe :: (a -> Bool) -> a -> Maybe a

Filtering a singleton list.

filterMaybe p a = listToMaybe (filter p [a])

Conditionals and loops.

forMaybe :: [a] -> (a -> Maybe b) -> [b]

Version of mapMaybe with different argument ordering.

caseMaybe :: Maybe a -> b -> (a -> b) -> b

Version of maybe with different argument ordering. Often, we want to case on a Maybe, do something interesting in the Just case, but only a default action in the Nothing case. Then, the argument ordering of caseMaybe is preferable.

caseMaybe m d f = flip (maybe d) m f

Monads and Maybe.

maybeM :: Monad m => m b -> (a -> m b) -> m (Maybe a) -> m b

Monadic version of maybe.

fromMaybeM :: Monad m => m a -> m (Maybe a) -> m a

Monadic version of fromMaybe.

caseMaybeM :: Monad m => m (Maybe a) -> m b -> (a -> m b) -> m b

Monadic version of caseMaybe. That is, maybeM with a different argument ordering.

ifJustM :: Monad m => m (Maybe a) -> (a -> m b) -> m b -> m b

caseMaybeM with flipped branches.

whenJust :: Monad m => Maybe a -> (a -> m ()) -> m ()

A more telling name for forM_ for the Maybe collection type. Or: caseMaybe without the Nothing case.

whenNothing :: Monad m => Maybe a -> m () -> m ()

caseMaybe without the Just case.

whenJustM :: Monad m => m (Maybe a) -> (a -> m ()) -> m ()

caseMaybeM without the Nothing case.

whenNothingM :: Monad m => m (Maybe a) -> m () -> m ()

caseMaybeM without the Just case.

allJustM :: Monad m => [m (Maybe a)] -> m (Maybe [a])

Lazy version of allJust . sequence. (allJust = mapM for the Maybe monad.) Only executes monadic effect while isJust.

allJustsOrNothings :: [Maybe a] -> Maybe (Maybe [a])

Precondition: list not empty. allJustsOrNothings [Nothing, Nothing] = Just Nothing allJustsOrNothings [Just 0, Just 1] = Just $ Just [0,1] allJustsOrNothings [Just 0, Nothing] = Nothing

module Data.Maybe