content: {-# OPTIONS_GHC -Wunused-imports #-}
  
  -- | A strict version of the 'Maybe' type.
  --
  --   Import qualified, as in
  --   @
  --     import qualified Agda.Utils.Maybe.Strict as Strict
  --   @
  
  module Agda.Utils.Maybe.Strict
    ( module Data.Strict.Maybe
    , module Data.Strict.Classes
    , module Agda.Utils.Maybe.Strict
    ) where
  
  import Prelude hiding (Maybe(..), maybe)
  
  import qualified Data.Maybe as Lazy
  import Data.Strict.Classes
  import Data.Strict.Maybe
  
  import Agda.Utils.Functor ( (<.>) )
  import Agda.Utils.Lens    ( Lens' )
  import Agda.Utils.Null
  
  -- | Note that strict Maybe is an 'Applicative' only modulo strictness.
  --   The laws only hold in the strict semantics.
  --   Eg. @pure f <*> pure _|_ = _|_@, but according to the laws for
  --   'Applicative' it should be @pure (f _|_)@.
  --   We ignore this issue here, it applies also to 'Foldable' and 'Traversable'.
  
  instance Applicative Maybe where
    pure              = Just
    Just f <*> Just x = Just $ f x
    _      <*> _      = Nothing
  
  instance Null (Maybe a) where
    empty = Nothing
    null = isNothing
  
  -- * Conversion to lazy @Maybe@.
  
  lensMaybeLazy :: Lens' (Maybe a) (Lazy.Maybe a)
  lensMaybeLazy f = toStrict <.> f . toLazy
  
  -- * Collection operations.
  
  -- | @unionWith@ for collections of size <= 1.
  unionMaybeWith :: (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a
  unionMaybeWith f Nothing mb      = mb
  unionMaybeWith f ma      Nothing = ma
  unionMaybeWith f (Just a) (Just b) = Just $ f a b
  
  -- | Unzipping a list of length <= 1.
  
  unzipMaybe :: Maybe (a,b) -> (Maybe a, Maybe b)
  unzipMaybe Nothing      = (Nothing, Nothing)
  unzipMaybe (Just (a,b)) = (Just a, Just b)
  
  -- | Filtering a singleton list.
  --
  --   @filterMaybe p a = 'listToMaybe' ('filter' p [a])@
  
  filterMaybe :: (a -> Bool) -> a -> Maybe a
  filterMaybe p a
    | p a       = Just a
    | otherwise = Nothing
  
  -- * Conditionals and loops.
  
  -- | Version of 'mapMaybe' with different argument ordering.
  
  forMaybe :: [a] -> (a -> Maybe b) -> [b]
  forMaybe = flip mapMaybe
  
  -- | 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 err f = flip (maybe err) m f@
  caseMaybe :: Maybe a -> b -> (a -> b) -> b
  caseMaybe m err f = maybe err f m
  
  -- * Monads and Maybe.
  
  -- | Monadic version of 'maybe'.
  
  maybeM :: Monad m => m b -> (a -> m b) -> m (Maybe a) -> m b
  maybeM n j mm = maybe n j =<< mm
  
  -- | Monadic version of 'fromMaybe'.
  
  fromMaybeM :: Monad m => m a -> m (Maybe a) -> m a
  fromMaybeM m mm = maybeM m return mm
  
  -- | Monadic version of 'caseMaybe'.
  --   That is, 'maybeM' with a different argument ordering.
  caseMaybeM :: Monad m => m (Maybe a) -> m b -> (a -> m b) -> m b
  caseMaybeM mm err f = maybeM  err f mm
  
  -- | 'caseMaybeM' with flipped branches.
  ifJustM :: Monad m => m (Maybe a) -> (a -> m b) -> m b -> m b
  ifJustM mm = flip (caseMaybeM mm)
  
  -- | A more telling name for 'Traversable.forM' for the 'Maybe' collection type.
  --   Or: 'caseMaybe' without the 'Nothing' case.
  whenJust :: Monad m => Maybe a -> (a -> m ()) -> m ()
  whenJust m k = caseMaybe m (return ()) k
  
  -- | 'caseMaybeM' without the 'Nothing' case.
  whenJustM :: Monad m => m (Maybe a) -> (a -> m ()) -> m ()
  whenJustM c m = c >>= (`whenJust` m)
  