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1. traceMaybe :: forall (m :: Type -> Type) a b . Monad m => (a -> Maybe b) -> Tracer m b -> Tracer m a

   contra-tracer	Control.Tracer

   Run a tracer only for the Just variant of a Maybe. If it's Nothing, the nullTracer is used (no output). The arrow representation allows for proper laziness: if the tracer parameter does not produce any tracing effects, then the predicate won't even be evaluated. Contrast with the simple contravariant representation as a -> m (), in which the predicate _must_ be forced no matter what, because it's impossible to know a priori whether that function will not produce any tracing effects. It's written out explicitly for demonstration. Could also use arrow notation:

   traceMaybe p tr = Tracer $ proc a -> do
   case k a of
   Just b  -> use tr        -< b
   Nothing -> Arrow.squelch -< ()
   

2. traceMaybeM :: Monad m => (a -> m (Maybe b)) -> Tracer m b -> Tracer m a

   contra-tracer	Control.Tracer

   A monadic version of traceMaybe.

3. mapMaybeWithKey :: GCompare k2 => (forall (v :: k1) . () => k2 v -> f v -> Maybe (g v)) -> MonoidalDMap k2 f -> MonoidalDMap k2 g

   dependent-monoidal-map	Data.Dependent.Map.Monoidal

   O(n). Map keys/values and collect the Just results.

4. fromMaybe :: a -> Maybe a -> a

   distribution-opensuse	OpenSuse.Prelude

   The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

   Examples

   Basic usage:

   >>> fromMaybe "" (Just "Hello, World!")
   "Hello, World!"
   

   >>> fromMaybe "" Nothing
   ""
   

   Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

   >>> import GHC.Internal.Text.Read ( readMaybe )
   
   >>> fromMaybe 0 (readMaybe "5")
   5
   
   >>> fromMaybe 0 (readMaybe "")
   0
   

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

   distribution-opensuse	OpenSuse.Prelude

   Monadic generalisation of fromMaybe.

6. mapMaybeM :: Monad m => (a -> m (Maybe b)) -> [a] -> m [b]

   distribution-opensuse	OpenSuse.Prelude

   A version of mapMaybe that works with a monadic predicate.

7. whenMaybe :: Applicative m => Bool -> m a -> m (Maybe a)

   distribution-opensuse	OpenSuse.Prelude

   Like when, but return either Nothing if the predicate was False, or Just with the result of the computation.

   whenMaybe True  (print 1) == fmap Just (print 1)
   whenMaybe False (print 1) == pure Nothing
   

8. whenMaybeM :: Monad m => m Bool -> m a -> m (Maybe a)

   distribution-opensuse	OpenSuse.Prelude

   Like whenMaybe, but where the test can be monadic.

9. toMaybe :: Bool -> a -> Maybe a

   dsp	DSP.Basic

   generates a Just if the given condition holds

10. throwMaybe :: forall (m :: Type -> Type) e a . Monad m => MSF (ExceptT e m) (Maybe e) (Maybe a)

    dunai	Control.Monad.Trans.MSF.Except

    When the input is Just e, throw the exception e. (Does not output any actual data.)

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