Modulerelude-1.2.0.0Haskell2010

Relude.Bool.Guard

SPDX-License-Identifier : MIT Maintainer : Kowainik xrom.xkov@gmail.com Stability : Stable Portability : Portable

Monadic boolean combinators.

7 values

Packagerelude-1.2.0.0
Exports7
LanguageHaskell2010
LicenceMIT
SourceGuard.hs

valueguarded :: Alternative f => (a -> Bool) -> a -> f a

Either lifts a value into an alternative context or gives a minimal value depending on a predicate. Works with Alternatives.

Example4 expressions

>>> guarded even 3 :: [Int][]>>> guarded even 2 :: [Int][2]>>> guarded (const True) "hello" :: Maybe StringJust "hello">>> guarded (const False) "world" :: Maybe StringNothing

You can use this function to implement smart constructors simpler:

newtype HttpHost = HttpHost
    { unHttpHost :: Text
    }

mkHttpHost :: Text -> Maybe HttpHost
mkHttpHost host = HttpHost <$> guarded (not . Text.null) host

valueguardM :: MonadPlus m => m Bool -> m ()

Monadic version of guard that help to check that a condition (Bool) holds inside. Works with Monads that are also Alternative.

Example3 expressions

>>> guardM (Just True)Just ()>>> guardM (Just False)Nothing>>> guardM NothingNothing

Here some complex but real-life example:

findSomePath :: IO (Maybe FilePath)

somePath :: MaybeT IO FilePath
somePath = do
    path <- MaybeT findSomePath
    guardM $ liftIO $ doesDirectoryExist path
    return path

valueifM :: Monad m => m Bool -> m a -> m a -> m a

Monadic version of if-then-else.

Example2 expressions

>>> ifM (pure True) (putTextLn "True text") (putTextLn "False text")True text>>> ifM (pure False) (putTextLn "True text") (putTextLn "False text")False text

valueunlessM :: Monad m => m Bool -> m () -> m ()

Monadic version of unless. Reverse of whenM. Conditionally don't execute the provided action.

Example2 expressions

>>> unlessM (pure False) $ putTextLn "No text :("No text :(>>> unlessM (pure True) $ putTextLn "Yes text :)"

valuewhenM :: Monad m => m Bool -> m () -> m ()

Monadic version of when. Conditionally executes the provided action.

Example5 expressions

>>> whenM (pure False) $ putTextLn "No text :(">>> whenM (pure True)  $ putTextLn "Yes text :)"Yes text :)>>> whenM (Just True) (pure ())Just ()>>> whenM (Just False) (pure ())Just ()>>> whenM Nothing (pure ())Nothing

value(&&^) :: Monad m => m Bool -> m Bool -> m Bool

Monadic version of (Data.Bool.&&) operator.

It is lazy by the second argument (similar to (Data.Bool.||)), meaning that if the first argument is False, the function will return False without evaluating the second argument.

Example5 expressions

>>> Just False &&^ Just TrueJust False>>> Just True &&^ Just TrueJust True>>> Just True &&^ NothingNothing>>> Just False &&^ NothingJust False>>> Just False &&^ error "Shouldn't be evaluated"Just False

value(||^) :: Monad m => m Bool -> m Bool -> m Bool

Monadic version of (Data.Bool.||) operator.

It is lazy by the second argument (similar to (Data.Bool.||)), meaning that if the first argument is True, the function will return True without evaluating the second argument.

Example5 expressions

>>> Just False ||^ Just TrueJust True>>> Just False ||^ Just FalseJust False>>> Just False ||^ NothingNothing>>> Just True ||^ NothingJust True>>> Just True ||^ error "Shouldn't be evaluated"Just True