Bool

Convenient commonly used and very helpful functions to work with Bool and also with monads.

Documentation

bool :: a -> a -> Bool -> a

Reversed version of if-then-else.

>>> bool 5 10 True
10
>>> bool 5 10 False
5

guard :: Alternative f => Bool -> f ()

guard b is pure () if b is True, and empty if b is False.

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

Monadic version of guard. Occasionally useful. 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 @

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

Monadic version of if-then-else.

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

unless :: Applicative f => Bool -> f () -> f ()

The reverse of when.

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

Monadic version of unless.

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

when :: Applicative f => Bool -> f () -> f ()

Conditional execution of Applicative expressions. For example,

when debug (putStrLn "Debugging")

will output the string Debugging if the Boolean value debug is True, and otherwise do nothing.

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

Monadic version of when.

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