guard -package:pandoc

Conditional failure of Alternative computations. Defined by

guard True  = pure ()
guard False = empty

Examples

Common uses of guard include conditionally signaling an error in an error monad and conditionally rejecting the current choice in an Alternative-based parser. As an example of signaling an error in the error monad Maybe, consider a safe division function safeDiv x y that returns Nothing when the denominator y is zero and Just (x `div` y) otherwise. For example:

>>> safeDiv 4 0
Nothing

>>> safeDiv 4 2
Just 2

A definition of safeDiv using guards, but not guard:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y | y /= 0    = Just (x `div` y)
| otherwise = Nothing

A definition of safeDiv using guard and Monad do-notation:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y = do
guard (y /= 0)
return (x `div` y)

Conditional failure of Alternative computations. Defined by

guard True  = pure ()
guard False = empty

Examples

Common uses of guard include conditionally signaling an error in an error monad and conditionally rejecting the current choice in an Alternative-based parser. As an example of signaling an error in the error monad Maybe, consider a safe division function safeDiv x y that returns Nothing when the denominator y is zero and Just (x `div` y) otherwise. For example:

>>> safeDiv 4 0
Nothing
>>> safeDiv 4 2
Just 2

A definition of safeDiv using guards, but not guard:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y | y /= 0    = Just (x `div` y)
| otherwise = Nothing

A definition of safeDiv using guard and Monad do-notation:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y = do
guard (y /= 0)
return (x `div` y)

Conditional failure, returning only if the condition is True.

Generalization of guard

Monadic boolean combinators.

This module contains monadic predicates.

g `guards` f : when the predicate g holds, f is applied, else none

like whenP

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

>>> guardM (Just True)
Just ()

>>> guardM (Just False)
Nothing

>>> guardM Nothing
Nothing

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

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

>>> guarded even 3 :: [Int]
[]

>>> guarded even 2 :: [Int]
[2]

>>> guarded (const True) "hello" :: Maybe String
Just "hello"

>>> guarded (const False) "world" :: Maybe String
Nothing

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

Guard using an arbitrary function on the Request.

Interior editing: g guards f applies f only when the predicate g succeeds, otherwise the content is discarded.