guard

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.

Succeed only if the extension is disabled.

Succeed only if the extension is enabled.

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