fromMaybe package:from-sum

The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; 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 Text.Read ( readMaybe )

>>> fromMaybe 0 (readMaybe "5")
5

>>> fromMaybe 0 (readMaybe "")
0

Lift a Maybe to an ExceptT with a default value for the case when the Maybe is Nothing. If the Maybe is Just, then just return the value like normal:

>>> let justVal = Just True :: Maybe Bool

>>> fromMaybeExceptT 5 justVal :: ExceptT Int Identity Bool
ExceptT (Identity (Right True))

If the Maybe is Nothing, then use the default value as the error value:

>>> let nothingVal = Nothing :: Maybe Bool

>>> fromMaybeExceptT 5 nothingVal :: ExceptT Int Identity Bool
ExceptT (Identity (Left 5))

A monadic version of fromMaybe.

fromMaybeM nothingAction === maybe nothingAction pure

>>> fromMaybeM [] $ Just 5
[5]

>>> fromMaybeM [] Nothing
[]

Similar to fromMaybeExceptT except the Maybe value is lifted in a Monad.

>>> let identityNothing = Identity Nothing :: Identity (Maybe Bool)

>>> fromMaybeMExceptT 5 identityNothing :: ExceptT Int Identity Bool
ExceptT (Identity (Left 5))

Similar to fromMaybeM but the Maybe argument is also a monadic value.

>>> fromMaybeMM [] [Just 6, Just 5]
[6,5]

>>> fromMaybeMM [] [Just 6, Nothing, Just 7]
[6,7]

NOTE: I don't particularly like the name of this function. If you have a suggestion for a better name, please submit a PR or issue.

Similar to fromMaybeM, but only run the monadic nothingAction if the Maybe argument is Nothing. Otherwise, return pure mempty.

fromMaybeM_ nothingAction === maybe nothingAction (const $ pure mempty)

>>> fromMaybeM_ (putStrLn "hello" >> pure "bye") $ Just 5
""

>>> fromMaybeM_ (putStrLn "hello" >> pure "bye") Nothing
hello
"bye"

This can be convenient when you want to run some sort of logging function whenever a Maybe is Nothing. If you imagine the logging function is IO '()', then the effective type of fromMaybeM_ becomes fromMaybeM_ :: IO '()' -> Maybe a -> IO '()', because '()' has a Monoid instance, and IO, has an Applicative instance.

>>> fromMaybeM_ (putStrLn "hello") Nothing
hello

A fliped version of fromMaybe.

Just like fromMaybeExceptT but with the arguments flipped.

A fliped version of fromMaybeM.

>>> fromMaybeOrM (Just 5) []
[5]

This can be nice to use as an error handler.

>>> fromMaybeOrM (Just 5) $ putStrLn "some error occurred" >> undefined
5

>>> fromMaybeOrM (Nothing) $ putStrLn "some error occurred" >> undefined
some error occurred
...

Just like fromMaybeMExceptT but with the arguments flipped.

A fliped version of fromMaybeMM.

A fliped version of fromMaybeM.