Documentation

Basic usage:

>>> maybe False odd (Just 3) True 

>>> maybe False odd Nothing False 

Read an integer from a string using readMaybe. If we succeed, return twice the integer; that is, apply (*2) to it. If instead we fail to parse an integer, return 0 by default:

>>> import GHC.Internal.Text.Read ( readMaybe ) >>> maybe 0 (*2) (readMaybe "5") 10 >>> maybe 0 (*2) (readMaybe "") 0 

Apply show to a Maybe Int. If we have Just n, we want to show the underlying Int n. But if we have Nothing, we return the empty string instead of (for example) "Nothing":

>>> maybe "" show (Just 5) "5" >>> maybe "" show Nothing "" 

Basic usage:

>>> isJust (Just 3) True 

>>> isJust (Just ()) True 

>>> isJust Nothing False 

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing) True 

Basic usage:

>>> isNothing (Just 3) False 

>>> isNothing (Just ()) False 

>>> isNothing Nothing True 

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing) False 

Basic usage:

>>> fromJust (Just 1) 1 

>>> 2 * (fromJust (Just 10)) 20 

>>> 2 * (fromJust Nothing) *** Exception: Maybe.fromJust: Nothing ... 

WARNING: This function is partial. You can use case-matching instead.

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 GHC.Internal.Text.Read ( readMaybe ) >>> fromMaybe 0 (readMaybe "5") 5 >>> fromMaybe 0 (readMaybe "") 0 

Basic usage:

>>> listToMaybe [] Nothing 

>>> listToMaybe [9] Just 9 

>>> listToMaybe [1,2,3] Just 1 

Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:

>>> maybeToList $ listToMaybe [5] [5] >>> maybeToList $ listToMaybe [] [] 

But not on lists with more than one element:

>>> maybeToList $ listToMaybe [1,2,3] [1] 

Basic usage:

>>> maybeToList (Just 7) [7] 

>>> maybeToList Nothing [] 

One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:

>>> import GHC.Internal.Text.Read ( readMaybe ) >>> sum $ maybeToList (readMaybe "3") 3 >>> sum $ maybeToList (readMaybe "") 0 

Basic usage:

>>> catMaybes [Just 1, Nothing, Just 3] [1,3] 

When constructing a list of Maybe values, catMaybes can be used to return all of the "success" results (if the list is the result of a map, then mapMaybe would be more appropriate):

>>> import GHC.Internal.Text.Read ( readMaybe ) >>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ] [Just 1,Nothing,Just 3] >>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ] [1,3] 

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import GHC.Internal.Text.Read ( readMaybe ) >>> let readMaybeInt = readMaybe :: String -> Maybe Int >>> mapMaybe readMaybeInt ["1", "Foo", "3"] [1,3] >>> catMaybes $ map readMaybeInt ["1", "Foo", "3"] [1,3] 

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3] [1,2,3]