The Haskell 98 Report
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18  Maybe Utilities


module Maybe(
    isJust, isNothing,
    fromJust, fromMaybe, listToMaybe, maybeToList,
    catMaybes, mapMaybe,

    -- ...and what the Prelude exports
    Maybe(Nothing, Just),
    maybe
  ) where

isJust, isNothing    :: Maybe a -> Bool
fromJust             :: Maybe a -> a
fromMaybe            :: a -> Maybe a -> a
listToMaybe          :: [a] -> Maybe a
maybeToList          :: Maybe a -> [a]
catMaybes            :: [Maybe a] -> [a]
mapMaybe             :: (a -> Maybe b) -> [a] -> [b]

The type constructor Maybe is defined in Prelude as

data Maybe a = Nothing | Just a

The purpose of the Maybe type is to provide a method of dealing with illegal or optional values without terminating the program, as would happen if error were used, and without using IOError from the IO monad, which would cause the expression to become monadic. A correct result is encapsulated by wrapping it in Just; an incorrect result is returned as Nothing.

Other operations on Maybe are provided as part of the monadic classes in the Prelude.

18.1  Library Maybe


module Maybe(
    isJust, isNothing,
    fromJust, fromMaybe, listToMaybe, maybeToList,
    catMaybes, mapMaybe,

    -- ...and what the Prelude exports
    Maybe(Nothing, Just),
    maybe
  ) where

isJust                 :: Maybe a -> Bool
isJust (Just a)        =  True
isJust Nothing         =  False

isNothing        :: Maybe a -> Bool
isNothing        =  not . isJust

fromJust               :: Maybe a -> a
fromJust (Just a)      =  a
fromJust Nothing       =  error "Maybe.fromJust: Nothing"

fromMaybe              :: a -> Maybe a -> a
fromMaybe d Nothing    =  d
fromMaybe d (Just a)   =  a

maybeToList            :: Maybe a -> [a]
maybeToList Nothing    =  []
maybeToList (Just a)   =  [a]

listToMaybe            :: [a] -> Maybe a
listToMaybe []         =  Nothing
listToMaybe (a:_)      =  Just a
 
catMaybes              :: [Maybe a] -> [a]
catMaybes ms           =  [ m | Just m <- ms ]

mapMaybe               :: (a -> Maybe b) -> [a] -> [b]
mapMaybe f             =  catMaybes . map f


The Haskell 98 Report
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December 2002