Data/Maybe.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Maybe
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  stable
    -- Portability :  portable
    --
    -- The Maybe type, and associated operations.
    --
    -----------------------------------------------------------------------------

    module Data.Maybe
       (
         Maybe(Nothing,Just)

       , maybe

       , isJust
       , isNothing
       , fromJust
       , fromMaybe
       , listToMaybe
       , maybeToList
       , catMaybes
       , mapMaybe
       ) where

    import GHC.Base

    -- $setup
    -- Allow the use of some Prelude functions in doctests.
    -- >>> import Prelude ( (*), odd, show, sum )

    -- ---------------------------------------------------------------------------
    -- Functions over Maybe

    -- | The 'maybe' function takes a default value, a function, and a 'Maybe'
    -- value.  If the 'Maybe' value is 'Nothing', the function returns the
    -- default value.  Otherwise, it applies the function to the value inside
    -- the 'Just' and returns the result.
    --
    -- ==== __Examples__
    --
    -- 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 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
    -- ""
    --
    maybe :: b -> (a -> b) -> Maybe a -> b
    maybe n _ Nothing  = n
    maybe _ f (Just x) = f x

    -- | The 'isJust' function returns 'True' iff its argument is of the
    -- form @Just _@.
    --
    -- ==== __Examples__
    --
    -- Basic usage:
    --
    -- >>> isJust (Just 3)
    -- True
    --
    -- >>> isJust (Just ())
    -- True
    --
    -- >>> isJust Nothing
    -- False
    --
    -- Only the outer constructor is taken into consideration:
    --
    -- >>> isJust (Just Nothing)
    -- True
    --
    isJust         :: Maybe a -> Bool
    isJust Nothing = False
    isJust _       = True

    -- | The 'isNothing' function returns 'True' iff its argument is 'Nothing'.
    --
    -- ==== __Examples__
    --
    -- Basic usage:
    --
    -- >>> isNothing (Just 3)
    -- False
    --
    -- >>> isNothing (Just ())
    -- False
    --
    -- >>> isNothing Nothing
    -- True
    --
    -- Only the outer constructor is taken into consideration:
    --
    -- >>> isNothing (Just Nothing)
    -- False
    --
    isNothing         :: Maybe a -> Bool
    isNothing Nothing = True
    isNothing _       = False

    -- | The 'fromJust' function extracts the element out of a 'Just' and
    -- throws an error if its argument is 'Nothing'.
    --
    -- ==== __Examples__
    --
    -- Basic usage:
    --
    -- >>> fromJust (Just 1)
    -- 1
    --
    -- >>> 2 * (fromJust (Just 10))
    -- 20
    --
    -- >>> 2 * (fromJust Nothing)
    -- *** Exception: Maybe.fromJust: Nothing
    --
    fromJust          :: Maybe a -> a
    fromJust Nothing  = errorWithoutStackTrace "Maybe.fromJust: Nothing" -- yuck
    fromJust (Just x) = x

    -- | 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
    --
    fromMaybe     :: a -> Maybe a -> a
    fromMaybe d x = case x of {Nothing -> d;Just v  -> v}

    -- | The 'maybeToList' function returns an empty list when given
    -- 'Nothing' or a singleton list when not given 'Nothing'.
    --
    -- ==== __Examples__
    --
    -- 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 Text.Read ( readMaybe )
    -- >>> sum $ maybeToList (readMaybe "3")
    -- 3
    -- >>> sum $ maybeToList (readMaybe "")
    -- 0
    --
    maybeToList            :: Maybe a -> [a]
    maybeToList  Nothing   = []
    maybeToList  (Just x)  = [x]

    -- | The 'listToMaybe' function returns 'Nothing' on an empty list
    -- or @'Just' a@ where @a@ is the first element of the list.
    --
    -- ==== __Examples__
    --
    -- 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]
    --
    listToMaybe           :: [a] -> Maybe a
    listToMaybe []        =  Nothing
    listToMaybe (a:_)     =  Just a

    -- | The 'catMaybes' function takes a list of 'Maybe's and returns
    -- a list of all the 'Just' values.
    --
    -- ==== __Examples__
    --
    -- 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 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]
    --
    catMaybes              :: [Maybe a] -> [a]
    catMaybes ls = [x | Just x <- ls]

    -- | The 'mapMaybe' function is a version of 'map' which can throw
    -- out elements.  In particular, the functional argument returns
    -- something of type @'Maybe' b@.  If this is 'Nothing', no element
    -- is added on to the result list.  If it is @'Just' b@, then @b@ is
    -- included in the result list.
    --
    -- ==== __Examples__
    --
    -- Using @'mapMaybe' f x@ is a shortcut for @'catMaybes' $ 'map' f x@
    -- in most cases:
    --
    -- >>> import 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]
    --
    mapMaybe          :: (a -> Maybe b) -> [a] -> [b]
    mapMaybe _ []     = []
    mapMaybe f (x:xs) =
     let rs = mapMaybe f xs in
     case f x of
      Nothing -> rs
      Just r  -> r:rs
    {-# NOINLINE [1] mapMaybe #-}

    {-# RULES
    "mapMaybe"     [~1] forall f xs. mapMaybe f xs
                        = build (\c n -> foldr (mapMaybeFB c f) n xs)
    "mapMaybeList" [1]  forall f. foldr (mapMaybeFB (:) f) [] = mapMaybe f
      #-}

    {-# INLINE [0] mapMaybeFB #-} -- See Note [Inline FB functions] in GHC.List
    mapMaybeFB :: (b -> r -> r) -> (a -> Maybe b) -> a -> r -> r
    mapMaybeFB cons f x next = case f x of
      Nothing -> next
      Just r -> cons r next