{-# 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 -- --------------------------------------------------------------------------- -- The Maybe type, and instances -- | The 'Maybe' type encapsulates an optional value. A value of type -- @'Maybe' a@ either contains a value of type @a@ (represented as @'Just' a@), -- or it is empty (represented as 'Nothing'). Using 'Maybe' is a good way to -- deal with errors or exceptional cases without resorting to drastic -- measures such as 'error'. -- -- The 'Maybe' type is also a monad. It is a simple kind of error -- monad, where all errors are represented by 'Nothing'. A richer -- error monad can be built using the 'Data.Either.Either' type. data Maybe a = Nothing | Just a deriving (Eq, Ord) instance Functor Maybe where fmap _ Nothing = Nothing fmap f (Just a) = Just (f a) instance Monad Maybe where (Just x) >>= k = k x Nothing >>= _ = Nothing (Just _) >> k = k Nothing >> _ = Nothing return = Just fail _ = Nothing -- --------------------------------------------------------------------------- -- 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. 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 _@. isJust :: Maybe a -> Bool isJust Nothing = False isJust _ = True -- | The 'isNothing' function returns 'True' iff its argument is 'Nothing'. 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'. fromJust :: Maybe a -> a fromJust Nothing = error "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'. 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'. 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. 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. 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 just @'Just' b@, then @b@ is -- included in the result list. 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 #-} {-# NOINLINE [0] mapMaybeFB #-} 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