module Data.Maybe
(
Maybe(Nothing,Just)
, maybe
, isJust
, isNothing
, fromJust
, fromMaybe
, listToMaybe
, maybeToList
, catMaybes
, mapMaybe
) where
#ifdef __GLASGOW_HASKELL__
import GHC.Base
#endif
#ifdef __NHC__
import Prelude
import Prelude (Maybe(..), maybe)
import Maybe
( isJust
, isNothing
, fromJust
, fromMaybe
, listToMaybe
, maybeToList
, catMaybes
, mapMaybe
)
#else
#ifndef __HUGS__
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
maybe :: b -> (a -> b) -> Maybe a -> b
maybe n _ Nothing = n
maybe _ f (Just x) = f x
#endif /* __HUGS__ */
isJust :: Maybe a -> Bool
isJust Nothing = False
isJust _ = True
isNothing :: Maybe a -> Bool
isNothing Nothing = True
isNothing _ = False
fromJust :: Maybe a -> a
fromJust Nothing = error "Maybe.fromJust: Nothing"
fromJust (Just x) = x
fromMaybe :: a -> Maybe a -> a
fromMaybe d x = case x of {Nothing -> d;Just v -> v}
maybeToList :: Maybe a -> [a]
maybeToList Nothing = []
maybeToList (Just x) = [x]
listToMaybe :: [a] -> Maybe a
listToMaybe [] = Nothing
listToMaybe (a:_) = Just a
catMaybes :: [Maybe a] -> [a]
catMaybes ls = [x | Just x <- ls]
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
#endif /* else not __NHC__ */
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