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{-# LANGUAGE NoImplicitPrelude #-}
module Data.Maybe where
import Data.Bool
import Data.Eq
data Maybe a = Just a | Nothing
maybe n _ Nothing = n
maybe _ f (Just x) = f x
isJust Nothing = False
isJust _ = True
isNothing Nothing = True
isNothing _ = False
fromJust (Just x) = x
-- fromJust Nothing = error "Maybe.fromJust: Nothing"
fromMaybe d x = case x of Nothing -> d
Just v -> v
maybeToList Nothing = []
maybeToList (Just x) = [x]
listToMaybe [] = Nothing
listToMaybe (x:_) = Just x
-- listToMaybe = foldr (const . Just) Nothing
-- GHC uses this to fuse via the foldr/build rule.
catMaybes ls = [ x | Just x <- ls]
mapMaybes _ [] = []
mapMaybes f (x:xs) = let rs = mapMaybes f xs
in case f x of Nothing -> rs
Just r -> r:rs
instance Eq a => Eq (Maybe a) where
Nothing == Nothing = True
Just x == Just y = x==y
_ == _ = False
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