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data List a = Nil
| Cons a (List a)
deriving Show
xs = Cons 1 (Cons 2 (Cons 3 Nil))
reduceList f z Nil = z
reduceList f z (Cons x xs) = f x (reduceList f z xs)
idList = reduceList Cons Nil
mapList f = reduceList (Cons . f) Nil
filterList p = reduceList (\x xs ->
if p x then Cons x xs
else xs)
mapListIf p f = reduceList (\x xs ->
if p x then Cons (f x) xs
else Cons x xs)
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