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jaspervdj/list-vs-non-empty.hs

Created April 3, 2019 07:37
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Is List or NonEmpty "simpler"? Neither.
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list-vs-non-empty.hs
--------------------------------------------------------------------------------
-- Is List or NonEmpty "simpler"? Neither:
type NonEmpty a = Fix (Compose ((,) a) Maybe)
type List a = Fix (Compose Maybe ((,) a))
--------------------------------------------------------------------------------
newtype Fix f = Fix {unFix :: f (Fix f)}
newtype Compose f g a = Compose {unCompose :: f (g a)}
instance (Functor f, Functor g) => Functor (Compose f g) where
fmap f = Compose . fmap (fmap f) . unCompose
--------------------------------------------------------------------------------
ne1, ne2 :: NonEmpty Int
ne1 = Fix (Compose (1, Nothing))
ne2 = Fix (Compose (1, Just (Fix (Compose (2, Nothing)))))
l0, l1, l2 :: List Int
l0 = Fix (Compose Nothing)
l1 = Fix (Compose (Just (1, Fix (Compose Nothing))))
l2 = Fix (Compose (Just (1, Fix (Compose (Just (2, Fix (Compose Nothing)))))))
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