Functor . Haskell

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-- definition
class Functor f where
    fmap :: (a -> b) -> f a -> f b
 
-- methods
    (<$) :: a -> f b -> f a
    (<$) = fmap . const

fmap id $\ \leftrightsquigarrow\ $ id

fmap f . fmap g $\ \leftrightsquigarrow\ $ fmap (f . g)

In Haskell, the first law implies the second.