functor_._haskell

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Functor . Haskell

Haskell class

-- 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)

Discussion

In Haskell, the first law implies the second.

Notes

Reference

Haskell, Functor, Functor category