Build a
Lens from a getter and a setter.
lens :: Functor f => (s -> a) -> (s -> b -> t) -> (a -> f b) -> s -> f t
>>> s ^. lens getter setter
getter s
>>> s & lens getter setter .~ b
setter s b
>>> s & lens getter setter %~ f
setter s (f (getter s))
lens :: (s -> a) -> (s -> a -> s) -> Lens' s a