nua.hs

{-# LANGUAGE RankNTypes #-}

import Data.Traversable (for)

import Data.Functor.Identity
import Data.Functor.Constant

type Lens s t a b = forall f . Applicative f => (a -> f b) -> (s -> f t)

views :: Lens s t a b -> s -> [a]
views lens s = getConstant (lens (Constant . (:[])) s)

over :: Lens s t a b -> (a -> b) -> s -> t
over lens partial s = runIdentity (lens (Identity . partial) s)

only :: (a -> Bool) -> Lens [a] [a] a a
only pred action list = 
    for list $ \elem ->
        if pred elem 
        then action elem
        else pure elem

at :: Int -> Lens [a] [a] a a
at i action list = 
    for (zip [0..] list) $ \(j, x) ->
        if i == j
        then action x
        else pure x

fromString = read
toString = show

asNumber :: Lens String String Int Int
asNumber action str = fmap toString (action (fromString str))

test = over (only (> "3") . asNumber) (+10) ["1","2","3","4","5"]
-- evals to ["1","2","3","14","15"]