indexed.hs

{-# LANGUAGE OverloadedStrings #-}
import qualified Data.Text as T
import Control.Lens
import Data.Functor.Compose

newtype Mailbody = Mailbody [Paragraph]
  deriving (Show)

data Line = Line Int T.Text
  deriving (Show)

newtype Paragraph = Paragraph [Line]
  deriving (Show)


mailbody :: Mailbody
mailbody = Mailbody [ Paragraph [Line 0 "This is a long", Line 0 "line and so forth", Line 0 "bla"]
                    , Paragraph [Line 0 "And here", Line 0 "is another", Line 0 "long", Line 0 "one blah"]
                    ]

mbBody :: Traversal' Mailbody [Paragraph]
mbBody f (Mailbody xs) = fmap (\xs' -> Mailbody xs') (f xs)

pLines :: Traversal' Paragraph [Line]
pLines f (Paragraph xs) = fmap (\xs' -> Paragraph xs') (f xs)

lNumber :: Lens' Line Int
lNumber f (Line n t) = fmap (\n' -> Line n' t) (f n)

main :: IO ()
main = do
  let mb' = iover (indexing (mbBody . traversed . pLines . traversed)) (set lNumber) mailbody
  print mb'