sed.hs

import qualified Data.Text.Lazy as T (Text, lines, unlines)
import qualified Data.Text.Lazy.IO as TIO (interact)

data Command = Next
             | Print

sed :: T.Text -> T.Text
sed t = T.unlines $ concat $ map (runCommands [Print]) $ T.lines t

runCommands :: [Command] -> T.Text -> [T.Text]
runCommands cs xs = map (runCommand xs) cs

runCommand :: T.Text -> Command -> T.Text
runCommand t Print = t
runCommand t Next = undefined

main :: IO ()
main = TIO.interact sed