Calculate string length with the Reader monad

StrLength.hs

import qualified Control.Monad.Trans.Reader as R

strLength :: R.Reader String Int
strLength = do
  s <- R.ask
  case s of
    [] -> return 0
    (_:cs) -> fmap (+1) (R.local (const cs) strLength)

main = do
  print (R.runReader strLength "abc") -- 3
  print (R.runReader strLength "abcde") -- 5