haskell script to show your reading progress of the haskellbook

hbprogress.hs

#!/usr/bin/env stack
-- stack runghc --resolver lts-11.2 --install-ghc
import Text.Read (readMaybe)
import System.Environment (getArgs)

progress :: Integer -> String
progress pag = show $ fromIntegral ( div (pag * 10000) 1971) / 100

showProgress :: [String] -> IO()
showProgress args =
  case (readMaybe $ head args) of
    Nothing -> putStrLn $ "Can't parse a page number from args\n\n" ++ usageMsg
    Just page -> putStrLn ("You have completed " ++ (progress page)
                          ++ "% of the book")

usageMsg :: String
usageMsg = ("Usage: hbprogress [the page you are reading]\n"
         ++ "note: n / 1971 in case of the e-reader version")


main :: IO()
main = do
  args <- getArgs
  if args == [] then
    putStrLn usageMsg
  else
    showProgress args