Add CAI project ID to Bruker file (demo)

AddProjToBruker.hs

module AddProjToBruker where

-- ##$SUBJECT_comment=( 2048 )
-- <CAI:10001>

import Control.Monad (when)
import Data.Either
import Data.List
import Data.Maybe
import System.Environment
import System.FilePath

is5digits :: String -> Bool
is5digits s = (length s == 5) && (isJust $ (readMaybe s :: Maybe Integer))
  where
    readMaybe :: (Read a) => String -> Maybe a
    readMaybe s =
      case reads s of
          [(a, "")] -> Just a
          _         -> Nothing

addProjectIdLine :: Integer -> [String] -> Either [String] [String]
addProjectIdLine projID [] = Left []
addProjectIdLine projID (x:xs) = if "##$SUBJECT_comment=" `isPrefixOf` x
                                    then Right (x:newProjectLine:xs)
                                    else fmap (x:) (addProjectIdLine projID xs)
    where newProjectLine = "<CAI:" ++ show projID ++ ">"

go :: String -> FilePath -> IO ()
go projID dir = do
    let f = dir </> "before"

    x <- readFile f

    if is5digits projID
        then case addProjectIdLine (read projID) (lines x) of
                Left _   -> putStrLn $ "Error: did not find a SUBJECT line in " ++ f
                Right x' -> putStrLn $ unlines x'
        else
            putStrLn $ "Error: did not get a 5 digit project ID: " ++ projID

main :: IO ()
main = do
    args <- getArgs

    case args of
        ["--id", projID, dir] -> go projID dir
        _                      -> putStrLn $ "Bad arguments? " ++ show args

That's not even code, it reads like a 1980's choose your own adventure book.