gistfile1.hs

import System.Environment
import System.IO
import System.Exit
import Control.Monad (filterM)
import qualified Data.Text as T
import qualified Data.Text.Lazy as TL
import qualified Data.Text.ICU.Regex as TIR
import qualified Data.Text.Lazy.IO as TILO

main :: IO ()
main = do
    args <- getArgs
    if length args < 1 
        then do
            hPutStrLn stderr "Usage: mygrep pattern [file1] [file2] ..."
            exitWith (ExitFailure 1)
        else
            do
                reg  <- TIR.regex [] (T.pack . head $ args)
                if length args == 1
                    then doesGrepLinesFromHandle reg stdin
                    else
                        mapM_ (\fn ->
                                withFile fn ReadMode $ \inh ->
                                    doesGrepLinesFromHandle reg inh) $ drop 1 args
                exitSuccess

matchesLine:: TIR.Regex -> TL.Text -> IO Bool
matchesLine reg line =
    do
        TIR.setText reg $ TL.toStrict line
        TIR.find reg 0


doesGrepLinesFromHandle:: TIR.Regex -> Handle -> IO ()
doesGrepLinesFromHandle reg inh =
    do
        contents <- TILO.hGetContents inh
        filtered <- filterM (matchesLine reg) $ TL.lines contents
        mapM_ TILO.putStrLn filtered