read from ag handle - ran with `stack test :target`

spec.hs

-- | The `ag` search process.
agSearchProcess :: Maybe FilePath -> CreateProcess
agSearchProcess mdir = ag { std_out = CreatePipe
                          , std_err = CreatePipe
                          , cwd     = mdir
                          }
  where ag = proc "ag" ["--noheading", "TODO"]


main :: IO ()
main = do
  hspec $ describe "prerequisites" $
    it "should be able to locate and use ag - silver searcher" $ do
      (code, _, _) <- waitForProcessExit checkForAg
      unless (code == ExitSuccess) $ fail "ag could not be found."
      (_, mayStdOut, _, _) <- createProcess $ agSearchProcess Nothing
      void $ flip (maybe (fail "could not get ag stdout")) mayStdOut $ \stdOut -> do
        hSetBuffering stdOut NoBuffering 
        race_
          -- delay three seconds
          (threadDelay 3000000 >> fail "timed out waiting for input from ag stdout handle")
          $ fix $ \loop -> do
            isReady <- hReady stdOut
            unless isReady loop
        bytes <- B8.hGetContents stdOut
        bytes `shouldSatisfy` (not . B8.null)