Running haskell executable using Java ProcessBuilder: Why does hGetLine only return when I close outputStream?

Question

I'm wrapping the FM-SBLEX Saldo program in a Java library.

Saldo is written in Haskell and does lookups in a lexicon for lines given on stdin, e.g.

echo "ord"|./sblex/bin/saldo dicts/saldo.dict

prints something like the following to stdout

{"ord":{"s_1":{"word":"ord","head":"sanna mina ord","pos":"abm","param":"invar 1:3-3","inhs":[],"id":"sanna_mina_ord..abm.1","p":"abm_i_till_exempel","attr":"3"},...

If I run it with

./sblex/bin/saldo dicts/saldo.dict

it does a lookup for each line I enter at the console until I send EOF.

In my Java library, I start it with ProcessBuilder and set up a thread to dump stdout and stderr to my program's stdout, and another thread writes a word and a newline, then flushes outputStream

On the console, saldo returns results each time I press return, but in my wrapper, it returns results for all my input only once I close outputStream (see .close() is commented out in the next code block)

    ProcessBuilder pb = new ProcessBuilder(binPath, dictPath);

    pb.redirectErrorStream(true);
    saldoProcess = pb.start();

    new Thread(new Reader(saldoProcess.getInputStream())).start();
    new Thread(new Writer(saldoProcess.getOutputStream())).start();

    saldoProcess.waitFor();
    System.out.println("saldo exited.");
    Thread.sleep(2000);

Writer's run override:

    public void run() {
        try {
            outputStream.write("ord\n".getBytes());
            outputStream.flush();
            //outputStream.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

The Haskell code that reads the input

run' :: Language a => a -> (String -> [Tok]) -> (String -> [[String]]) -> AnaType -> Stats -> IO Stats
run' l tokenizer f a st = 
 do b <- hIsEOF stdin
    if b then return st 
     else do 
       s <- hGetLine stdin
       analyze l a f (tokenizer s) st >>= run' l tokenizer f a

If binPath="cat" and dictPath="-" my java program outputs the input after each flush. Any idea why this haskell program is only dealing with the input after I close the outputStream?

NB: as the answer shows, it was not hGetLine that didn't return (as I assumed) but the output that was buffered because the Haskell implementation I'm using defaults to block buffer if it's not run from the console.

Answer 1

Your Haskell program is probably buffering its output. (Line-buffered when it's writing to a terminal, block-buffered when writing to anything else.)

Try adding

hSetBuffering stdout LineBuffering

near the start of the program.

More about buffering in Haskell.

(Edited in response to Daniel Wagner's comment.)