Java – Simple haskell script with java

Try running a simple script in Haskell using Java code. ( window 10)

The haskell script looks like this:

import Data.Char(toUpper)
main = interact (map toUpper)

I made a simple.exe file with ghc and it works as expected for the cmd module. I wrote a simple string that replies with the same string in uppercase, and I can repeat this until I choose to stop the program.

But when I try to run this program through java, it won’t work like that. I can provide input, but in order to get the output, I need to close the input feed.

public class Main {
    public static void main(String[] args) {
        try {
            Process process = new ProcessBuilder("simple").start();

InputStream processInputStream = process.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(processInputStream));
            OutputStream processOutputStream = process.getOutputStream();
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(processOutputStream));
            char[] bt = ("Hello").toCharArray();
            writer.write(bt);
            writer.flush();
            writer.newLine();
            processOutputStream.close(); -- Only works if I close output stream,.
            System.out.println(reader.readLine());

}catch (IOException e) {
            e.printStackTrace();
        }
    }
}

How to get output without closing the output feed in a Java program.

Restarting the process works, but the execution time is terrible.

I tried threading the program and the result is the same.

BufferedWriter does not automatically refresh its output when you execute newLine(). So your Haskell program will not receive a newline and wait forever.

When you call close(), all remaining buffered output is flushed (of course, you don’t want to do that).

So the solution should be

writer.write(bt);
writer.newLine();
writer.flush();

You can also use PrintWriter, which has an auto-refresh line wrap option and understands strings (via the print method), so there’s no need to mess with the characters[].

Finally, if you plan to send

or send a large amount of text at the same time, you may need to resize the buffer or use multiple threads to avoid deadlocks because the receive Java-side buffer is full and you are not currently reading it. Look at this thread for solutions.