Using Haskell's GHCI through a Java Program

Question

What I would like to do is wrap my Java program around the GHCI. In my mind it should work like this:

Starting my Java program

Write down some Haskell function as Input for Java (i.e. reverse [1,2,3,4])

See the appropriate Haskell Output on my Java Console

Because I did not want to mess around with any language bridges I tried the clumsy way and used the Runtime.exec() approach.

This is my Code:

public static void main(String[] args) throws Exception {
Runtime r = Runtime.getRuntime();
Process p = r.exec("ghci");
OutputStream output = p.getOutputStream();
output.write("let x = 5\r\n".getBytes());
output.write("x".getBytes());

int tmp;
String result = "";
while ((tmp = p.getInputStream().read()) != -1) {
  result += (char) tmp;
}

System.out.println(result);
p.destroy();  }

My problem here is that the read() method always returns a -1 and I cannot get the output. I dont even know if what I wrote created any Output.

Help would be appreciated. Thanks!

Possible duplicate of Printing Runtime exec() OutputStream to console — Ami Hollander
– Ami Hollander, Commented Feb 23, 2018 at 16:13
That Answer did not solve my problem because if I read from another Thread the read() method still returns -1 — Mythos
– Mythos, Commented Feb 23, 2018 at 16:18

Abhijit Pritam Dutta · Accepted Answer · 2018-02-23 17:29:56Z

2

It is clear that Process p = r.exec("ghci"); did not successful for which read() method always returns a -1. Provide full path and check.

Process p = r.exec("/fullpath/ghci  2>&1");
p.waitFor();//You need to use this line of code

For confirmation first execute ls command first

Process p = r.exec("ls 2>&1");

Also modify your codes like below and try:-

public static void main(String[] args) throws Exception {
    Runtime r = Runtime.getRuntime();
    Process p = r.exec("ghci");
            p.waitFor();
    OutputStream output = p.getOutputStream();
    ByteArrayOutputStream byte1=new ByteArrayOutputStream();
    output.write(byte1.toByteArray());
    String result=byte1.toString();
    System.out.println(result);
    p.destroy();  
}

edited Feb 23, 2018 at 17:29

answered Feb 23, 2018 at 16:23

Abhijit Pritam Dutta

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9 Comments

Mythos Over a year ago

Adding the full path resulted in an error message that it wasnt able to find the specified file. I tried typing in just "ghci" in cmd (that worked) and in the bash VM (that worked as well).

Abhijit Pritam Dutta Over a year ago

what it returns in standard ouput?

Mythos Over a year ago

When I type the keyword "ghci" in my CMD I get this CMD output: GHCi, version 8.0.1: haskell.org/ghc :? for help Prelude> Same for bash

Abhijit Pritam Dutta Over a year ago

I have modified my answer. First just check with ls command.

Mythos Over a year ago

Thank you for taking your time! Unfortunately my read() method still returns -1 immediately when doing as you said. When I type the ls command in cmd it outputs the appropriate list of directorys Edit: to clarify: neither the "/myPathToGHCI/ghci 2>&1" nor the "ls 2>&1" worked

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