Making input and output command in programming language

Question

so I'm making some basic programming language (just for exercise). I've made simple grammar using ANTLR.

Let's use this as example of simple program.

begin
    int a;
    a = 3+4*4;
end

And let's test if that runs with following java class.

public class Test {
    public static void main(String[] args) throws RecognitionException {    
        CharStream charStream = new ANTLRStringStream("here goes the code");
        LangLexer lexer = new LangLexer(charStream);
        TokenStream tokeStream = new CommonTokenStream(lexer);
        LangParser parser = new LangParser(tokeStream);
        Lang.program();
        System.out.println("done");
    }
}

Now I'm stuck. I want to make, for example "input x" and "print x". So that when you have "input x" in your code, the program wants you to enter some number, and stores given value in var X. And with print X it outputs that value in console.

begin
    int a, b, c;
    input a;
    input b;
    c = a + b;
    print c;
end

Any ideas and suggestions? Thanks!

Answer 1

How to do that exactly is hard to explain since you need a lot of underlying stuff (grammar, AST walker, scope/symbol-table, etc.).

I've written a blog that explains how to create a small programming language. If you read through it and/or download the zip of the last part, you only need to add a couple of things to support user-input. These are the changes you need to make:

In file TL.g:

atom
  :  Number
  |  Bool
  |  Null
  |  lookup
  |  Input '(' String? ')' -> ^(Input String?) // added this line
  ;

Input : 'input'; // added this rule

In file TLTreeWalker.g:

expression returns [TLNode node]
  :  ^(TERNARY a=expression b=expression c=expression) {node = new TernaryNode($a.node, $b.node, $c.node);}
  |  ^(In a=expression b=expression)                   {node = new InNode($a.node, $b.node);}
  |  ^('||' a=expression b=expression)                 {node = new OrNode($a.node, $b.node);}
  |  ^('&&' a=expression b=expression)                 {node = new AndNode($a.node, $b.node);}
  |  ^('==' a=expression b=expression)                 {node = new EqualsNode($a.node, $b.node);}
  |  ^('!=' a=expression b=expression)                 {node = new NotEqualsNode($a.node, $b.node);}
  |  ^('>=' a=expression b=expression)                 {node = new GTEqualsNode($a.node, $b.node);}
  |  ^('<=' a=expression b=expression)                 {node = new LTEqualsNode($a.node, $b.node);}
  |  ^('>' a=expression b=expression)                  {node = new GTNode($a.node, $b.node);}
  |  ^('<' a=expression b=expression)                  {node = new LTNode($a.node, $b.node);}
  |  ^('+' a=expression b=expression)                  {node = new AddNode($a.node, $b.node);}
  |  ^('-' a=expression b=expression)                  {node = new SubNode($a.node, $b.node);}
  |  ^('*' a=expression b=expression)                  {node = new MulNode($a.node, $b.node);}
  |  ^('/' a=expression b=expression)                  {node = new DivNode($a.node, $b.node);}
  |  ^('%' a=expression b=expression)                  {node = new ModNode($a.node, $b.node);}
  |  ^('^' a=expression b=expression)                  {node = new PowNode($a.node, $b.node);}
  |  ^(UNARY_MIN a=expression)                         {node = new UnaryMinusNode($a.node);}
  |  ^(NEGATE a=expression)                            {node = new NegateNode($a.node);}
  |  Number                                            {node = new AtomNode(Double.parseDouble($Number.text));}
  |  Bool                                              {node = new AtomNode(Boolean.parseBoolean($Bool.text));}
  |  Null                                              {node = new AtomNode(null);}
  |  lookup                                            {node = $lookup.node;}
  |  ^(Input String?)                                  {node = new InputNode($String.text);} // added this line
  ;

Add the following class:

package tl.tree.functions;

import tl.TLValue;
import tl.tree.TLNode;

import java.io.PrintStream;
import java.util.Scanner;

public class InputNode implements TLNode {

    private String prompt;
    private PrintStream out;

    public InputNode(String p) {
        this(p, System.out);
    }

    public InputNode(String p, PrintStream o) {
        prompt = (p == null) ? "" : p;
        out = o;
    }

    @Override
    public TLValue evaluate() {
        out.println(prompt);
        Scanner keyboard = new Scanner(System.in);

        if(keyboard.hasNextDouble()) 
            return new TLValue(Double.valueOf(keyboard.nextDouble()));
        else if(keyboard.hasNextInt()) 
            return new TLValue(Integer.valueOf(keyboard.nextInt()));
        else if(keyboard.hasNextBoolean()) 
            return new TLValue(Boolean.valueOf(keyboard.nextBoolean()));
        else 
            return new TLValue(keyboard.nextLine().trim()); // else it's a plain string
    }
}

And if you now evaluate the input:

a = 10;
b = input("Enter a number: ");
println(a + b);

you will be prompted with the message Enter a number: and this number is then added to a and will be printed to the console.