Entries tagged intepreters

Recently I've had an overwhelming desire to write a BASIC intepreter. I can't think why, but the idea popped into my mind, and wouldn't go away.

So I challenged myself to spend the weekend looking at it.

Writing an intepreter is pretty well-understood problem:

• Parse the input into tokens, such as "LET", "GOTO", "INT:3"
  • This is called lexical analysis / lexing.
• Taking those tokens and building an abstract syntax tree.
  • The AST
• Walking the tree, evaluating as you go.
  • Hey ho.

Of course BASIC is annoying because a program is prefixed by line-numbers, for example:

 10 PRINT "HELLO, WORLD"
 20 GOTO 10

The naive way of approaching this is to repeat the whole process for each line. So a program would consist of an array of input-strings each line being treated independently.

Anyway reminding myself of all this fun took a few hours, and during the course of that time I came across Writing an intepreter in Go which seems to be well-regarded. The book walks you through creating an interpreter for a language called "Monkey".

I found a bunch of implementations, which were nice and clean. So to give myself something to do I started by adding a new built-in function rnd(). Then I tested this:

let r = 0;
let c = 0;

for( r != 50 ) {
   let r = rnd();
   let c = c + 1;
}

puts "It took ";
puts c;
puts " attempts to find a random-number equalling 50!";

Unfortunately this crashed. It crashed inside the body of the loop, and it seemed that the projects I looked at each handled the let statement in a slightly-odd way - the statement wouldn't return a value, and would instead fall-through a case statement, hitting the next implementation.

For example in monkey-intepreter we see that happen in this section. (Notice how there's no return after the env.Set call?)

So I reported this as a meta-bug to the book author. It might be the master source is wrong, or might be that the unrelated individuals all made the same error - meaning the text is unclear.

Anyway the end result is I have a language, in go, that I think I understand and have been able to modify. Now I'll have to find some time to go back to BASIC-work.

I found a bunch of basic-intepreters, including ubasic, but unfortunately almost all of them were missing many many features - such as implementing operations like RND(), ABS(), COS().

Perhaps room for another interpreter after all!

Tags: golang, intepreters, monkey | 4 comments

So I challenged myself to writing a BASIC intepreter over the weekend, unfortunately I did not succeed.

What I did was take an existing monkey-repl and extend it with a series of changes to make sure that I understood all the various parts of the intepreter design.

Initially I was just making basic changes:

• Added support for single-line comments.
  • For example "// This is a comment".
• Added support for multi-line comments.
  • For example "/* This is a multi-line comment */".
• Expand \n and \t in strings.
• Allow the index operation to be applied to strings.
  • For example "Steve Kemp"[0] would result in S.
• Added a type function.
  • For example "type(3.13)" would return "float".
  • For example "type(3)" would return "integer".
  • For example "type("Moi")" would return "string".

Once I did that I overhauled the built-in functions, allowing callers to register golang functions to make them available to their monkey-scripts. Using this I wrote a simple "standard library" with some simple math, string, and file I/O functions.

The end result was that I could read files, line-by-line, or even just return an array of the lines in a file:

 // "wc -l /etc/passwd" - sorta
 let lines = file.lines( "/etc/passwd" );
 if ( lines ) {
    puts( "Read ", len(lines), " lines\n" )
 }

Adding file I/O was pretty neat, although I only did reading. Handling looping over a file-contents is a little verbose:

 // wc -c /etc/passwd, sorta.
 let handle = file.open("/etc/passwd");
 if ( handle < 0 ) {
   puts( "Failed to open file" )
 }

 let c = 0;       // count of characters
 let run = true;  // still reading?

 for( run == true ) {

    let r = read(handle);
    let l = len(r);
    if ( l > 0 ) {
        let c = c + l;
    }
    else {
        let run = false;
    }
 };

 puts( "Read " , c, " characters from file.\n" );
 file.close(handle);

This morning I added some code to interpolate hash-values into a string:

 // Hash we'll interpolate from
 let data = { "Name":"Steve", "Contact":"+358449...", "Age": 41 };

 // Expand the string using that hash
 let out = string.interpolate( "My name is ${Name}, I am ${Age}", data );

 // Show it worked
 puts(out + "\n");

Finally I added some type-conversions, allowing strings/floats to be converted to integers, and allowing other value to be changed to strings. With the addition of a math.random function we then got:

 // math.random() returns a float between 0 and 1.
 let rand = math.random();

 // modify to make it from 1-10 & show it
 let val = int( rand * 10 ) + 1 ;
 puts( "math.random() -> ", val , "\n");

The only other signification change was the addition of a new form of function definition. Rather than defining functions like this:

 let hello = fn() { puts( "Hello, world\n" ) };

I updated things so that you could also define a function like this:

 function hello() { puts( "Hello, world\n" ) };

(The old form still works, but this is "clearer" in my eyes.)

Maybe next weekend I'll try some more BASIC work, though for the moment I think my monkeying around is done. The world doesn't need another scripting language, and as I mentioned there are a bunch of implementations of this around.

The new structure I made makes adding a real set of standard-libraries simple, and you could embed the project, but I'm struggling to think of why you would want to. (Though I guess you could pretend you're embedding something more stable than anko and not everybody loves javascript as a golang extension language.)

Tags: golang, intepreters, monkey | No comments