Theta is a Lisp interpreter for Ruby (though it’s really just a small subset of Lisp it interprets). It was heavily influenced by lis.py and flea. It was primarily made as a learning exercise.

Theta is distributed as a gem, so install by typing:

% gem install theta

After installation, simply type theta to get to the interactive interpreter prompt.

% theta
theta> (define a 2)
theta> (  a 3)
5
theta> (define square 
theta>   (lambda (x) 
theta>     (* x x)))
theta> (square 3)
9

To exit, simply type ‘exit’ at the prompt.

Or, you can use the -f command line parameter to evaluate a file.

% theta -f test.scm

Alternately, you can use the -c parameter and pass in code directly.

% theta -c "(  3 2)"

And finally, you can use Theta inside your own Ruby programs, if that sounds like something you’d do.

require 'rubygems'
require 'theta'

t = Theta::Interpreter.new
t.run "(define n 15)"
puts t.run "(  10 n)"

Theta uses a very simplified Lisp-like syntax.

Theta has your basic mathematical functions.

theta> (  2 2 5)
9
theta> (- 3 2)
1
theta> (* 4 2)
8
theta> (/ 4 2)
2

You also have the basic variable definition stuff.

theta> (define a 2)
theta> (  a 3)
5

And functions.

theta> (define square (lambda (x) (* x x)))
theta> (square 3)
9

We have your basic if statement that will evaluate the first piece of code if true, otherwise the second (if you give it one).

theta> (if (= 2 2) (display "true") (display "not true"))
true
theta> (if (= 1 2) (display "true") (display "not true"))
not true

And some basic Boolean statements.

theta> (= 2 2)
true
theta> (> 1 2)
false
theta> (< 2 3)
true
theta> (>= 3 3 2)
true
theta> (<= 4 5 2)
false

Theta also has basic input and output.

theta> (display "poop")
poop
theta> (define a (input))
hi <this is user input>
theta> (display a)
hi

And a function to turn user input into an integer.

theta> (define a (string-to-num (input)))
123 <this is user input>
theta>(  a 3)
126