Theta¶ ↑
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.
How to use¶ ↑
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)"
Syntax¶ ↑
Theta uses a very simplified Lisp-like syntax.
Math¶ ↑
Theta has your basic mathematical functions.
theta> (+ 2 2 5) 9 theta> (- 3 2) 1 theta> (* 4 2) 8 theta> (/ 4 2) 2
Defining Functions and Variables¶ ↑
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
Conditionals and Truthiness¶ ↑
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
Input and Output¶ ↑
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