Reduced Lisp

A dynamic, functional, object oriented programming language. I can't believe it's not Scheme!

Quick Example

Here's a short program that demonstrates how RL integrates with python extension modules, in this case examples/gtk/gtk.py.

(include gtk)


(class Application
    (defun new (self)
        (begin
            (::self builder ((:gtk.Builder new)))
            ((:(:self builder) add_from_file) "test.glade")
            ((:(:self builder) connect_signals) self)))
    (defun quit (self widget)
        (gtk.main_quit))
    (defun run (self)
        (gtk.main)))


((:(Application) run))

What is RL?

Reduced Lisp is an interpreted, functional programming language written in Python 3. The syntax is based on the lisp family of languages:

• 12 is an integer
• 3-1j is a complex number
• (^ 3-1j 12) is a function call - it calls the "^" function, exponentiation, with two arguments.

Here's a small program:

(defun f (x)
    (* x x))

(print (f 12))

If you run it, it will print (integer 144).

RL optimizes tail recursion. If you can write a recursive function so that the last expression is a recursive call, you don't have to worry about stack overflows:

; naive factorial, not tail-recursive (will crash for large n)
(defun factorial (n)
    (if (= n 0)
            1
        (* n (factorial (- n 1)))))


; this is a tail-recursive function (won't crash).
(defun factorial' (n)
    (let (f (lambda (n acc)
        (if (= n 0)
                acc
            (f (- n 1) (* acc n)))))
        (f n 1)))

(print (factorial' 2000))

You can just type all of this into a terminal:

python3 rl.py

Using RL

In order to run the interpreter, simply type python3 rl.py. If you want to use the gtk extension module, you'll also have to install the python3-gi package (this is Gtk3 with Gobject-Introspection).

Note: at the moment modules are loaded from the current working directory.

In order to run the gtk example, you'd therefore type

cd examples/gtk
python3 ../../rl.py gtk-example.rl

Syntax

Defining functions

To define functions, the defun macro is called with three arguments: a name, a list of arguments and an expression - the function body.

(defun <= (a b)
    (or (< a b) (= a b)))

The function, <=, will be available in the scope of the function itself and in the scope in which it was run (but it is not a global variable).

Defining variables

The defun macro is really just syntactic sugar for binding a variable, in this case a function, to a name. You can use define to bind anything to a name in this way:

(define (greeting "hello"))
(print greeting)

Using values

If you don't want a value to be in the current namespace, you can use let. The very last argument is an expression that is executed, the ones before it are pairs of (name value).

(let
    (name "lisp")
    (greeting "hello")
    (print greeting name))

The value of a let expression is the value of the last expression in it. In this case, it will be the value of a call to print, which is null.

Anonymous functions

Anonymous functions, lambdas, are objects just like integers or strings, they can be used as arguments, returned from functions and so on.

(lambda (x) (* x x))

Executing multiple expressions

To do multiple things, you can use the (begin) construct:

(begin
    (print "hello")
    (print "world"))

The value of this expression will be the value of the last expression in it.

Conditional Evaluation

Conditionals works like you'd expect:

(if (< 4 8)
        (print "four is less than eight")
    (print "eight is less than four.. huh?"))

Looping constructs

You can loop by simply writing a tail recursive function, but there are also while and for loops built in:

(while true
    (print "are we there yet?"))

For loops take three arguments: a variable, or list of variables, an iterable objects, such as a list, set, or range, and a body:

(for i (range 10)
    (print i))

(for (i j) (zip (range 10) (range 0 20 2))
    (print i j))

Including modules

Extension modules can either be written in Python or reduced-lisp. Running (include gtk) will make the interpreter look for a module named either gkt.py or gtk.rl. If it is found, all the variables defined in it will be assigned to gtk.variablename.

(include maths)
(print (maths.square 12))

With maths.rl being

(defun square (n)
    (* n n))

Assertions

Assertions merely raise an error if their condition is not true:

(assert (= result 144))

Classes and Attributes

Classes are lists of expressions. Their namespace will become the class namespace, and any function inside will become a method - meaning it will receive the instance as the first argument (called self).

Attributes of classes can be got using : and set using ::, i.e. (:my_point x) will return the member x of the object my_point, and (::my_moint x 12) will set it to 12.

Every class should have a method called new. It is automatically run after the object has been created, and it can be used to set object attributes.

To create an instance of the class, simply call the class with the arguments for new. Here's an example:

(class Point

    (defun new (self x y)
        (begin
            (::self x x)
            (::self y y)))

    (defun distance (self other)
        (sqrt (+
            (^ (- (:other x) (:self x)) 2)
            (^ (- (:other y) (:self y)) 2))))

    (defun get_x (self)
        (:self x))

    (defun get_y (self)
        (:self y)))


(define (p
    (Point 1.34 7)))
(define (q
    (Point 2.34 8)))


(print ((:p distance) q))

Comments

Inline comments are inside of curly braces, line comments begin with a semicolon.

Types

These are the built in types:

• null (always null)
• boolean (true and false)
• integer (i.e. 12, -9, 0b101101011)
• real (i.e. 0.5, .3, 2e-99)
• complex (i.e. 1j, 7.5-4j)
• list (i.e. (list 1 2 3 4))
• set (i.e. (set), (set 1 2 3 4))
• functions (i.e. (lambda (x) x))

Built-in Functions

RL comes with loads of built-in functions. Type (help "functions") in the interactive prompt to get a list of them. Here's just one example on how to use them:

(defun even-squares-up-to (n)
    ; print the even squares of all numbers up to n
    (filter (lambda (j) (= (mod j 2) 0))
        (map (lambda (i) (* i i))
            (range n))))

(defun sum (a)
    (reduce + a))

(defun sum-of-even-squares-up-to (n)
    (sum (even-squares-up-to n)))

Arithmetic functions, such as + or mod take any number of arguments - i.e. (+ 1 2 3) is equivalent to (+ (+ 1 2) 3). However, predicates such as =, and, or, not only take two arguments.

The functions map, reduce and filter take a function as their first argument and a list, or set, as their second argument.

(defun add-one (n) (+ n 1))
(map add-one (list 1 2 3)) ; returns (list 2 3 4)

List-related functions (head, tail, get, pop, push, append, extend, length) never modify the actual object: lists are immutable.

(define (a (list 1 2 3)))
(print (pop a 0)) ; will return (list 2 3)
(print (push a 4 0)) ; will return (list 4 1 2 3)
(assert (= a (list 1 2 3)))

For operations on sets, there are five functions pre-defined: union, difference, symmetric-difference, intersection, and cross-product. To convert a set into a list, use (to-list myset), and (to-set mylist) to convert a list into a set.

Note: At the moment, language semantics change pretty much every time I make a new commit. I'll will try to keep the documentation up to date.