# Mandelbrot: SETL & Lisp

SETL is a general-purpose programming language developed by Jack Schwartz back in the late 1960s. The language is multiparadigm – both Procedural (containing subroutines and control structures like if statements) and Set Theoretical. The latter is not often mentioned when talking about language paradigms – and is not to be confused with Logical Programming. The concept is outlined in the 1974 paper An Introduction to the Set Theoretical Language SETL. Importantly though data in SETL is of three main types: Atoms, Sets, and Sequences (also referred to as tuples or ordered sets).

A good overview of SETL can be found in the GNU SETL Om.

Pixels inside the Mandelbrot Set are marked in black, the code used to generate them (mandelbrot.setl) can be found here.

Generation of the Mandelbrot Set
On the Wikipedia page for the Mandelbrot Set you will find its formal definition, which looks like so: $M = \left\{c\in \mathbb C : \exists s\in \mathbb R, \forall n\in \mathbb N, |P_c^n(0)| \le s \right\}.$

The most fantastic thing about SETL is how powerful its Set Builder Notation is. Almost exactly as you would in the formal case (removing ‘there exists some real s’ for the practical application):

M := {c : c in image | forall num in {0..iterations} | cabs(pofc(c, num)) &lt;= 2};


For some reason SETL doesn’t support complex numbers, but they are easily handled by writing the necessary procedures we need like cabs, ctimes, and cplus dealing with tuples in the form [x, y]. The variable ‘image’ is a sequence of these tuples. Another procedure is written, pofc, which is a more practical version of $P_c^n(0)$.

Interaction with the Lisp graphical display
The goal of the SETL program is to produce a set of points that lie inside the Mandelbrot set. To actually display the image I used Common Lisp and displayed a widget with the image using CommonQt. In a very rudimentary way I had mandelbrot.setl take arguments about the image then print the set of pixel co-ordinates. All lisp had to do was swap and ‘{}’ or ‘[]’ for ‘()’ and read it as lisp code then update the image.

(setf in-set (read-from-string (parse-line (uiop:run-program "setl mandelbrot.setl -- -250 -250 250 8" :output :string))))
(draw-mandel instance in-set)


An after-thought was to make a Mandelbrot searcher where you could zoom and move using the mouse but the SETL code is such an inefficient way of doing it that it’s not worth it. As an attempt to mimic the formal definition it was highly successful and fun. Though much quicker SETL code could be written for generating the Mandelbrot Set.

Source file for mandelbrot.setl and the Lisp/CommonQt front end can be found here.