Tittle

Turtles in Scittle.

See Papert, S: Mindstorms.

State of Play

As a proof of concept, this proves the concept. There's a lot more to do.

I have not (yet) managed to get a scittle REPL running.

Clojure (and thus Scittle) is a fairly pure functional language. It's not good at imperative things, although of course it can do them. Turtle Graphics is by its nature imperative: you're commanding the turtle. So this is not a very idiomatic use of Scittle, and it's not very efficient; it invites you to write recursive functions (and of course I wish to encourage you to write recursive functions), but recursive functions in Scittle are not fast.

Usage

At this stage, edit index.html

Functions

Generally speaking, functions whose names end in exclamation points are imperative functions: they change the state of things by side-effect, and the return value may not be particularly relevant. I would normally consider imperative functions bad practice in Clojure.

(cos angle)

Return the cosine of this angle, considered to be expressed in degrees.

(degrees->radians angle)

Return the equivalent, in radians, of this angle espressed in degrees.

(draw-polygon! sides side-length)

Draw a regular polygon, with this number of sides, each of this side-length.

(draw-tree length left-branch right-branch curvature branch-fraction trunk-fraction depth)

Draw a tree. This is a fairly crude tree-drawing algorithm; there's lots of ways it can be improved, consider it a plac to start. Parameters (all numeric) as follows:

• length: the length of the current segment of the tree;
• left-branch: the amount to turn left, with respect to the parent segment, to draw a left branch, in degrees;
• right-branch: the amount to turn right, with respect to the parent segment, to draw a right branch, in degrees;
• curvature: the amount to turn to draw the next segment of the current stem, in degrees;
• branch-fraction: the length of a branch segment, as a proportion of the length of its parent segment;
• trunk-fraction: the length of the next segment of a stem, as a proportion of the length of its parent segment;
• depth: the maximum depth of recursion to allow.

(log-turtle!)

Prints the state of the turtle to the browser console.

(move! distance)

Move the turtle forward on its current heading by distance units.

NOTE THAT this function invokes move-to!, q.v., which will create a line element if the pen is down.

(move-to! x y)

Move the turtle absolutely to the coordinates x, y. If the pen is down, create a line element.

NOTE THAT this is the only function which creates new graphical elements.

(number-or-error! n)

If n is a number, return it, else throw an exception.

(pen-down!)

Put the turtle's pen down (i.e., cause line segments to be created).

(pen-down?)

Return true if the turtle's pen is down, else false.

(pen-up!)

Lift the turtle's pen up (line segments will not be created).

(pen-up?)

Return true if the turtle's pen is not down, else false.

(radians->degrees angle)

Return the equivalent, in degrees, of this angle espressed in radians.

(sanitise-angle angle)

Take this angle, and return a number between 0 and 360 that represents it as an angular measurement.

(set-ink! colour)

Set the ink with which the turtle draws to this colour, which should be a string representation of a colour known to CSS.

(set-nib! n)

Set the nib (width) of the pen to this n, which should be a number.

(sin angle)

Return the sine of this angle, considered to be expressed in degrees.

(turn! angle)

Turn the turtle clockwise by this angle, expressed in degrees with respect to the X axis. If angle is not a number, throw an exception.

(turn-to! angle)

Turn the turtle to face angle, expressed in degrees with respect to the X axis. If angle is not a number, throw an exception.

TODO: Note that 180° is currently straight up; this is not intended, it is intended that 0° should be straight up, and this change will be made before version 1.0.

Licence

Copyright © Simon Brooke 2025

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.