The language is similar to PILOT in concept, using single-letter commands who's primary purpose is to cause the turtle to move and create drawings. In contrast to PILOT, WSFN allows the construction of more complex macros using parenthesis, which can then be combined into larger programs. These macros can call themselves recursively, which allows it to draw complex images like fractals in a few lines of code.
Extended WSFN was an implementation created for the Atari 8-bit family by Harry Stewart and published by the Atari Program Exchange (APX) in 1981. The main differences were to add another set of commands to provide access to some of the Atari platform's capabilities, notably graphics, color, sound and joystick support. The language was otherwise similar to the original, although some of the command letters changed to make room for new commands.
A unique feature of the language is that the keyboard is active at all times, and users can interrupt programs as they run. For instance, if one makes a macro to draw an arc on the screen and names it "A", typing A will begin the drawing of the arc, but the user can press A in the middle of the process to draw a new arc at the current location, and so on. This makes all programs interactive without any specific code like an event loop.
Editor's note: One clearly missing feature is the ability to define macros that run on events other than the keyboard. If one could define macros that were triggered by the joystick, perhaps by defining the macro with a specific name like "!UP", one could write joystick-based drawing programs in a few lines of code.
The following Extended WSFN code draws a square in the center of the screen:
U2L12FND12F3(2R24F)2R12FThe code starts with the U which lifts the pen (Up) so the following commands will not draw to the screen. This is followed by a 2, which means the next instruction should be run twice. In this case, the next instructions is Left. Since each step of a turn is 45 degrees, this causes the turtle to rotate 90 degrees to the left so it points to the left side of the screen. Next, the turtle moves Forward 12 steps, is pointed North (up). Since the turtle was formerly pointed left, 2R would have the same end effect as the N. Finally the pen is put back Down so the following commands will cause drawing on the screen.
Next, the 12F draws 12 steps, and since the turtle was pointing north, this causes a short line segment to be drawn up the screen. Then comes a 3, meaning the following instruction should be run three times. In this case it is not a single instruction, but all of the instructions in the parens. These rotate 2Right, or 90 degrees, and then draws a segment 24 long. So the first iteration draws the horizontal line across the top, the next the vertical line down the right side, and then across the bottom. Finally, the ending 2R12F finishes off the square by drawing the missing segment on the bottom of the left vertical side.
As you can see, WSFN code can become almost unreadable even in simple examples!