Where you go from here

Introduction

S1 said this course would be a series of small programs, each showing one thing Dr Logo can do, and that it would not build towards anything.

That was deliberate, and here is the payoff: you did not spend twenty-six sections on scaffolding for a single artefact. You have twenty-odd working programs, and none of them needs the others.

What you can actually do now

Not a list of section titles - a list of things you can sit down and do.

You can make the machine draw. Lines, squares, circles that are really thirty-six straight lines, arcs, flowers, charts with real numbers in them, patterns that repeat and shift. You know why a shape that closes is worth more than one that does not.

You can teach it new words. to and end, inputs, op to hand a value back, and a procedure that calls itself with a condition that stops it. That is the whole of the language's abstraction, and you have all of it.

You can handle data. Words, numbers, lists, lists of lists, variables, and property lists. A tune is a list. A chart is a list. A quiz answer is a list.

You can talk to whoever is sitting there. Ask a question, read a line or a single key, put text where you want it on the screen, make a noise, read a joystick.

You can keep your work and get it back. save, load, dir, erasefile, and the screen editor when you need to change three characters in the middle of a procedure.

You can find out why it went wrong. Which is the one that separates people who program from people who have programmed.

What this course left alone

Some of it on purpose, and some of it because it is not there.

Four primitives the manual documents do not exist on this machine. savepic, loadpic, dirpic and erpic - the picture-file ones - all answer I don't know how to. The manual marks them as belonging to a version this is not. So there is no way to save a drawing as a picture; you save the procedure that draws it, which is smaller anyway.

Two turtle modes were never tried. sf reports the turtle as being in WINDOW mode, and the manual names two others, WRAP and FENCE. This course never changes it. If you want the turtle to reappear on the other side of the screen, or to refuse to leave it, that is where to start looking - and you now know how to find out what they do rather than take anyone's word for it.

Two ways of writing that this course avoids. Read SKETCH.LOG and you will meet label and go, which jump about inside a procedure, and setscrunch, which squashes the drawing. Both work. Neither is taught here, because a procedure that calls itself will do everything go does and stays readable.

The manual lists more primitives than this course covers. That is normal. You now have the two things you need to deal with an unfamiliar one: type it and see, and po anything that claims to be a procedure.

Three things worth doing next

Read the two programs that came with the machine. Appendix II covers them. SKETCH and QUIZ were written at Tatung in October 1984 to show what this language could do, and everything in them is made of things you now know. Load one, pots it, and po whatever looks interesting. Watching somebody else solve the same problems differently is worth more than another section.

Write something that is yours. The honest advice is the boring advice: pick something small that you actually want to see on the screen, and make it happen. A name in big letters. A clock face. A pattern you can change while it runs with the joystick. Dr Logo rewards small things that compose.

Take the lid off. If you want to know what the machine is doing underneath all this - how the screen really works, what the sound chip is, why the turtle is 256 pixels wide and not 320 - there is a companion course on this site that writes to that hardware directly in Z80 assembly. Dr Logo and assembly are the two ends of the same machine, and each one makes more sense once you have seen the other.

A last word about the way this was written

Every listing in this course was run on the machine before it was printed, and every result you were told to expect was read off a screen rather than off a manual. That mattered more than it should have: the manual that came with Dr Logo is wrong about the version number, wrong about the syntax of tones, wrong about which primitives this release has, and silent about the fact that the bracket keys are not on the keyboard.

That is not a complaint about the book. It is the reason the course is shaped the way it is, and it is worth carrying with you: the machine is the authority. When something you read disagrees with what the Einstein does in front of you, the Einstein is right.

Go and find out what it does.