Turtle

Turtle is a web application intended to teach programming. It is inspired by the Logo programming language, and includes a turtle that can be moved around the canvas to draw shapes and patterns. It supports variants of six languages:

• BASIC
• C
• Java
• Pascal
• Python
• TypeScript

Each language includes a core set of commands implementing the turtle graphics metaphor, as well as a library of mathematical functions. These follow the conventions of the respective languages as far as possible, but with some diversion for the sake of overall consistency and owing to the constraints of the system (for example, the system only supports integers, not floating point numbers).

The Virtual Machine

Turtle works by first compiling the source code into an intermediate representation, which is then executed by a virtual machine. This serves two educational purposes in this context:

• For beginners, it allows for very clear and targetted error messages from code errors.
• For more advanced students, it supports teaching about advanced programming concepts such as compilers, machine code, virtual machines, and memory management. Every aspect of the virtual machine is inspectable by the user (behind a UI toggle for advanced mode).

Getting started

The codebase is written in TypeScript, and managed and run by Deno.

deno task start    # development server, with file watching
deno task test     # the fast test suites (~8s)
deno task build    # bundle the frontend JavaScript and CSS into assets/build/
deno task fmt      # format everything with Prettier
deno task lint     # deno's built-in linter

deno task build must have run at least once before the app is served, since the pages reference assets/build/index.js and assets/build/screen.css.

Where things are

Path	What's in it
src/	all the application code — see src/README.md
test/	the test suite — see test/README.md
style/	the CSS
assets/	images, example programs, and the built JavaScript and CSS
app.ts	the server entry point
build.ts	the build script

src/README.md is the entry point for reading the source: how the compiler, the virtual machine and the UI are laid out, and the decisions that shape each of them. test/README.md covers the four test suites, when to run each, and the conventions for writing new tests.