teliva/tour.md

A guided tour of Teliva

Teliva is intended to be a safe environment for running shareable little text-mode Lua apps that are easy to modify. Apps aren't sandboxed yet, but I'm working on that.

This page is an in-progress guided tour through Teliva's Readme and Lua's reference documentation. We'll start out really slow and gradually accelerate.

Prerequisites

You will need the following to build Teliva:

• A non-mobile computer running some sort of Unix variant. Teliva has been tested on Linux, OpenBSD and Mac OS X. (Other BSD variants and Windows Subsystem for Linux should require minor changes at most. Please contact me if you run into issues running Teliva on your computer.)
• Git.
• A working C toolchain.
• Some fluency in typing commands at the terminal and interpreting their output.

Task 1: running a Teliva app

Read the first question in the Readme. Try running the commands to download and build Teliva. If you have any trouble at this point, don't waste any time thinking about it. Just get in touch.

Run the simplest example app:

src/teliva counter.tlv

Can you figure out what this app does, what you can do with it? There's a number on screen. Hit enter. The number increments by 1. Hit ctrl-x to exit (press ctrl and x on your keyboard at the same time).

Run the app again. Try editing the app by hitting ctrl-e. You see a "big picture" view of the app. Spend a few moments scanning this page. All programs consist of data and functions: code that operates on data in some way. Teliva apps always start by running the special function main. The big picture orders functions from the top down. It shows main up top, functions that main calls below, functions called by those functions next, and so on.

Try browsing to some of the names visible on screen. Don't be afraid to experiment. The menu at the bottom always shows the hotkeys available to you at any point in time. Don't worry, everything you do can be undone.

Task 2: modifying a Teliva app

The first section of the Lua book starts with this one-line example:

print("Hello World")

Can you figure out where to add this line to the app's code so it's visible when the app runs?

Start by looking inside main. Most Teliva apps tend to share a basic structure in main:

• some initialization, followed by
• a loop that repeatedly:
  • updates the screen, and then
  • waits for the user to press a key

Can you map the lines of main to this structure? Which function describes how the app updates the screen?

Some hints:

• The function responsible for updating the screen is render.
• render begins by clearing the screen (window in Teliva).

After you make a change, can you figure out how to run your program?

Does it do what you expect? Feel free to edit your programs as often as necessary. Programming is just long sessions of repeatedly editing (ctrl-e) and running (ctrl-e) your program.

Once you're happy with your change, try exiting Teliva and restarting it with the same app. Do you still see your changes?

You could save the file counter.tlv anywhere you like at this point. Or share it with someone else. Everything needed for the app is in that file.` If you get stuck or have a question, send it to me!

Task 3: variables and arithmetic

Can you figure out how to modify this app to increment by 2 each time you hit enter? Again, don't be afraid to experiment. The menu at the bottom always shows the hotkeys available to you at any point in time. Don't worry, everything you do can be undone.

Divide the problem into two parts in your head: where to make your change and what change to make there. You've already gotten some practice selecting a place to modify in Task 2. Repeat that process. Go back to the render page. What is the name of the variable (box containing a number) that decides what number gets printed to screen? Go back to the big picture. Where is this variable defined? How does it get modified when you press enter?

If you're stuck, some short sections from the Lua book might help at this point: getting started; assignment; what you can do with numbers.

(Buy the Lua book to support the creators of Lua. Teliva is a tiny molehill on the mountain of awesome that is Lua.)

Some hints:

• The function responsible for processing keystrokes is update.
• The variable that tracks what number to print on screen is n.