Lua/Tutorial

How to use this tutorial

This tutorial was originally developed for the 2012 Berlin Hackathon, a face-to-face event. It has been adapted for on-wiki, remote use, as you see it here. As all things wiki, it is a living document and we encourage you to participate in developing and maintaining it.

The content hereafter is focused on established MediaWiki template developers. Everyone is welcome to use it. If you're new to templates on MediaWiki, but have Lua experience, you may want to find out more about templates. Note that some things in Scribunto do not apply to the Lua distribution. If you're a MediaWiki template guru, but new to Lua, you will learn some cool, new magic. If you're new to both, you will find that the MediaWiki community has many people willing to help you learn.

Introduction to Lua

Brad Jorsch’s short presentation for a basic example of how to convert a wikitext template into a Lua module.

• Lua scripting general information and brief historical context of its planned use in MediaWiki.
  • Status of Lua scripting on MediaWiki.
  • What is Lua? Check out the Lua.org demo if you don't want to download Scribunto just yet.
  • Lua evolution PDF and Lua version history.
• What is different from JavaScript and OO languages we could have used?
  • Scribunto: how is the Lua implementation in MediaWiki different? Wikitext is not parsed the way you would normally expect. Parser frames are arguments in Scribunto templates. Strip markers replace certain markup, which, if it is passed, will result in placeholders instead of markup. You will need to use the new Namespace:Module.
• How you adapt for these differences.

Demonstration

Suggested templates you want to see adapted during the demo

Please post the link to a template that you'd like to see adapted to Lua as a demo, on-stage during the tutorial.

1. (First template suggestion)

Hands-on Lua scripting

• Create a page in the Module:Sandbox namespace, called for example Module:Sandbox/your username/my_module like Module:Sandbox/ExampleUsername/my_module
• Put Lua code in it, like this example:

local p = {}
function p.hello(frame)
    return 'Hello'
end
return p

Explanation of Code
local	this tells lua the "scope" of the variable that follows. It can only be accessed right here, locally. The opposite is "global".
p	this is a (local) object (variable) we are creating. We have decided to call it p. We could call it anything we wanted. We will address it as p later, whenever we want to do something with it.
= {}	The equal sign means we're assigning what comes after it to what came before it. The {} means "object", in this and many other script languages like JavaScript. It could have had stuff inside it like {foo = 'bar'}, but we're making it empty and will assign stuff inside it later.
function	Now we're creating a function, which like in calculus is a piece of code that takes values and returns other values. The function will include all code until we get to a keyword "end". This is similar to most popular programming languages.
p.hello	Remember when we made the object "p"? Now we are creating a property inside of that object, called "hello", which is this function. The dot means "property inside" in this and many other script languages.
(frame)	The parens() wrap whatever values we are passing to the function we are creating, the one called "hello". "frame" is the name we give a container of all parameters we'll pass to it. We could call it anything, as long as we consistently refer to it by that name later.
return 'Hello'	"return" says we send this back out of the function we're in, which is called p.hello, and it will be returned to the object we created, p. 'Hello' is the thing we're returning. The single quotes around it mean it's a string, not a variable named Hello that contains some value.
end	Remember when we said that the function would go on until we got to a keyword end? This is that keyword.
return p	Remember the other "return" that sent 'Hello' out of the function called p.hello? Well THIS return takes whatever happened inside the entire object (which we called p) and sends it outside of that object. In this case, it'll end up being printed to the screen for the user to see or wiki server to parse.

• Go to the module talk page.
• Put a #invoke in it to call that new module:

{{#invoke: Sandbox/your username/my_module | hello }}

Accessing template parameters

Here's a simple example of a function that uses the parameters:

local p = {}
function p.hello(frame)
    return 'Hello, my ' .. frame.args[1] .. ' is ' .. frame.args[2]
end
return p

Such a function can access the frame object to get access to the parameters that the template was invoked with. For example, let's change the previous module so that it takes two arguments, like this:

{{#invoke: Sandbox/your username/my_module | hello | hair | brown }}

Our Lua function can use the parameters "hair" and "brown" in either of two ways:

1. Access them directly as frame.args[1] and frame.args[2], respectively (named arguments can also be used, such as frame.args['title'] or frame.args.title).
2. Iterate them using the pairs( frame.args ) function, which returns a (name, value) pair for each; this is the best way to code a template that can take a variable number of parameters.

A few pitfalls to avoid:

1. The parameter values are always strings, even if they are numeric in form; you can use the Lua function tonumber() if you need to treat a value as a number.
2. However, numbered parameter keys are numbers, even if they are given explicitly in the template invocation (for example, the parameters in both {{#invoke:Sandbox/your username/my_module | hello | world}} and {{#invoke:Sandbox/your username/my_module | hello | 1=world}} are indexed by the number 1, not the string '1'.
3. With explicitly named (or numbered) parameters, leading and trailing whitespace is stripped from name and value. This is similar to template parameters.
4. An empty string value is not the same as a missing value, which is nil; for example,

   {{#invoke:Sandbox/your username/my_module | hello | world }}

   results in frame.args[2] being nil (which will cause a script error in the example code hereinbefore), but

   {{#invoke:Sandbox/your username/my_module | hello | world | }}

   results in frame.args[2] being an empty string (which will not, although the resulting text will look odd). One way to work around this is to assign the desired parameters to a local variable, and replacing nils with empty strings at this step; e.g.,

    local arg2 = frame.args[2] or ''

Lua implementation plan as part of MediaWiki roadmap

• Return value expansion, access to the frame object and bug fixing

External resources for general/non-wiki Lua scripting

• Lua 5.1 Reference Manual
• Programming in Lua
• Lua Quick Reference
• #Lua-Support IRC channel for support questions

Tutorial feedback

If you saw this tutorial live, in Berlin, please give us your feedback on the Talk page, so we can refine it for future hackathons and other events. You can give anonymous feedback if you prefer.

Tutorial slide deck from Berlin.