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Lua introduction for newprogrammers

Download slides at:http://tstarling.com/presentations

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Hello world

In [[Module:Hello]] put:

Then in a wiki page:

{{#invoke: Hello | hello }}

Try it now at http://scribunto.wmflabs.org/

local p = {}function p.hello()

return'Hello, world!'endreturn p

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if

if colour == 'black'thencssColour = '#000'

end

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if


elsecssColour = colour

end

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if


elseif colour == 'white'thencssColour = '#fff'

elsecssColour = colour

end

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Equality

if x = y then-- Error: 'then' expected near '=' return'something'

end

if x == y then return'something'end

Single equals is only for assignment

Use double equals for equality

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for

f = 1

for i = 1, 5dof = f * iendreturn'The factorial of 5 is ' .. f

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Types

String

Number

Boolean (true/false)

nil

A few other things

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Logic

and: both are true

or: one or the other or both

not: the following thing is false

if beans and toast then return'breakfast'end

if chicken or beef then return'dinner'end

ifnot hungry then return'nothing'end

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Functions

Calling functions

Defining functions

colour = getDivColour()

localfunction getDivColour()

return'blue'end

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Functions

Functions let you avoid duplication

Functions can have arguments:

localfunction plural(word) return word .. 's'end

German version left as an exercise to thereader

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Functions

Two types of functions Local functions for private use within the

module

Exported functions

localfunction plural(word) return word .. 's'end

local p = {}function p.hello() return'hello'endreturn p

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Local variables

function getDivStart()colour = getDivColour()

return''

end

colour = 'Fuschia'

return getDivStart() .. colour .. ''

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Local variables

function getDivStart() local colour = getDivColour()

return''

end

colour = 'Fuschia'

return getDivStart() .. colour .. ''

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Local variables

If you don't set a variable to something, it willbe nil by default

local x

if ready thenx = 'GO!'

end-- Now x has "GO!"or nil

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Tables

Creating a table

Accessing a table element

numbers = {one = 1,two = 2,

three = 3}

return numbers.one -- returns 1return numbers['one'] -- also returns 1

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Numbered tables

africanFlatbreads = { 'Aish Mehahra', 'Injera',

'Lahoh', 'Ngome'}

return africanFlatbreads[2] -- returns 'Injera'

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Visiting each table element

pairs: key/value pairs in random order

ipairs: Numeric keys in ascending order

for name, number inpairs(numbers) do...

end

for index, bread inipairs(africanFlatbreads) do...

end

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Strings

Length

sub

s = 'hello'return #s -- returns 5

s = 'hello'return s:sub(2, 3) -- returns 'el'return s:sub(2) -- returns 'ello'return s:sub(-2) -- returns 'lo'

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Further reading

Programming in Lua: http://www.lua.org/pil/

Reference manual:http://www.lua.org/manual/5.2/

Scribunto:https://www.mediawiki.org/wiki/Extension:Scribunto

lua-users.org

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Lua introduction for programmers

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Lexical

Comments reminiscent of SQL

Line breaks ignored

Semicolons to terminate statements optional, discouraged

-- Single line comment

--[[

long comment--]]

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Data types

nil

Numbers

Single type, floating point

Strings

8-bit clean

boolean

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Data types

Functions

First class values

Return multiple values

Multiple return values are not bundled into a datatype

Anonymous syntax:

x = function ()...

end

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Data types

Tables

Implemented as a hashtable

Used for OOP, like JavaScript

Literal syntax: {name = value} or {a, b, c} Access with foo.bar or foo['bar']

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Operators

Not equals: ~= instead of!=

Concatenation: ..

Length: #

Logical: and, or, not

Exponentiation: ^

Usual meanings: , =, ==, +, -, *, /, %

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Operator omissions

No assignment operators like +=

Even plain = is not really an operator

No bitwise operators

No ternary ?:

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Assignment

Like BASIC, assignment is a complete statement, not anexpression

Multiple assignment:

Assignment with local variable declaration:

a, b = c, d

a, b = foo()

Not like this!

local a, b = c, d

local a = b, c = d

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Control structures Explicit block

Precondition loop

Postcondition loop

do...

end

while cond do...

end

repeat...

until cond

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Control structures

If

if cond then...

elseif cond then

...else

...end

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Control structures

Numeric for

Generic for

Index variable is local to the loop

for i = start, stop do...

end

for i in iterator do...

end

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Variables

Lexical scoping, almost identical to JavaScript

An unset variable is identical to a nil variable

No special syntax for deletion, just x = nil

No error raised for access to undefined variables

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Objects

Made from tables using a variety of syntaxes,similar to JavaScript

Private member variables implemented using

lexical scoping, as in JavaScript Dot for static methods: obj.func()

Colon for non-static methods: obj:func()

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Objects

Factory function style example

function newObj() local private = 1

local obj = {}

function obj:getPrivate() return private end

return objend

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Metatables

Each table may have an attached metatable Provides operator overloading

"index" metatable entry is used for object

inheritance and prototype-based OOP

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MediaWiki/Lua interface

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Module namespace

All Lua code will be inside the Modulenamespace

Code editor provided

"ace" JavaScript code editor Automatic indenting

Syntax highlighting

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Invocation

{{ #invoke: module_name | function_name |arg1 | arg2 | name1 = value1 }}

#invoke instances are isolated, globals defined

in one are not available in another Only caches are shared

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Module structure

A module is a Lua chunk that returns an exporttable

require() provided

not isolated package library provided

R l

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Return value

The exported function returns a wikitext string Multiple return values are concatenated

Non-string return values are converted to

string Metatable entry "tostring" supported

F th d

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Frame methods

Argument access: args

argumentPairs()

getParent()

Provides access to the parent frame, i.e. thearguments to the template which called #invoke

local name1 = frame.args.name1

local t = {}for name, value in frame:argumentPairs() do

t[name] = valueend

F th d

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Frame methods

Wikitext preprocessing

Structured template invocation

frame:preprocess('{{template}}')

frame:expandTemplate{ title = 'template',

args = {foo = foo}}

A idi d bl i

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Avoiding double-expansion

Arguments are already expanded Don't construct preprocessor input from

arguments

Use frame:expandTemplate()

F t di ti

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Future directions

Gabriel Wicke's interface: frame:getArgument()

frame:newParserValue()

frame:newTemplateParserValue() All provided but only with stub functionality

F t di ti

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Future directions

Interwiki module invocation Languages other than Lua

Date/time functions

Direct access to other core parser functionsand variables

{{PAGENAME}}

{{#ifexist:}} etc.

Further reading

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Further reading

Programming in Lua: http://www.lua.org/pil/ Reference manual:

http://www.lua.org/manual/5.2/

Scribunto:https://www.mediawiki.org/wiki/Extension:Scribunto

lua-users.org

Try it out

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Try it out

Go to http://scribunto.wmflabs.org/ Create a function that takes several

arguments and does something to them

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

return p

Tim Lua 2012

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