Help:Lua for beginners

Overview
Lua is a programming language implemented on Wikipedia with some substantial restrictions via Scribunto. Its purpose is to allow you to process the data which is available on Wikipedia content pages to allow various sorts of customized display of information.

The most important help file is the MediaWiki Scribunto Lua reference manual, which provides a concise summary of the language and standard library calls as implemented on MediaWiki.

The general, non-MediaWiki Lua reference manual can—while being very well written, comprehensive, and informative—be problematic for beginners, because certain features don't work in Wikipedia—beginning with, which appears in standard Lua "hello world" programs.

Issues with the current implementation
Besides the lack of, there are other features missing – see Differences from standard Lua for a complete list.

At the present time, it is advisable to use mw.ustring functions instead of string, because the latter sometimes fails with Unicode characters.

Input
The programs are run only when the page is "parsed" (when it or a page it incorporates is changed or previewed), not every time you view the output. Therefore there can be no convenient Lua module that allows you to type in a Fahrenheit temperature in an input box and get back the corresponding Celsius temperature when you press a button, or allows you to click on a segment of a Mandelbrot set visualization on a page to expand it as often as you like. There has to be an actual Wiki page (or at least a page you have submitted for preview) containing the input data.

However, it is possible to use library functions like mw.title.new to import content from any text content page on the Wiki. You cannot, however, import data from files, not even .svg files which contain XML text data.

Calling a Lua module
Lua calls look much like templates, and consist of a small block of text like

This text calls the Lua script itself, which is housed in the  namespace. The effect of this call is to send the information within the  block to the Lua module, and to replace everything within the brackets with a piece of text that it sends back in return. (Literally, in the "return" statement)

Note that the first "parameter", in this case, is actually a function called within the Lua module. This field must always be included in any. To those unfamiliar with modules, especially Wikipedia template coders who expect anything after  to be a parameter, the need for this extra field is surprising, especially if all uses of the module depend on its presence. When documenting your work for them it is useful to include an explicit usage instruction like  so that they understand not to omit it.

For many existing modules, an example  of the script (and little else) is provided on the   page. It is convenient for authors to be able to flip quickly to the talk tab to look at the effects of their changes, but you should never transclude the talk page as a template - people might actually talk on it!

Another example: Using LuaCall to perform a single Lua instruction
As a beginner, or in casual talk page conversation, you might only have one little calculation you want to use Lua for but don't want to write a full module. You might find Module:LuaCall convenient for this. For example, you can test how a greedy Lua pattern works:



or count up the length of a Did you know hook or the text portion of a Did you know candidate:



In these specific examples, however, Module:String could do both of these tasks.

The script at Module:LuaCall has been written to accept any set of named parameters, for each one storing the string   in the variable with the name  , and then allowing you to use these variables as parameters for any function available in Lua. The script then returns only the first value returned by the function (remember that Lua functions can return multiple values, but in this case, only the first is returned from the module).

Errors
Lua errors appear as red "Script error" messages. If Javascript is enabled, the red script error message is a link which usually allows you to follow it back to the line in the module where the error occurred. There are some exceptions, for example "Module not found", if the name of the module itself is mistyped, or "The function you specified did not exist" if the function name given is invalid.

Lua program structure: Output
The most fundamental part of a Wikipedia Lua program is a return statement which carries its output back to the page that had the. You can have a Lua function that runs without error even though it doesn't contain a return statement, but on Wikipedia it is pointless, as Lua programs cannot generally have side effects on Wikipedia.

The module itself must return a Lua table of values. A Lua table is expressed as a list of values separated by commas, within curly braces. When the module is called by, the function it names (the first argument after |) is looked for in that table. That function, in turn, is expected to return something that can be represented as a string.

Therefore,  is actually a complete Lua module (though a very strange one) - it returns the function returned by mw.ustring.gmatch (an iterator function listed in the Lua reference cited above) as the one and only element in an array (represented within  )—which when executed using   yields the string "Hello world". However, things are not usually done this way. Typically we use the overall form:

Note that  is equivalent to   or. The function is just another type of value, retrieved with the key  from table. If you want to allow users to invoke the same module with  instead of , you can write   to copy the reference to this function to a new key in the table. You can even write, which causes   to produce the same output as. Learning to think of functions as a data type becomes very important later on for working with library functions like, and constructing iterator functions.

Lua program structure: Input
The   parameter above (which is pretty much always given this name in Wikipedia Lua modules) receives another table, which is passed from the page that makes the call to the Lua module. It contains a surprising amount of stuff, of which just a few things concern the novice.

Arguments
contains another table, namely, all the content sent by the user within the  brackets except the first argument which states the name of the function to be executed. So in, the string  is the content of   (which is the same as   but not the same as   or  ). Unnamed parameters come out with numbers as keys (, , etc.), while named parameters come out with the parameter names (strings) as keys ( ,  , etc.).

Example:""results in"=3377, =4, =3,  =."

Parent frame
Within  there is a parent frame, referring to the page that called the page that gives the script, and you can pull out arguments from that also. Just write

and  will contain those arguments.

It is popular in Lua to use the synonymous statement, cancelling the need to write   twice. Note the colon instead of the dot. means exactly the same as. For novices this can be confusing, and it is important to be aware of this idiom. If you use it in the wrong way, though, the script errors are pretty good at pointing out that this was the mistake.

Basic debugging
Debugging can start as soon as you write programs, and can be done simply with string concatenation. Just set up a variable with some recognizable name like "debuglog" in your main function (p.main) with a statement like. This initial "" definition helps because otherwise it will be nil and concatenating a string to nil gets you an error. Now whenever you have a variable you'd like to test, say x, just write, and have at the end of your program    The "tostring" is a function to ensure x is interpreted as a string, so that if it is an table, nil, etc. it will display as "table", "nil", etc. rather than as Script error.

Format
The WP:Lua style guide gives some basic formatting suggestions expected by the Javascript module editor, such as using four-space indentations and keeping if, then, else, end at the same level of indentation.

Comments to the end of a line are marked by --. Use them. Many modules for Wikipedia have a straightforward, linear design, but that doesn't mean it won't help to have your sections clearly labelled when you go back to the code for the hundredth time. The Lua style guide gives additional recommendations for using functions to keep your work more organized.

Exasperating bugs
Some bugs you might want to keep in mind:


 * Attempt to call a string value. It means you forgot the .. between a string and a variable somewhere in a mess of stuff you're concatenating.
 * A variable ignores all your efforts to assign stuff to it. You may have inadvertently written two local statements - the one sets the value of the variable within a limited region, and when the program leaves that region, you're back to the old value.
 * A numbered table entry ignores all your efforts to assign to it. This is because a["50"] is not a[50].  Typically you have processed a parameter (which you may have received from the invoke as a string) with string functions in one place, but performed numeric operations in another, leaving you with two different types of variable to use for an index.
 * Some graphics you're trying to display are heading off to the hills. (actually a HTML error) You didn't close your   s, so all the top: and left: styles keep adding up.
 * ... nil ... There are all sorts of things you can't do to a nil variable, like assign   or x[n] if   is nil, concatenate   if a or b is nil, or evaluate a[x] if x is nil.  Initialize such variables with (local) x={}, a="", etc.  Often "global" is mentioned in these errors because you didn't have a   statement for the nil variable.
 * string expected, got function. Some important things like mw.ustring.gmatch actually return functions, not strings - see Functions below.
 * no such module. You #invoked a module that didn't exist -- or wrote #invoke:Module:x instead of #invoke:x.
 * the function specified did not exist. You #invoked a module, but the field after the name of the module is wrong.  Often this field expects a standard name like "main", and you've forgotten it and gone straight to the first data parameter.  If you're unsure of the function name, check the module documentation, or look for what function(s) in the code accept a "frame" parameter.

Understanding tables and related concepts

 * An expression list is a set of values separated by commas. The values can be strings, numbers, tables, functions, etc.
 * A sequence is a set of entries with indices from 1 to N, where N is a positive integer. They can be created by placing brackets around an expression list.  For example, if   then  =1,  ="quotation",   is the function returned by , and   is the table  .  An expression list can also be recovered from a table using  :    will set  =1,  ="quotation", and   as the function returned by  ;   will be discarded in this case.
 * A table is a sequence, optionally supplemented by named keys: .  Several table functions like   will only work with the numbered values and ignore named keys.
 * The metatable offers a large, optional set of methods for altering table behavior. For example, you can define a table to be callable like a function.

Initializing a table
It is often useful to create a whole table at once in a statement. There are many equivalent ways to do this, but the shortcuts don't work for every kind of value. To begin with, the most general way is to assign each key and value explicitly:

If sequence keys (positive integers) are given in order, only the values need to be given, so the following will assign  to  :

If a key has only letters, digits, and underscores, and begins with a non-digit, the brackets and quotation marks can be omitted:

This is identical to.

However, this will fail for keys that begin with a digit:  will produce an error; use   instead.

Note that when given within brackets, or to the right of the equal sign, quotation marks are needed, or else string values will be taken as variables:

assigns the value of variable c to the key contained in variable b.

Functions

 * Functions can return any kind of value -- including a function. This is a powerful feature that can readily confuse the beginner.  If you set , the result assigned to   will be a function, not a string character! However, assigning   by calling the function stored in   will return the first match (a string). Every time you set   after that you'll get another match (string) into  , until you run out of matches and get  .  Many iterator functions act this way.
 * You can keep separate counts for iterator functions by using different variables. For example, if you set  in the same module, you can pull characters from the same piece of text  by evaluating   without losing your place in.
 * Tail calls offer substantial benefits in performance for those who master the language.
 * Function names are often of the form "p.myFunctionName", where p is the table from the "return p" at the bottom of your program. The reason for this is that you can only access functions that are entries in this table from the original #invoke statement.  Functions for local use within the program can have any name.

Understanding patterns
Note: Lua patterns are not regular expressions in the traditional POSIX sense, and they are not even a subset of regular expressions. But they share many constructs with regular expressions (more below).

Lua patterns are used to define, find and handle a pattern in a string. It can do the common search and replace action in a text, but it has more options that doing plain text only. For example, in one go it can change the errors 'New yorker', 'New-Yorker', and 'NewYorker' into 'New Yorker'.


 * To begin with, a pattern works like a plain string so long as it doesn't contain the special characters ^ $ % . [] * + - ?


 * Square brackets [ ] are used to match one single character in the string from a list of choices. [abc] matches the letters a, b, or c. With ^ right after [ they indicate "anything but": [^abc] = not a, b, or c.  Inside brackets and when not the first character, a minus - indicates a range: [a-z] matches one single character from a, b, c, …, z.


 * Period . matches any character.


 * Percent % indicates a large set (class) of possible character matches when it is followed by a letter. See  for a full list.   When followed by punctuation (whether a special character above or not) the % is removed and the punctuation is taken as a literal character; %% = literal %.  Special classes include a balanced class %bxy and %f[set]; see the link above for more.


 * Parentheses  indicate captures. The captures can be accessed later in the search string or in the string.gsub replacement string as %1 to %9, and are returned by string.match as an expression list of results.


 * The qualifiers ? - * + specify repetitions of a single character (not a longer string).
 * ? means 0 or 1 repetitions: "a?" matches "a" or "".
 * - means 0 or more repetitions, choosing as few as possible to achieve a match ("non-greedy"). For example string.match("bbbb", "(.-)") yields "", which is less than useful because there is nothing to root the ends of the expression and prevent it from matching zero characters.
 * * means 0 or more repetitions, choosing as many as possible ("greedy"). For example string.match("bbbb", ".*") yields bbbb.
 * + means 1 or more repetitions, choosing as many as possible ("greedy").

Note that the greediness of the leftmost qualifier rules over all others when there is a choice: (.*)b(.*) when matched on "bbb" will return "bb", "", while a(.-)b(.-)a when matched on "abbba" will return "", "bb".

The reference manual for Lua patterns is at metawiki.
 * ^ and $ indicate the beginning and end of the string if they occur in the appropriate place in the pattern. Otherwise they are literal characters.  ^ is not used in the string.gmatch function.

Note on Lua patterns versus regular expressions
Lua patterns are loosely based on regular expressions (sometimes shortened to regex or regexp). Lua patterns deliberately lacks the most complex regular expression constructs (to avoid bloating the Lua code base), where many other computer languages or libraries use a more complete set. Lua patterns are not even a subset of regular expressions, as there are also discrepancies, like Lua using the escape character  instead of , and additions, like Lua providing   as a non-greedy version of.

Here is a list of some of the things that Lua patterns lack compared to regular expressions: There are Lua libraries that offer more powerful options, including regular expressions, but the support on Wikipedia is pretty basic.
 * You cannot search for alternations between anything else than single characters (you cannot say  to choose between   and , you can only say   to choose between single characters  ,  , or  ).
 * You cannot look for multiples of multi-letter constructs such as  to match ,   or  . There is no way to do this with Lua patterns.
 * You cannot specify the minimum and maximum number of repetitions like  (to match 3 to 5 digits); in Lua you would say   instead in this case.

Wikipedia help for regular expressions (which Lua, as mentioned, does not support) is at AutoWikiBrowser/Regular expression.