Difference between revisions of "SyntaxWML"

Revision as of 06:44, 29 January 2018

The Wesnoth Markup Language (WML) is used to code almost everything in Wesnoth, including scenarios, units, savefiles, and the user interface layout. WML files are simple, human-readable text files, usually with the .cfg extension, with similarities to INI files and XML. For guidelines on keeping these files easily human-readable, see ConventionsWML.

Tag and Attribute Structures

WML has a syntax containing two basic elements: tags and attributes. Furthermore, attributes consist of keys and values. For example:

[tag]
    key=value
[/tag]

Tags are used to partition information, while the data is contained in the attributes. Keys identify the type of data to be stored and values are the actual data stored. When WML is processed, the tag identifies some unit of information, such as an action to perform or even an entire campaign. This gives a context for the attributes within the tag. For each key=value line within a tag, the attribute identified by key has its data set to value. Also allowed inside a tag is another tag. The inner tag is considered the child of the outer tag, as in the following example.

[parent_tag]
    key1=value1
    [child_tag]
        key2=value2
    [/child_tag]
[/parent_tag]

Every tag describes something different about the game; different tags work differently, with the allowed tags defined by context. There are several "top-level tags" that are allowed when not inside any other tag, and each tag defines which child tags (and which keys) it recognizes. Unrecognized tags and keys, such as the result of typos, sometimes produce error messages, but at other times they are ignored.

Keys should not be confused with variables! A common mistake among beginners is to make up undocumented key names. Instead, consult the WML Reference to find allowed key names for each tag. For a list of all tags with links to their documentation, see the "WML Tags" navigation box.

Also, tag and key names follow a special format. They will contain only alphanumeric characters and underscores; in particular, they will not contain +, -, or whitespace. The values, however, may contain such characters when needed.

Tag Amendment Syntax

Inserting a plus sign (+) before a tag name allows one to append to an earlier tag (the most recent with the same name) rather than starting a new tag. This allows attributes to be added or replaced.

[tag]
    key=value
[/tag]
[+tag]
    key=value
[/tag]

All keys in the +tag will be set to the given values. If the keys did not exist in the most recent [tag] then they are added to that [tag]; otherwise their values will replace the old values in the most recent [tag].

Any child tags of the +tag will be appended to the children of the most recent [tag]. To be clear: none of those original child tags will be altered by this operation, since this is an "append" and not a "merge."

It is even possible to make tag amendments to a child tag after the parent tag has already closed. Using [+tag] syntax multiple times in a row (first for the parent, then for the child) will allow you to amend the more inward scopes.

Multiple Assignment Syntax

It is possible to set multiple attributes on a single line. This is done by listing the associated keys, followed by an equal sign, followed by the desired values.

[tag]
    key1,key2,key3=value1,value2,value3
[/tag]

would be the same as:

[tag]
    key1=value1
    key2=value2
    key3=value3
[/tag]

If there are extra keys, they will be set to an empty value. If there are extra values the last key will be set to the comma-separated list of all remaining values.

Special Attribute Values

Although an attribute's value can be just text corresponding to the function of its key, a value can also be encoded in many other ways, each with a specific purpose.

key = "value": a quoted value is a value surrounded by quotes. This is often unnecessary as single-line values are typically interpreted as intended. However, quotes are required in order to specify multiple-line values (a line break inside quotes does not end the value). Quotes may also be required to cancel the special meaning of other characters, and they prevent spaces from being stripped. It is never wrong to use quotes with correct WML.
key = _"value": a translatable value is a value that is subject to translations, and should be used for all text intended to be shown to a player (most notably seen in [story], [message], and the name= key in unit definitions). A translatable value is surrounded by quotes and preceded by an underscore (_). In terms of WML syntax, it behaves very much like a quoted value, other than being unsuitable for comparisons to other values. Translatable values are intended for display on the screen, not for internal data. See to TranslationsWML for more information.
key = "value1" + "value2": string concatenation is performed with the plus sign (+). If a plus sign appears outside quotes in a value, it means that the string/value on its right will be appended to the string/value on its left. To have an actual plus sign in a value, the string containing the + character must be surrounded by quotes (a quoted value or a translatable value). Quotes are not strictly necessary around the pre-concatenated values, but they are advisable so that it is easy to tell where the values begin and end and to spot some kinds of mistakes.
key = "quoted ""double quoted value"" value": doubled quotes can be used to create quote marks within a quoted or translatable value. The doubled quote mark in the value produces one quote mark in the stored data and does not terminate the quoted value. (These do not necessarily need to be used in pairs.)
key = $variable: a variable substitution sets the key to the value of the indicated WML variable. This is indicated by the dollar sign ($) and is really just a special case of general variable substitution, as variables can be substituted within other values. See below for more information on values based on WML variables. (Note that some keys require their data to be a variable name, not the variable's value; in that case there would be no dollar sign.) Variable substitution is supported in only a few contexts, such as in IntroWML and EventWML.
key = "$(formula-expression)": a formula expression sets the key to the value of the processed formula. This is indicated by a dollar sign ($) followed by a parenthesized expression. See Wesnoth Formula Language for more information on formula basics, data types, and built-in functions. Quotes around the formula are not strictly necessary in all cases, but they are advisable, particularly since quotes are the only way to use a plus sign (+) within a formula (without quotes, the plus sign represents string concatenation). Formula expressions are only supported where variable substitution is supported.
key = <<value>>: similar to a quoted value but stronger: the value is hidden from the preprocessor, which will see all text therein as literal. Useful with LuaWML or whenever a literal curly brace ("{", a US-ASCII 0x7B) is needed in any text string.

Variables

Variables in WML are used to store data for later retrieval. Each variable is identified by its name. Once created, a variable persists until the end of a campaign unless explicitly cleared. Variable name should contain only alphanumerics and underscores, and the first character should be non-underscore. Regular expression for variable name: [a-zA-Z0-9][a-zA-Z0-9_]*.

The three basic manipulations of WML variables are assigning a value, querying the value, and clearing the variable.

Assigning to a variable: stores a value in the variable. This is done with tags like [set_variable] or with macros like {VARIABLE}.
Querying a variable: returns the last value stored in the variable (or the empty string, if no value was). This is done by prefixing the variable name with a dollar sign, as in $variable, and sometimes ending the variable name with a pipe character, as in $variable|.
Clearing a variable: makes the WML engine forget about that variable. This is useful for reducing overhead, since all used variables are stored in saved games. This is done with [clear_variable] or the {CLEAR_VARIABLE} macro.

Kinds of Variables

Scalar

A scalar variable can store a single string or number.

[set_variable]
    name=my_variable
    value="sample value"
[/set_variable]

The full name of a scalar variable is its given name, in this case my_variable. Note that the value of the variable can be translatable or even a formula expression (Special Attribute Values).

Array

An array variable is a numbered sequence of container variables. There are some specific tags that assign array information, for example [store_unit] and [store_locations]. One could create an array using [set_variable] like this:

[set_variable]
    name=my_awesome_array[0].x
    value=10
[/set_variable]
[set_variable]
    name=my_awesome_array[1].x
    value=12
[/set_variable]
[set_variable]
    name=my_awesome_array[2].x
    value=14
[/set_variable]

However, when working with arrays, it is usually easier to make use of [set_variables]. This would be written as follows:

[set_variables]
    name=my_awesome_array
    [value]
        x=10
    [/value]
    [value]
        x=12
    [/value]
    [value]
        x=14
    [/value]
[/set_variables]

If foo is the name of an array, foo[0] is the full name of its first container variable, foo[1] the full name of its second, and so on. foo.length is the special variable that always stores the number of containers in the array foo. Hence, if the value stored in foo.length is 18, the last container in the array would be foo[17]. If you try to query an array as if it were a container, then it will simply use the first index[0]. Thus $foo.bar would be the same as $foo[0].bar

Note: Do not attempt to store a scalar value to the explicit index of an array, which is a container of scalar variables. Hence referring to a variable named foo[3] as if it were a scalar one is illegal; instead, you would use foo[3].value to store a scalar value. (While it may appear to work to an extent if you ignore this rule, it may also cause undefined behavior. For example, loading a text save of a game that contains such variables will fail with a WML error.)

Container

A container variable can store any number of scalar and/or array variables. There are tags to assign specific information, for instance [store_side]. To refer to a variable bar stored in a container foo you would write foo.bar. An explicit index inside an array is also considered a container.

Conditionals

Variables may be compared by using [variable] within an [if] or [while] tag. For more information, please refer to ConditionalActionsWML.

Variable Substitution

Whenever using a $ in front of a variable name, the content which has previously been put into this variable name is used instead of the name of the variable. For example:

[event]
    name=turn 1
    [set_variable]
        name=my_variable
        value= _ "Konrad"
    [/set_variable]
    [message]
        speaker=Delfador
        message= _ "Hello, $my_variable|... How are you?"
    [/message]
[/event]

The WML code above will cause Delfador to say "Hello, Konrad... How are you?" on turn 1.

When writing scenario events (EventWML), a scalar variable can generally be substituted into the right-hand of any key=value assignment. If the provided value contains a $, the WML engine with interpret what is between the rightmost $ and the next | as a full variable name to be queried, and replace $variable| with the result of this query.

In certain situations, the | that marks the end of the variable name to be queried can be omitted. The exact rule is: if there is no |, variable names span letters, digits, underlines, balanced square brackets and some periods. Doubled periods and some periods that would result in an illegal variable name will not be included. If the variable name ends up being empty (e.g. when using $|), then it will be replaced by just $, giving you an easy way to include a dollar sign in an interpolated string.

(Version 1.13.2 and later only) If you want to substitute a default value when the variable is uninitialized or empty, you can use the syntax $varname?default text|. In this case, the | is required.

Literal Mode

There are a few places where the substitution mode is literal. In these places, attribute value are used exactly as provided, nothing is substituted, and the $ will not have special significance. The following places use the literal mode:

value of literal= inside [set_variable]
contents of [literal] inside [set_variables]
the special [variables] tag, used to give initial values to many variables upon scenario start

Automatically Stored Variables

side_number: the number of the current player's side (may be empty during start or prestart events)
turn_number: the number of the current turn (may be empty during start or prestart events)
x1: this is the x-coordinate of the location where the most recent event was triggered
y1: this is the y-coordinate of the location where the most recent event was triggered
x2: this is the x-coordinate of the location that assisted in triggering the most recent event
y2: this is the y-coordinate of the location that assisted in triggering the most recent event
unit: inside an event, this is the unit at $x1,$y1
second_unit: inside an event, this is the unit at $x2,$y2
this_unit: inside a standard unit filter, this is the unit currently being considered for a possible match
other_unit: inside some standard unit filters, this is an adjacent unit relevant to the match
damage_inflicted: inside attacker_hits and defender_hits events, this is the amount of damage that was inflicted
weapon: inside attack, attack_end, attacker_hits, attacker_misses, defender_hits, defender_misses, die and last_breath events, this is some information about the weapon that is/was being used by the unit at $x1,$y1. It contains the attributes from [attack], see UnitTypeWML.
second_weapon: inside attack, attack_end, attacker_hits, attacker_misses, defender_hits, defender_misses, die and last_breath events, this is some information about the weapon that is/was being used by the unit at $x2,$y2. It contains the attributes from [attack], see UnitTypeWML.

Note: Automatically stored container and array variables are only stored once that one of their attributes is accessed for the first time. This means that one can sometimes get wrong results, for instance by killing the unit at $x1,$y1 as first action in a moveto event and then accessing $unit.something. This can be worked around by previously making a dummy access, such as adding 0 to hitpoints.

The [variables] tag

The [variables] tag is used in saved games to describe the current value of each variable, and in scenario files for assigning initial values to variables at scenario start.

A scalar variable is assigned using an attribute, where the attribute's key is the variable's given name, and the attribute's value is the value to be stored in the variable.

A container variable with given name foo is assigned using a [foo] tag that contains the definitions for the contained variables.

An array variable with given name foo is assigned using several [foo] tags, where the first tag describes foo[0], the second foo[1], ...

Storing variables inside units

Sometimes it is useful to store a custom WML variable inside a unit. Units stored with the [store_unit] command have a unit.variables sub-container where custom variables related to that unit may be saved. (Remember to [unstore_unit] for the changes to be kept.) One benefit of this approach is that the unit may then be filtered based on the value, for example:

[filter]
  [filter_wml]
    [variables]
      my_variable="test"
    [/variables]
  [/filter_wml]
[/filter]

Variable Usage Examples

Consider a saved game with the following [variables] tag (or a freshly started scenario with that tag)

[variables]
    attitude_of_elves=hate
    attitude_of_dwarves=love
    attitude_of_humans=like
    current_opponent=elves
[/variables]

Then,

[message]
   message="Oh, I see $current_opponent|! They surely $attitude_of_$current_opponent|| us!"
[/message]

displays the message

Oh, I see elves! They surely hate us!

Consider another game with variables

[variables]
    our_side=1
    their_side=2
[/variables]

where side 1 has 75 gold, and side 2 50 gold. Then,

[store_side]
    side=$our_side
    variable=we
[/store_side]
[store_side]
    side=$their_side
    variable=they
[/store_side]
[message]
    message=We have $we.gold gold, they have $they.gold gold.
[/message]
[if]
    [variable]
        name=we.gold
        greater_than=$they.gold
    [/variable]
    [then]
        [message]
            message=This should be easy!
        [/message]
    [/then]
    [else]
        [message]
            message=This will not be easy!
        [/message]
    [/else]
[/if]
[clear_variable]
    name=we
[/clear_variable]
[clear_variable]
    name=they
[/clear_variable]

displays the messages

We have 75 gold, they have 50 gold.
This should be easy!

If side 2 had 100 gold instead, the same code would display the messages

We have 75 gold, they have 100 gold.
This will not be easy!

The code

[store_unit]
    [filter]
        canrecruit=yes
        side=1
    [/filter]
    variable=leader
[/store_unit]
[message]
    message=Our leader's first attack does $leader[0].attack[0].damage damage per hit.
[/message]
[clear_variable]
    name=leader
[/clear_variable]

always displays a true sentence.

You may find more complicated examples of variable use in the UsefulWMLFragments section.

Comments

Comments are indicated by starting a line with a pound sign (#). Unless the line forms a valid preprocessor directive, all text after the pound sign will be ignored by the WML engine.

It is a very good coding convention to always add a space immediately following a pound sign for every comment. Not only does it avoid accidentally calling a preprocessor directive (for example, a commented line that begins with the word “define”) but it also makes comments stand further apart from the code.

Tutorial

How to use variables

@@ Line 81: / Line 81: @@
 == Variables ==
-Variables in WML are used to store data for later retrieval. Each variable is identified by its name. Variable name should contain only alphanumerics and underscores, it also should contain at least one non-underscore character. Once created, a variable persists until the end of a campaign unless explicitly cleared.
+Variables in WML are used to store data for later retrieval. Each variable is identified by its name. Once created, a variable persists until the end of a campaign unless explicitly cleared.
+Variable name should contain only alphanumerics and underscores, and the first character should be non-underscore. Regular expression for variable name: '''[a-zA-Z0-9][a-zA-Z0-9_]*'''.
 The three basic manipulations of WML variables are assigning a value, querying the value, and clearing the variable.