ParserFunctions

This MediaWiki extension is a collection of parser functions. Parser functions typically have the syntax:

{{#functionname: argument 1 | argument 2 | argument 3...}}

Functions

This module defines six functions at present: expr, if, ifeq, ifexist, ifexpr, and switch.

#expr:

The expr function computes mathematical expressions based on permutations of numbers (or variables/parameters that translate to numbers) and operators. It does not work with strings; use ifeq below instead. The syntax is:

{{ #expr: expression }}

A list of supported operators follows. For more details about the operator precedence see Help:Calculation, it's roughly (1) grouping (parentheses), (2) unary (+/- signs and NOT), (3) multiplicative (*, /, div, mod), (4) additive (+ and -), (5) round, (6) comparative (=, !=, <, >, etc.), (7) logical AND, (8) logical OR. Within the same precedence class operators are evaluated left to right. As always some redundant parentheses are better than erroneous terse code.

The boolean operators consider 0 to be "false" and any other number to be "true", on output "true" is shown as {{#expr: 30 and 7}}.

Numbers are given in decimal with "." for the decimal point. Scientific notation with E plus exponent is not yet supported on input for expressions, but used on output, for details see Help:Calculation.

#if:

The if function is an if-then-else construct. The syntax is:

{{ #if: <condition> | <then text> | <else text> }}

If the condition is an empty string or consists only of whitespace, then it is considered false, and the else text is returned. Otherwise, the then text is returned. The else text may be omitted, in which case the result will be blank if the condition is false.

An example:

                      {{Template|parameter=something}}  {{Template}} {{Template|parameter=}}
                                     |                        |                |
                                     |                        |                |
                                     |                        |                |
{{ #if: {{{parameter|}}} | Parameter is defined. | Parameter is undefined, or empty }}

Note that the if function does not support "=" signs or mathematical expressions. {{#if: 1 = 2|yes|no}} will return "yes", because the string "1 = 2" is not blank. It is intended as an "if not empty" structure.

#ifeq:

ifeq compares two strings or numbers, and returns another string depending on the result of that comparison. The syntax is:

{{ #ifeq: <text 1> | <text 2> | <equal text> | <not equal text> }}

If both strings can be interpreted as numbers the comparison is numerical. To force a string comparison add tokens that can't be interpreted as numbers:

{{ #ifeq: +07 | 007 | 1 | 0 }} gives {{#ifeq: +07 | 007 | 1 | 0 }}

{{ #ifeq:"+07"|"007"| 1 | 0 }} gives {{#ifeq:"+07"|"007"| 1 | 0 }}

For compatibility with older templates #if: cannot directly distinguish defined and undefined parameter values, it's a shorthand for a comparison with the empty string. With #ifeq: it's directly possible to identify undefined parameters:

{{ #if: {{{x| }}}|not blank|blank}} = {{#if: |not blank|blank}},

{{ #ifeq: {{{x| }}}| |blank|not blank}} = {{#ifeq: | |blank|not blank}},

{{ #ifeq: {{{x| }}}|{{{x|u}}}|defined|undefined}} = {{#ifeq: |u|defined|undefined}}.

An undefined parameter without default counts in the comparison as a string consisting of the tag:

{{ #ifeq: {{{x}}}|{{ concat|{|{|{x}|}|} }}|1|0}} = {{ #ifeq: {{{x}}}|Template:Concat|1|0}}.

#ifexist:

ifexist returns one of two results based on whether or not a named title exists, e.g.

{{#ifexist:Foo|Bar|RFC 3092}} gives {{#ifexist:Foo|Bar|RFC 3092}}, because Foo {{#ifexist:Foo|exists|doesn't exist}}.

{{#ifexist:ParserFunctions|Thanks|No}} gives {{#ifexist:ParserFunctions|Thanks|No}}.

{{#ifexist:m:Help:Calculation|Yes|Oops}} gives {{#ifexist:m:Help:Calculation|Yes|Oops}} although m:Help:Calculation exists, because of the interwiki prefix.

The first parameter is the title to check for, the second is the positive result, and the third, the negative result. If the parameter passed does not produce a valid title object, then the result is negative.

Template:Tim gives the same result, except that the result is positive for an interwiki link. Template:Tim exploits this difference.

#ifexpr:

ifexpr evaluates a mathematical expression and returns one of two strings depending on the result.

{{#ifexpr: <expression> | <then text> | <else text> }}

If the expression evaluates to zero, then the else text is returned, otherwise the then text is returned. Expression syntax is the same as for expr.

At the moment the else text is also returned for an empty expression:

{{#ifexpr: {{ns:0}}|Toast|'''or else'''}} gives {{#ifexpr: |Toast|or else}}

Omitting both then text and else text gives no output except possibly an error message; this can be used to check the correctness of an expression, or to check the wording of the error message (emulated assertions, forced errors):

{{#ifexpr: 1/{{#ifeq: {{ns:4}}|Meta|1|0}}}} {{#ifexpr: 1/{{#ifeq: SikhiWiki|Meta|1|0}}}}

{{#ifexpr: 1/{{#ifeq: {{ns:4}}|Meta|0|1}}}} {{#ifexpr: 1/{{#ifeq: SikhiWiki|Meta|0|1}}}}

{{#if:{{#ifexpr: 1=2}}|wrong|correct}} {{#if:{{#ifexpr: 1=2}}|wrong|correct}}

{{#if:{{#ifexpr: 1E2}}|wrong|correct}} {{#if:{{#ifexpr: 1E2}}|wrong|correct}}

{{#if:{{#ifexpr: 1/0}}|wrong|correct}} {{#if:{{#ifexpr: 1/0}}|wrong|correct}}

{{#if:{{#ifexpr: a=b}}|wrong|correct}} {{#if:{{#ifexpr: a=b}}|wrong|correct}}

For an application, see also Template:Tim.

#switch:

switch compares a single value against multiple others, returning a string if a match is found. The syntax is basically:

{{ #switch: <comparison value>
| <value1> = <result1>
| <value2> = <result2>
| ...
| <valuen> = <resultn>
| <default result>
}}

switch will search through each value passed until a match is found with the comparison value. When found, the result for that value is returned (the text string after the equal sign). If no match is found, but the last item has no equal sign in it, it will be returned as the default result. If your default result must have an equal sign, you may use #default:

{{ #switch: <comparison value>
| <value> = <result>
| #default = <default result>
}}

Note that it's also possible to have "fall through" for values (reducing the need to duplicate results). For example:

{{ #switch: <comparison value>
| <value1>
| <value2>
| <value3> = <result3>
| ...
| <valuen> = <resultn>
| <default result>
}}

Note how value₁ and value₂ contain no equal sign. If they're matched, they are given the result for value₃ (that is, whatever is in result₃).

As for #ifeq: the comparison is numerical where possible:

{{ #switch: +07 | 7 = Yes | 007 = Bond | No }} gives {{ #switch: +07 | 7 = Yes | 007 = Bond | No }}

{{ #switch:"+07"|"7"= Yes |"007"= Bond | No }} gives {{ #switch:"+07"|"7"= Yes |"007"= Bond | No }}

The matched value can be empty, therefore the following constructs are equivalent:

{{ #if: {{ns:0}} | not empty | empty }} gives {{ #if: | not empty | empty }}

{{ #switch:{{ns:0}}|=empty|not empty }} gives {{ #switch:|=empty|not empty }}

require_once( "$IP/extensions/ParserFunctions/ParserFunctions.php" );

 $wgParser->setFunctionHook( 'expr', array( &$wgExtParserFunctions, 'expr' ) );
 $wgParser->setFunctionHook( 'if', array( &$wgExtParserFunctions, 'ifHook' ) );
 $wgParser->setFunctionHook( 'ifeq', array( &$wgExtParserFunctions, 'ifeq' ) );
 $wgParser->setFunctionHook( 'ifexpr', array( &$wgExtParserFunctions, 'ifexpr' ) );
 $wgParser->setFunctionHook( 'switch', array( &$wgExtParserFunctions, 'switchHook' ) );
 $wgParser->setFunctionHook( 'ifexist', array( &$wgExtParserFunctions, 'ifexist' ) );

 $wgParser->setFunctionHook( '#expr', array( &$wgExtParserFunctions, 'expr' ) );
 $wgParser->setFunctionHook( '#if', array( &$wgExtParserFunctions, 'ifHook' ) );
 $wgParser->setFunctionHook( '#ifeq', array( &$wgExtParserFunctions, 'ifeq' ) );
 $wgParser->setFunctionHook( '#ifexpr', array( &$wgExtParserFunctions, 'ifexpr' ) );
 $wgParser->setFunctionHook( '#switch', array( &$wgExtParserFunctions, 'switchHook' ) );
 $wgParser->setFunctionHook( '#ifexist', array( &$wgExtParserFunctions, 'ifexist' ) );