View source for Template:If empty/doc

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{{Cascade-protected template}}
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{{Lua|Module:If empty}}

This template is used inside other templates. It takes any number of parameters and returns the first found to be defined and non-empty. If none is found, nothing is returned. By writing <code><nowiki>{{if empty|a|b|c}}</nowiki></code> instead of <code><nowiki>{{#if:a|a|{{#if:b|b|c}}}}</nowiki></code>, expression a and b do not have to be [[Don't repeat yourself|repeated]] and evaluated twice. The template provides a [[fallback]] order, similar to a [[try catch]] based "It is easier to ask for forgiveness than permission" ([[Time-of-check to time-of-use#Preventing TOCTOU|EAFP]]) programming style.

Typical usage is like this:
: <code><nowiki>{{If empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}</nowiki></code>
: — <nowiki>Return {{{logo|}}} if it has a value; else {{{image|}}} if it has a value; else {{{picture|}}} if it has a value; else return "default.svg".</nowiki>
This returns the first of the parameters ''logo'', ''image'' and ''picture'' that is defined and non-empty, otherwise "default.svg".

===Rationale===
The MediaWiki parameter default function doesn't return the default value for empty parameters. That is, <code><nowiki>{{{logo|default.svg}}}</nowiki></code> does not return "default.svg" if the template was called like this: <code><nowiki>{{template|logo=}}</nowiki></code>.

The usual workaround for a single parameter is:
: <code><nowiki>{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}</nowiki></code>
: — <nowiki>If {{{logo}}} has a value, return it, else return "default.svg".</nowiki>

But this becomes complex when several parameters are to be checked:
: <code><nowiki>{{#if:{{{logo|}}} |{{{logo}}} |{{#if:{{{image|}}} |{{{image}}} |{{#if:{{{picture|}}} |{{{picture}}} |default.svg}} }} }}</nowiki></code>
: — <nowiki>If {{{logo}}} has a value, return it; else if {{{image}}} has a value, return that; else if {{{picture}}} has a value, return that; else return "default.svg".</nowiki>
In these cases, {{tlf|if empty}} produces the simpler syntax (as above):
: <code><nowiki>{{if empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}</nowiki></code>

====Piping====
Parameters used with {{tlf|if empty}} must be [[Pipe (Unix)|pipe]]d&nbsp;– i.e. include the [[Vertical bar|vertical bar (pipe) symbol]] {{nowrap|("{{thin space}}&#124;{{thin space}}")}} as a trailing character&nbsp;– so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example, <code><nowiki>{{{logo|}}}</nowiki></code>, <code><nowiki>{{{image|}}}</nowiki></code> and <code><nowiki>{{{picture|}}}</nowiki></code> rather than {{param|logo}}, {{param|image}} and {{param|picture}} in the above examples.

===Examples===
{{Aligned table |class=wikitable
 |cols=3 |col1style=padding-right:1.0em; |col2style=padding-left:0.5em;padding-right:1.0em; |col3style=padding-left:0.5em;
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 | Code | Result |
 | {{tlc|if empty}} | {{if empty}} | Returns an empty string.
 | {{tlc|if empty|one}} | {{if empty|one}} | Returns the first parameter that is defined and not empty.
 | {{tlc|if empty|one|two|three|four}} | {{if empty|one|two|three|four}} | 
 | <code><nowiki>{{if empty||two}}</nowiki></code> | {{if empty||two}} | The first parameter is empty/undefined, so is passed over.
 | <code><nowiki>{{if empty||two||four}}</nowiki></code> | {{if empty||two||four}} | 
 | <code><nowiki>{{if empty||||||||||ten}}</nowiki></code> | {{if empty||||||||||ten}} | Unlike in previous versions, this template is no longer limited by 9 parameters.
 | {{tlc|if empty{{!}}}} | {{if empty|}} | The only parameter is empty or undefined, so the template returns an empty string.
 | {{tlc|if empty|{{!}}{{!}}{{!}}{{!}}}} | {{if empty||||}} | Returns an empty string.
 | <code><nowiki>{{if empty|{{{1|}}}|{{{2|}}}|three}}</nowiki></code> | {{if empty|{{{1|}}}|{{{2|}}}|three}} | Neither 1 nor 2 are defined, so "three" is returned.
 | <code><nowiki>{{if empty|{{{1}}}|{{{2}}}|three}}</nowiki></code> | {{if empty|{{{1}}}|{{{2}}}|three}} | No pipe-characters following the names of the first two parameters ("1" and "2"), so the first is returned as text ("{{{1}}}").
 | <code><nowiki>{{if empty|{{{logo|}}}|two}}</nowiki></code> | {{if empty|{{{logo|}}}|two}} | 
 | <code><nowiki>{{if empty|{{{logo}}}|two}}</nowiki></code> | {{if empty|{{{logo}}}|two}} | No pipe-character following the parameter name "logo", so the text "{{{logo}}}" is returned.
 | <code><nowiki>{{if empty|p=q}}</nowiki></code> | {{if empty|p=q}} | The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc.
}}

===See also===
* {{tl|if all}}
* {{tl|if either}}
* [[Help:Parameter default]]
* {{tl|Category see also if exists 2}}

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