Template:If empty/doc

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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 {{if empty|a|b|c}} instead of {{#if:a|a|{{#if:b|b|c}}}}, expression a and b do not have to be 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" (EAFP) programming style.

Typical usage is like this:

{{If empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}

— 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".

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, {{{logo|default.svg}}} does not return "default.svg" if the template was called like this: {{template|logo=}}.

The usual workaround for a single parameter is:

{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}

— If {{{logo}}} has a value, return it, else return "default.svg".

But this becomes complex when several parameters are to be checked:

{{#if:{{{logo|}}} |{{{logo}}} |{{#if:{{{image|}}} |{{{image}}} |{{#if:{{{picture|}}} |{{{picture}}} |default.svg}} }} }}

— 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".

In these cases, {{if empty}} produces the simpler syntax (as above):

{{if empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}

Piping

Parameters used with {{if empty}} must be piped – i.e. include the vertical bar (pipe) symbol ("|") as a trailing character – so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example, {{{logo|}}}, {{{image|}}} and {{{picture|}}} rather than {{logo}}, {{image}} and {{picture}} in the above examples.

Examples

Code	Result
{{if empty}}	main}}	Returns an empty string.
{{if empty|one}}	main}}	Returns the first parameter that is defined and not empty.
{{if empty|one|two}}	main}}
{{if empty|one|two|three|four}}	main}}
{{if empty||two}}	main}}	The first parameter is empty/undefined, so is passed over.
{{if empty||two|three|four}}	main}}
{{if empty||two||four}}	main}}
{{if empty||||||||||ten}}	main}}	Unlike in previous versions, this template is no longer limited by 9 parameters.
{{if empty{{!}}}}	main}}	The only parameter is empty or undefined, so returns an empty string.
{{if empty|{{!}}{{!}}{{!}}{{!}}}}	main}}	Returns an empty string.
{{if empty|{{{1|}}}|{{{2|}}}|three}}	main}}
{{if empty|{{{1}}}|{{{2}}}|three}}	main}}	No pipe-characters following the names of the first two parameters ("1" and "2"), so the first of these returned as text ("{{{1}}}").
{{if empty|{{{logo|}}}|two}}	main}}
{{if empty|{{{logo}}}|two}}	main}}	No pipe-character following the parameter name "logo", so the text "{{{logo}}}" returned.
{{if empty|p=q}}	main}}	The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc.