Nhwɛdo:If empty/sandbox
This is the template sandbox page for Nhwɛdo:If empty (diff). See also the companion subpage for test cases. |
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This template is used in system messages. Changes to it can cause immediate changes to the Wikipedia user interface. To avoid major disruption, any changes should be tested in the template's /sandbox or /testcases subpages, or in your own user subpage. The tested changes can be added to this page in a single edit. Please discuss changes on the talk page before implementing them. |
This template uses Lua: |
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
sesa muThe 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
sesa muParameters 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
sesa muCode | Result | |
{{if empty}} | Returns an empty string. | |
{{if empty|one}} | one | Returns the first parameter that is defined and not empty. |
{{if empty|one|two}} | one | |
{{if empty|one|two|three|four}} | one | |
{{if empty|two}} | two | The first parameter is empty/undefined, so is passed over. |
{{if empty|two|three|four}} | two | |
{{if empty|two|four}} | two | |
{{if empty||||||||||ten}} | ten | Unlike in previous versions, this template is no longer limited by 9 parameters. |
{{if empty|}} | The only parameter is empty or undefined, so returns an empty string. | |
{{if empty|||||}} | Returns an empty string. | |
{{if empty|{{{1|}}}|{{{2|}}}|three}} | three | |
{{if empty|{{{1}}}|{{{2}}}|three}} | {{{1}}} | 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}} | two | |
{{if empty|{{{logo}}}|two}} | {{{logo}}} | No pipe-character following the parameter name "logo", so the text "{{{logo}}}" returned. |
{{if empty|p=q}} | The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc. |