mediawikisemantic-mediawiki

Where does the inconsistency in behaviour of Mediawiki templates come from?


This question pertains to the inconsistent behaviour of Mediawiki syntax and its templates and parser functions.

When placed directly in a wiki page, both these snippets work as expected:

{{#ifeq:{{{1|}}}|{{{2}}}|{{{1}}}}}

and

{{#ifeq:{{{1|{{FULLPAGENAME}}}}}|{{NAMESPACE:{{{1|{{FULLPAGENAME}}}}}}}:{{ROOTPAGENAME:{{{1|{{FULLPAGENAME}}}}}}}|{{{1|{{FULLPAGENAME}}}}}}}

The first snippet is simple. The second outputs the full name of the page, but only if it is not a sub-page.

However, when each snippet is placed in a template, let's say the first in Template:IsEqual, and the second in Template:IsRootPage, both of which are then included in some page, such as the output of each is the subject of an #if test, the first continues to behave as expected, while the second always yields the string not empty section of the #if statement.

So:

{{#if:{{IsEqual|A|B}}|equal|unequal}} will output "unequal"

while:

{{#if:{{IsRootPage}}|root|non-root}} will always output "root", even if placed in a subpage or if the name of a non-top-page is passed as a parameter to Template:IsRootPage, driving the template to return nothing.

According to the documentation of #if:

This function evaluates a test string and determines whether or not it is empty. A test string containing only white space is considered to be empty.

Moreover, the inconsistent behaviour persists for Template:IsRootPage even when it is amended to return an empty string when the #ifeq test fails, by introducing a fourth empty parameter to the #ifeq in the template, i.e by adding a third | followed by nothing/space.

It seems that something happens in between the output of the template and the #if parser function.

Is there a known explanation for this discrepancy in behaviour?

Is there some syntax hack to make it work as expected?

Or am I missing something that's otherwise obvious?


Edit: This behaviour is witnessed in Mediawiki 1.35.5


Solution

  • Your sample template does not return an empty string when the page has sub pages. It returns <nowiki/> for which {{#if:<nowiki/>|yes|no}} properly returns yes.

    If you remove the <nowiki/>, you'll get the behavior you want.