Template:If empty/doc

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  instead of , 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:
 * — Return if it has a value; else  if it has a value; else  if it has a value; else return "default.svg".
 * — Return if it has a value; else  if it has a value; else  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,  does not return "default.svg" if the template was called like this:.

The usual workaround for a single parameter is:
 * — If has a value, return it, else return "default.svg".
 * — If has a value, return it, else return "default.svg".

But this becomes complex when several parameters are to be checked:
 * — If has a value, return it; else if  has a value, return that; else if  has a value, return that; else return "default.svg".
 * — If has a value, return it; else if  has a value, return that; else if  has a value, return that; else return "default.svg".

In these cases, produces the simpler syntax (as above):

Piping
Parameters used with must be piped – i.e. include the vertical bar (pipe) symbol (" &#124; ") as a trailing character – so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example,,   and   rather than ,  and  in the above examples.