Template:Ifexpr
With this template #ifexpr can be used in a way such that it does not strip spaces and newlines from the "then" and "else" part.
Usage
{{ifexpr|2<3| p | q }}gives " p "{{ifexpr|2>3| p | q }}gives " q "{{ifexpr|2{{=}}3| p | q }}gives " q "{{ifexpr||1=2=3| p | q }}gives " q "{{ifexpr||1=7=7| ok true | no }}gives " ok true "{{ifexpr||1=3*4<=10+1| ok| oh no}}gives " oh no"
Compare:
{{#ifexpr:2<3| p | q }}gives "p"
Note that, unlike parser functions, templates require writing equals signs in values of unnamed parameters by using double-braces, as {{=}}. So, to allow "=" (or "<=" or ">=") then use the named parameter 1, as in: 1={{{x}}}>=7.
For full substitution, simply use subst: or safesubst:. If the condition contains a parser function, template, or variable, that should be substituted too. Optionally the "then" or "else" part can also be substituted.
Error messages (examples):
{{ifexpr|1234>e3|1|0}}gives
ERROR: invalid parameters for Template:Ifexpr:
{{Ifexpr...|1234>e3|1|0}}
Check for invalid numbers, or parameter 1 must be empty when using "1=" format.
{{#ifexpr:1234>e3|1|0}}gives Expression error: Unexpected number., for e3.{{#ifexpr:1234>1e3|1|0}}gives 1, because 1,234 > 1e3 (1,000).
Expansion depth limit
During 2011-2012, the nesting limit of {Ifexpr} was 12 nested structures (this is one of the template limits for the MediaWiki parser). For example, nesting 13 times can cause an error:
{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|{{ifexpr|1|a}}}}}}}}}}}}}}}}}}}}}}}}}}
- Result: {{safesubst:{{Expansion depth limit exceeded|p1=p1|p2=p2|if/errmsg}}|a||1}}
The expansion-depth error occurs when exceeding the depth limit of nested markup, typically 41 levels of nested templates or if-else structures, during 2009-2012.
Internal technical details
For evaluating the condition the template uses the actual parser functions; however, not directly with p and q, because this strips spaces and newlines from them, but with index values 1 and 2. The result can be 1, 2, or an error message. Using a switch these are converted to p1, p2, and if/errmsg, respectively. The template with the applicable name is called (Template:P1, Template:P2, or Template:If/errmsg), with as parameters p, q, and {{{1}}}. Templates p1 and p2 ignore {{{1}}}; template if/errmsg is only called in the case of a wrong expression {{{1}}}; it ignores p and q and uses {{{1}}} for display in the error message.