Feature Proposal: VAR{...} to get the value defined in a topic and the value in the current context

Motivation

As discussed in AddTopicAttributeToVAR, there is no way to get the members of a TWiki group. If there is a way to get the value of a preferences variable defined in a topic, we are golden.

%IF{"defined FOO" then="%FOO%"}% doesn't work well if FOO contains double quotes. %IF{"defined FOO" then="$percntFoo$percnt"}% works well but it's cumbersome. %VAR{"FOO"}% is close, but it does not retrieve the value of FOO if FOO is defined in the topic or Main.TWikiPreferences. There should be a straightforward way to retrieve the value of a variable meeting the following requirements:

• If the variable is defined in the current context, its value is returned even if it contains double quote characters
• Otherwise, a null string ("") is returned

Description and Documentation

%VAR{...}% will be enhanced in the following manner.

• If the topic parameter is specified, the value of the specified variable defined in the specified topic is returned.
• If the web parameter is specified and the topic parameter is not specified, %VAR{...}% works in the same manner as before.
• If neither the web nor topic parameter is specified, %VAR{...}% fetches the preferences variable or the session variable of the specified name in the current context.

Examples

Group members

%VAR{"GROUP" topic="Main.TWikiAdminGroup"}% returns the members of the TWikiAdminGroup

Hook in TWiki06x01 web topics

TWikiAccessControl can have %VAR{TWIKI_ACCESS_CONTROL_TOP}% at the beginning so that the TWiki admin can put some sentences there by defining TWIKI_ACCESS_CONTROL_TOP on Main.TWikiPreferences.

Skin

Let's say you are writing the "foo" skin. And you want to have the users specify a string inserted in the head tag with FOOSKIN_HTML_HEAD_HOOK. That can be achieved by the following:
%VARVAL{"FOOSKIN_HTML_HEAD_HOOK" default="<nop>" ignorenull="on"}%

Here, additional parameters default and ignorenull are assumed. The effect of the default parameter should be obvious. If ignorenull is "on", if the variable is defined but its value is null, %VAR{...}% behaves as if the variable is not defined.

Impact

Implementation

-- Contributors: Hideyo Imazu - 2013-07-11

Discussion

Sensible enhancement to make VarVAR handle the current topic scope in case the web="" is not present.

On your example, this should work even with double quotes: %IF{"defined FOO" then="$percntFOO$percnt"}%

-- Peter Thoeny - 2013-07-11

Ah, thank you for the tips. I didn't know $percnt and other escapes are processed in then and else parameter values. In TWiki 4.1, escapes in then and else parameters were not processed, which led me to believe that it's still the case.

-- Hideyo Imazu - 2013-07-12

See also related old AddTopicAttributeToVAR proposal.

-- Peter Thoeny - 2013-07-12

AddTopicAttributeToVAR feature is now added.

-- Hideyo Imazu - 2013-07-12