Optional parameter to <ant> task call?

Question

I would like to be able to pass an optional parameter to an ant task call, so that it appears as unset to the called target if it is omitted (important in conditions or when using the unless or if attributes). I don't know if optArg is set, so I cannot hard-code the parameter.

Example: The ant target someTarget requires a mandatory parameter mandatoryArg and supports an optional parameter optArg.

I can achieve this by cludgy duplication like so:

<if><equals arg1="${optArg}" arg2="true" />
    <then>
        <ant dir="someDir" target="someTarget" inheritAll="false">
            <property name="mandatoryArg" value="42" />
            <property name="optArg" value="true" />
        </ant>
    </then>
    <else>
        <ant dir="someDir" target="someTarget" inheritAll="false">
            <property name="mandatoryArg" value="42" />
        </ant>
    </else>
</if>

I would like to leave out the if statement so that the ant call is not duplicated, without losing the optional nature of optArg. I already found this question, which appears to cover a different problem.

I use Ant 1.7 and ant-contrib.

Any ideas? Thanks!

EDIT: I also accept answers that explain why it really isn't possible.

Answer 1

You might be able to get the behavior you want by using the <if> block to build a property set, and then referencing it inside a single <ant> task call later with <propertyset refid="whatever"/>