title: How to set 'importance' for finding variable importance for various regression learners in mlr3
body: <p>while using regr.ranger I'm getting an error message that says importance.mode must be one of &quot;impurity&quot; etc.</p>
  <p>While using regr.rfsrc it says I should specify 'importance' to one of 'TRUE' etc.</p>
  <p>I just want to understand at what stage should I assign the value to 'importance'.</p>
  <p>I get an error if I do it while creating the learner.</p>
  <pre><code>&gt; lrnr_ranger = mlr_learners$get(key = &quot;regr.ranger&quot;,importance=&quot;impurity&quot;)
  Error in initialize(...) : unused argument (importance = &quot;impurity&quot;)
  </code></pre>
  <p>or</p>
  <pre><code>&gt; lrnr_ranger = mlr_learners$get(key = &quot;regr.ranger&quot;,importance.mode=&quot;impurity&quot;)
  Error in initialize(...) : unused argument (importance.mode = &quot;impurity&quot;)
  </code></pre>
  <p>or should I try to set it using the param_set:</p>
  <pre><code>&gt; lrnr_ranger$param_set$add(p = list(&quot;importance.mode&quot;,&quot;impurity&quot;))
  Error in .__ParamSet__add(self = self, private = private, super = super,  : 
    Assertion on 'p' failed: Must inherit from class 'Param'/'ParamSet', but has class 'list'.
  </code></pre>
  <p>Any clue would be super helpful.</p>
  <p>I'm not really reporting a problem but asking how to do something. (Hence, I believe no need to create a reprex!)</p>
  <p>I wish this was addressed in the mlr3 book or some documentation but it is not.</p>