Sometimes it would be useful to specify how parameters are changed inside a Learner / PipeOp, e.g. as in mlr-org/mlr3pipelines#24. A typical example is the mtry parameter of a random forest, which should range from 1 to task$ncol. It would be nice if one could introduce an mtry.pexp parameter ranging from 0 to 1, so that the actual mtry is set to round(task$ncol ^ mtry.pexp).

The $trafo function, as it currently stands, is not a good fit for this, because it (1) operates before the Learner even sees the Task, so wouldn't know about task$ncol, and (2) would not be able to introduce a new parameter mtry.pexp, it would only be able to re-scale the present mtry, which is an integer between 1 and Inf, not a real number between 0 and 1.

I think the following UI would be quite nice:

lrn = mlr_learners$get("classif.ranger")
ps = lrn$param_set$clone()
ps$subset(setdiff(ps$ids(), "mtry"))
ps$add(ParamDbl$new("mtry.pexp", 0, 1))
ps$trafo = function(x, env, param_set) {
  x$mtry = round(env$task$ncol ^ x$mtry.pexp)
  x$mtry.pexp = NULL
  x
}

lrn$param_set$add_interface(ps)  # !!

# set effective `mtry` to `round(ncol(task) ^ 0.7)` when training happens
lrn$param_set$values$mtry.pexp = 0.7

lrn$param_set$values$mtry = 3 # ERROR

This would change the lrn$param_set to "look and feel" like the ps constructed / modified before, but internally the Learner (or e.g. a PipeOp) would get the parameter values as performed by the $trafo function.

A way to implement this would be the following:

1. Add a private$.learnerside = NULL slot that points to the ParamSet that the Learner / PipeOp should see.
2. Add a $has_interface active binding:

   has_interface = function() !is.null(private$.learnerside)

3. Add a self$learnerside(last = TRUE) function that gives the ParamSet that the Learner / PipeOp should see. Because private$.learnerside could point to a ParamSet that itself has a private$.learnerside set, it should be recursive if last is TRUE, and only give the "next" learnerside if last is FALSE.

   learnerside = function(last = TRUE) {
     if (!self$has_interface)
       return(self)
     if (last) {
       private$.learnerside$learnerside(last = TRUE)
     } else {
       private$.learnerside
     }
   }

4. Implement a private$copy_param_set() helper function. It copies all relevant items from its argument to the ParamSet itself, to turn the self into an effective copy of that argument:

   copy_param_set = function(param_set) {
     private$.params = param_set$params
     private$.deps = param_set$deps
     private$.values = param_set$values
     private$.trafo = param_set$trafo
     invisible(self)
   }

5. Implement the public $add_interface() function:

   add_interface = function(param_set) {
     private$.learnerside = self$clone(deep = TRUE)
     private$copy_param_set(param_set)
   }

6. Implement a public $remove_interface() function:

   remove_interface = function(param_set, all = FALSE) {
     if (!self$has_interface)
       stop("no interface to remove")
     replace_with = self$learnerside(last = all)
     private$copy_param_set(replace_with)
     private$.learnerside = replace_with$.learnerside
   }

7. How does the Learner / PipeOp get its value out of this? There probably should be a $get_values() function that gets the values for the operation, which should also have the filter functionality that ids currently has.

   get_values = function(class = NULL, tags = NULL, learnerside = FALSE, env) {
     if (learnerside && self$has_interface) {
       private$.learnerside$values = self$trafo(self$values, env)
       return(private$.learnerside$get_values(
         class = class, tags = tags, learnerside = learnerside, env = env
       ))
     }
     values = self$values
     values[intersect(names(values), self$ids(class = class, tags = tags))]
   }

8. Change the trafo active binding to also accept functions of the form function(x, env)

This implementation has the advantage that multiple interfaces can be "stacked" on top of each other: A user who gets a Learner does not need to know or care if something put an interface in front of its ParamSet. When the user sets a parameter using param_set$values$param = x, the value gets checked against the constraints of the interface parameter set. When he calls lrn$train(), the train() function calls get_values(tags = "train", learnerside = TRUE, env = list(task = task)), which recurses through the different interfaces that were added, and sets $values in each one of them after transforming. This automatically checks that the trafo function returns a feasible value for the original ParamSet.

This change would also be completely transparent to everything ParamSet is doing so far.

Things that I am not sure about:

• It is a bit inelegant to have the env parameter depend on what kind of object the ParamSet belongs to: Some PipeOps (e.g. PipeOpModelAvg) have parameters in a different context, where no task is present (and instead maybe a prediction). One would probably want to agree on an interface (always task in a Learner / preprocessing PipeOp, always prediction in a "post-processing" PipeOp, other contexts..?)
• There are no checks on the feasibility of the trafo function output until the actual training / predicting happens.
• Maybe one still wants to use the "train" / "predict" tags from the outside, e.g. maybe a tuning algorithm wants to train a model with one set of "train" parameters and then evaluate these with different "predict" parameters to get multiple performance datapoints with only a single train() call for efficiency. In that case it would be nice if the trafo could also respect the "train" / "predict" tags and work when only a subset of parameter values is present. In that case, the get_values would need to be adapted to only give self$values[intersect(names(self$values), set$ids(...tags = tags))] to self$trafo.
• I don't know if it would be useful to do this for ParamSetCollection. Maybe a GraphLearner would want to have an interface as well? I wouldn't know what the UI for that would look like, however. In that case it would probably be easiest to intervene with the individual PipeOps' ParamSet.