diff --git a/R/ParamSet.R b/R/ParamSet.R
index f5ede7c9..26944879 100644
--- a/R/ParamSet.R
+++ b/R/ParamSet.R
@@ -230,7 +230,34 @@ ParamSet = R6Class("ParamSet",
       private$.deps = rbind(private$.deps, data.table(id = id, on = on, cond = list(cond)))
       invisible(self)
     },
-
+    learnerside = function(last = TRUE) {
+      if (!self$has_interface)
+        return(self)
+      if (last)
+        return(private$.learnerside$learnerside(last = TRUE))
+      private$.learnerside
+    },
+    add_interface = function(param_set) {
+      private$.learnerside = self$clone(deep = TRUE)
+      private$copy_param_set(param_set)
+    },
+    remove_interface = function(param_set, all = FALSE) {
+      if (!self$has_interface)
+        stop("no interface to remove")
+      replace_with = self$learnerside(last = all)
+      private$copy_param_set(replace_with)
+      private$.learnerside = replace_with$.learnerside
+    },
+    get_values = function(class = NULL, tags = NULL, learnerside = FALSE, env) {
+      if (learnerside && self$has_interface) {
+        private$.learnerside$values = self$trafo(x = self$values, env = env)
+        return(private$.learnerside$get_values(
+          class = class, tags = tags, learnerside = learnerside, env = env
+        ))
+      }
+      values = self$values
+      values[intersect(names(values), self$ids(class = class, tags = tags))]
+    },
     # printer, prints the set as a datatable, with the option to hide some cols
     print = function(..., hide.cols = c("nlevels", "is_bounded", "special_vals", "tags", "storage_type")) {
       catf("ParamSet: %s", self$set_id)
@@ -290,7 +317,10 @@ ParamSet = R6Class("ParamSet",
       if (missing(f)) {
         private$.trafo
       } else {
-          assert_function(f, args = c("x", "param_set"), null.ok = TRUE)
+        assert(
+          check_function(f, args = c("x", "param_set"), null.ok = TRUE),
+          check_function(f, args = c("x", "env"), null.ok = TRUE)
+        )
         private$.trafo = f
       }
     },
@@ -304,6 +334,7 @@ ParamSet = R6Class("ParamSet",
       if (length(xs) == 0L) xs = named_list()
       private$.values = xs
     },
+    has_interface = function() !is.null(private$.learnerside),
     has_deps = function() nrow(private$.deps) > 0L
   ),
 
@@ -315,7 +346,14 @@ ParamSet = R6Class("ParamSet",
     .deps = data.table(id = character(0L), on = character(0L), cond = list()),
     # return a slot / AB, as a named vec, named with id (and can enfore a certain vec-type)
     get_member_with_idnames = function(member, astype) set_names(astype(map(self$params, member)), self$ids()),
-
+    .learnerside = NULL,
+    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)
+    },
     deep_clone = function(name, value) {
       switch(name,
         ".params" = map(value, function(x) x$clone(deep = TRUE)),
diff --git a/tests/testthat/test_interface.R b/tests/testthat/test_interface.R
new file mode 100644
index 00000000..4be8bb09
--- /dev/null
+++ b/tests/testthat/test_interface.R
@@ -0,0 +1,22 @@
+context("Interface")
+
+test_that("interface use case works", {
+  ps_orig = ParamSet$new(list(ParamInt$new("mtry", 0)))
+  ps = ps_orig$clone()
+  ps$subset(setdiff(ps$ids(), "mtry"))
+  ps$add(ParamDbl$new("mtry.pexp", 0, 1))
+  ps$trafo = function(x, env) {
+    x$mtry = round(env$task$ncol ^ x$mtry.pexp)
+    x$mtry.pexp = NULL
+    x
+  }
+
+  ps_orig$values$mtry = 3
+  ps_orig$add_interface(ps)
+  ps_orig$values$mtry.pexp = 0.7
+  expect_error({ps_orig$values$mtry = 3})
+
+  expect_equal(ps_orig$get_values(), list(mtry.pexp = 0.7))
+  expect_equal(ps_orig$get_values(learnerside = TRUE,
+    env = list(task = list(ncol = 200))), list(mtry = round(200^0.7)))
+})