Make Selector work for target column

[mb706]

Any ideas how we make Selectors work for the target column or columns of other roles? Some cases (e.g. PipeOpColRole, also #492) may want to select columns when they are the target.

One suggestion is to have a selector_role(), and possibly selector_target() because the target is special, and if more than just the target is wanted one could do selector_union(selector_target(), selector_xxx(...)).

But how to handle cases where one wants to select a specific subset of target columns?

• Should the selectors we have so far get an extra argument only_features = TRUE?
• Should there be a "modifier that changes a selector's view of a task (select_from_all_roles(selector_type("factor")))?

[pfistfl]

+1 for selector_target.

Perhaps have selector_union() and selector_intersect().
Or just selector_combine(..., combine = "or" | "and")

I think the cases where this will become complicated are rather rare, and at some point, people instead would just write their own selectors.

[mb706]

selector_union and selector_intersect already exist. I am wondering about how to handle multitarget, or cases where one wants to do something to one (out of several) columns with other roles (e.g. weights or blocking)

[pfistfl]

Multi-target would then be e.g. selector_intersect(selector_target(...), selector_name(...))

I guess we could use a selector_role(...), which would perhaps handle weights etc.?
I mean for most other things, I think people should just write their selectors?
It might be a little hard to anticipate all use-cases.

If we want to benchmark e.g. multi-target settings or something, we might have to start and annotate col_roles somehow, as this would just lead to huge complexity in the selector API.