Accept targets as inputs to cross

[vkehayas]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Description

Hi,

Thanks for this lifesaver of a package!
I am a long-time user trying to catch-up with the new way of creating large plans.
This is a minimal example of what I would like to do:

library(drake)

dPlan = drake_plan(
  kMu = c(0, 1, 10),
  kSD = c(1, 2, 3),
  x = target(rnorm(n = 1000, 
                   mean = mean_val, 
                   sd = sd_val),
             transform = cross(mean_val = kMu,
                               sd_val = kSD))
)

And I would expect it to expand to

> dPlan
# A tibble: 9 x 2
  target command                             
  <chr>  <expr>                              
1 x_0_1  "rnorm(n = 1000, mean = 0, sd = 1) "
2 x_1_1  "rnorm(n = 1000, mean = 1, sd = 1) "
3 x_10_1 rnorm(n = 1000, mean = 10, sd = 1)  
4 x_0_2  "rnorm(n = 1000, mean = 0, sd = 2) "
5 x_1_2  "rnorm(n = 1000, mean = 1, sd = 2) "
6 x_10_2 rnorm(n = 1000, mean = 10, sd = 2)  
7 x_0_3  "rnorm(n = 1000, mean = 0, sd = 3) "
8 x_1_3  "rnorm(n = 1000, mean = 1, sd = 3) "
9 x_10_3 rnorm(n = 1000, mean = 10, sd = 3)  

which I can produce if I move kMu, kSD outside of drake's control and use !!. But in my real plan these would be variables that are being used to build other targets as well and I would like them to trigger building the first plan whenever they change. Is this an abuse of drake or is there a way to achieve what I want?

Apologies if there was a pre-existing issue I missed or if the answer relies on knowledge of the tidyverse (of which I have close to none).

[wlandau]

Definitely a good question. This is currently not supported in drake. It is by far the most challenging feature request, and there are long-term plans to try. See #685 for the main thread.