Show used columns in col_vals_expr()

[yjunechoe]

Summary

This PR aims to have col_vals_expr() show columns used in expr in the agent report. Example inspired by fn docs:

tbl <-
  dplyr::tibble(
    a = c(1, 2, 1, 7, 8, 6),
    b = c(0, 0, 0, 1, 1, 1),
    c = c(0.5, 0.3, 0.8, 1.4, 1.9, 1.2),
  )
c <- 1

tbl %>%
  create_agent() %>% 
  col_vals_expr(expr = ~ a %% 1 == 0) %>% 
  col_vals_expr(expr = ~ case_when(
    b == 0 ~ a %>% between(0, 5) & c < 1,
    b == 1 ~ a > 5 & c >= 1
  )) %>% 
  col_vals_expr(expr(.data$a + b == !!c)) %>% 
  col_vals_expr(expr = ~ a + d > 0) %>% 
  interrogate()

This is a draft because it needs more tests.

Related GitHub Issues and PRs

• Ref: get_agent_x_list()$columns is empty for col_vals_expr() #477

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

[yjunechoe]

Apparently {covr} injects its own expressions into even quote()/expr()-ed code - r-lib/covr#381. This unfortunately interferes with the expr parsing process and makes the new feature impossible to test on the test-coverage/codecov GHA.

I'm trying to find a good way around this but the good thing is that this feature is purely a convenience of display in the agent report, so currently I have it failing gracefully in case of parsing errors (falls back to showing nothing for columns) and I'll think more about what I can do w.r.t. including it in (automated) tests.