phop-index-vec.md
In slider: Sliding Window Functions

phop_index_vec() errors if it can't simplify

Code
  (expect_error(phop_index_vec(list(1:2, 1:2), 1:2, 1:2, 1:2, fn, .ptype = NULL),
  class = "vctrs_error_incompatible_type"))
Output
  <error/vctrs_error_ptype2>
  Error in `phop_index_vec()`:
  ! Can't combine `out[[1]]` <double> and `out[[2]]` <character>.

`.ptype = NULL` validates that element lengths are 1

Code
  phop_index_vec(list(1:2, 1:2), 1:2, 1:2, 1:2, ~ if (.x == 1L) {
    1:2
  } else {
    1
  }, .ptype = NULL)
Condition
  Error:
  i In index: 1
  ! The result of `.f` must have size 1, not 2.

Code
  phop_index_vec(list(1:2, 1:2), 1:2, 1:2, 1:2, ~ if (.x == 1L) {
    NULL
  } else {
    2
  }, .ptype = NULL)
Condition
  Error:
  i In index: 1
  ! The result of `.f` must have size 1, not 0.