Feature request: .vary in expand_grid()

[lhdjung]

expand_grid() is meant to replace the purrr::cross() family. However, these functions vary fastest, and expand_grid() only varies slowest (in the sense of vec_expand_grid()).

Although varying slowest might be more useful overall, there is no other designated replacement for the cross() functions. Their current docs say that cross() and expand_grid() can be used equivalently, but as that very example shows, the order of combinations is different. This was first noted in tidyverse/purrr#768.

It would be truly equivalent if expand_grid() had a .vary parameter passed to vec_expand_grid() that could be set to "fastest" in this case. The default should still be "slowest". Alternatively, vec_expand_grid() itself could fulfill this role, which should then be advertised in the cross() docs.

Some use cases do require varying fastest (although I don't know how common this is). For example, the vectors creating the background patterns in these plots were made using cross2(), and the patterns wouldn't emerge when varying slowest.