Returning list_of<> vs list in "low level" functions

[DavisVaughan]

I have a bit of a tough time deciding when to return a list_of<> from a vctrs function vs just a bare list. For example, vec_chop() returns a list_of<>:

vctrs::vec_chop(letters[1:2])
#> <list_of<character>[2]>
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [1] "b"

But this is slightly problematic because it is often used as a "low level" function where I really just want a bare list back. Like here in as_df_row_impl()

vctrs/src/bind.c

Line 175 in 2c8dd41

SET_ATTRIB(x, R_NilValue);

And here where I planned to implement vec_cast.list.data.frame

vec_cast.list.data.frame <- function(x, to, ...) {
  row.names(x) <- NULL
  
  # returns list_of()
  out <- vec_chop(x)
  attributes(out) <- NULL
  
  # alternative - much slower
  #out <- map(vec_seq_along(x), vec_slice, x = x)
  
  out
}

@lionel- and I think that vec_chop() should just return a bare list, but we would like to be consistent about this and make a systematic change in all vctrs functions. For example, would it still make sense for vec_split() to return a list_of<>?

vctrs::vec_split(letters[1:4], c(1, 1, 2, 2))$val
#> <list_of<character>[2]>
#> [[1]]
#> [1] "a" "b"
#> 
#> [[2]]
#> [1] "c" "d"

@hadley what do you think about this?

[hadley]

I don't have a strong preference; I think in general it's better to return the most specific type (which would be list_of<> here), but we have relatively little experience with list_of<>, so we probably don't want to use it in too many places. (And it provides relatively little benefit over list)