Rewrite nth(), first(), and last() using vctrs

[DavisVaughan]

Revival of #4636
Technically closes tidyverse/funs#35

I am now quite confident that first() and friends should use [ / vec_slice() rather than [[ / vec_get(). I think the invariant for these functions should be that they return an object with:

• Size 1
• The same type as x

This guarantees they will be useful with summarise(), which is good because ?summarise already mentions them as "Useful Functions"

This change mainly affects lists, which now return a list of size 1 rather than the actual element of the list. But I think that change is correct - i.e. it feels weird for "first" to return something that could be size >1. It also affects data frames, which previously returned a single column but now return a single row (which again, feels more consistent).

I'll check revdeps to ensure this doesn't cause a big uproar, but I think these are relatively uncommon edge cases
"252be158-3a4a-4e70-a233-4a9a0a858637"

[DavisVaughan]

This one might actually be too extreme. It seems like a lot of people use first() and last() in a programmatic way outside of data frames. In particular, a number of people use it on lists and expect it to extract the first/last element of that list.

14 broken revdeps:

• 7 due to first() or last() on a list, expecting to get list element
• 5 due to first() or last() on a data frame, expecting to get a column. Should use pull() instead.
• 1 due to stricter casting rules (can't cast character to double)
• 1 unknown

See #6262 for details

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We might be able to make it use vec_slice2()-like behavior instead. i.e.:

• For most types, uses vec_slice() and then removes any names
• For lists (i.e. vec_is_list() is TRUE), uses [[

It depends on how strongly we feel about the invariants of these functions (i.e. should they always return an object the same type as x with size 1? or are we ok with lists being different here?)

I guess first(list()) would continue to return NULL

[hadley]

Worth incorporating #6257?

[DavisVaughan]

I thought about it but I think it'll be easier to me to keep it straight if I do that in a separate PR

This one is ready for review now though