Report array dimensionality in obj_type_friendly()

[DavisVaughan]

In vctrs, we are trying to be fairly strict about the type of condition in vec_if_else(). In particular, we disallow arrays from being passed as condition, even 1D arrays (we do this consistently across vec_if_else(), list_combine(), vec_case_when(), and vec_replace_when()`, all of which can take logical "condition" indices).

https://github.com/r-lib/vctrs/blob/f27ab24c7eb0b1d78d48bd1d39175a1ecf7d4411/src/slice-assign.c#L964-L971

Unfortunately, obj_type_friendly() currently reports 1D arrays as identical to vectors resulting in the following not-useful error message

     Error in `if_else()`:
     ! `condition` must be a logical vector, not a logical vector.

I'd argue that 1D arrays should be reported as arrays by obj_type_friendly(). In other words, we'd have:

• n_dim = 0 gives vector
• n_dim = 1 gives array
• n_dim = 2 gives matrix
• n_dim = 3+ gives array

This feels right to me and allows us to report a useful distinction between vectors and 1D arrays.

[lionel-]

Since array vectors print the same way as vectors, it seems useful to distinguish them from arrays (which print differently) in a more explicit way. Some suggestions inside.

[lionel-]

Suggested change

	return("a list array")
	return("an array vector of type list")

[DavisVaughan]

• n_dim = 0 gives vector
• n_dim = 2 gives matrix
• n_dim = N gives {N}D array

Here's what we agreed on

I like this because it also lets you differentiate between arrays of different dimensionality which we couldn't do before. Like "i expect a 3D array, not a 4D array"