Promotion monoid

Promotions (i.e. automatic coercions) should always transform inputs to their richer type to avoid losing values of precision. vec_ptype2() returns the richer type of two vectors, or throws an incompatible type error if none of the two vector types include the other. For example, the richer type of integer and double is the latter because double covers a larger range of values than integer.

vec_ptype2() is a monoid over vectors, which in practical terms means that it is a well behaved operation for reduction. Reduction is an important operation for promotions because that is how the richer type of multiple elements is computed. As a monoid, vec_ptype2() needs an identity element, i.e. a value that doesn’t change the result of the reduction. vctrs has two identity values, NULL and unspecified vectors.

The NULL identity

As an identity element that shouldn’t influence the determination of the common type of a set of vectors, NULL is promoted to any type:

vec_ptype2(NULL, "")
#> character(0)
vec_ptype2(1L, NULL)
#> integer(0)

The common type of NULL and NULL is the identity NULL:

vec_ptype2(NULL, NULL)
#> NULL

This way the result of vec_ptype2(NULL, NULL) does not influence subsequent promotions:

vec_ptype2(
vec_ptype2(NULL, NULL),
""
)
#> character(0)

Unspecified vectors

In the vctrs coercion system, logical vectors of missing values are also automatically promoted to the type of any other vector, just like NULL. We call these vectors unspecified. The special coercion semantics of unspecified vectors serve two purposes:

1. It makes it possible to assign vectors of NA inside any type of vectors, even when they are not coercible with logical:

x <- letters[1:5]
vec_assign(x, 1:2, c(NA, NA))
#> [1] NA  NA  "c" "d" "e"

2. We can’t put NULL in a data frame, so we need an identity element that behaves more like a vector. Logical vectors of NA seem a natural fit for this.

Unspecified vectors are thus promoted to any other type, just like NULL:

vec_ptype2(NA, "")
#> character(0)
vec_ptype2(1L, c(NA, NA))
#> integer(0)

Finalising common types

vctrs has an internal vector type of class vctrs_unspecified. Users normally don’t see such vectors in the wild, but they do come up when taking the common type of an unspecified vector with another identity value:

vec_ptype2(NA, NA)
#> <unspecified> [0]
vec_ptype2(NA, NULL)
#> <unspecified> [0]
vec_ptype2(NULL, NA)
#> <unspecified> [0]

We can’t return NA here because vec_ptype2() normally returns empty vectors. We also can’t return NULL because unspecified vectors need to be recognised as logical vectors if they haven’t been promoted at the end of the reduction.

vec_ptype_finalise(vec_ptype2(NULL, NA))
#> logical(0)

See the output of vec_ptype_common() which performs the reduction and finalises the type, ready to be used by the caller:

vec_ptype_common(NULL, NULL)
#> NULL
vec_ptype_common(NA, NULL)
#> logical(0)

Note that partial types in vctrs make use of the same mechanism. They are finalised with vec_ptype_finalise().