Internal FAQ -
NULL, and unspecified vectors
Promotions (i.e. automatic coercions) should always transform inputs to
their richer type to avoid losing values of precision.
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
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.
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:
The common type of
NULL is the identity
vec_ptype2(NULL, NULL) #> NULL
This way the result of
vec_ptype2(NULL, NULL) does not influence
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:
It makes it possible to assign vectors of
NAinside any type of vectors, even when they are not coercible with logical:
We can’t put
NULLin a data frame, so we need an identity element that behaves more like a vector. Logical vectors of
NAseem a natural fit for this.
Unspecified vectors are thus promoted to any other type, just like
vctrs has an internal vector type of class
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
vec_ptype2(NA, NA) #> <unspecified>  vec_ptype2(NA, NULL) #> <unspecified>  vec_ptype2(NULL, NA) #> <unspecified> 
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.
See the output of
vec_ptype_common() which performs the reduction and
finalises the type, ready to be used by the caller:
Note that partial types in vctrs make use of the same mechanism.
They are finalised with