as_vector() collapses a list of vectors into one vector. It checks that the type of each vector is consistent with .type. If the list can not be simplified, it throws an error. simplify will simplify a vector if possible; simplify_all will apply simplify to every element of a list.
.type can be a vector mold specifying both the type and the length of the vectors to be concatenated, such as numeric(1) or integer(4). Alternatively, it can be a string describing the type, one of: "logical", "integer", "double", "complex", "character" or "raw".