list_of: Construct a list of homogenous vectors

In vctrs: Vector Helpers

Construct a list of homogenous vectors

Description

A list_of is a restricted version of a list that adds constraints on the list elements.

• list_of(.ptype = ) restricts the type of each element.

  • ⁠.ptype = <type>⁠ asserts that each element has type ⁠<type>⁠.

  • .ptype = NULL infers the type from the original set of elements, or errors if no vector inputs were provided.

  • .ptype = rlang::zap() doesn't restrict the type.

• list_of(.size = ) restricts the size of each element.

  • ⁠.size = <size>⁠ asserts that each element has size ⁠<size>⁠.

  • .size = NULL infers the size from the original set of elements, or errors if no vector inputs were provided.

  • .size = rlang::zap() doesn't restrict the size.

The default behavior infers the element type and doesn't restrict the size.

Both .ptype and .size may be specified to restrict both the size and type of the list elements. You cannot set both of these to rlang::zap(), as that would be the same as a bare list() with no restrictions.

Modifying a list_of with ⁠$<-⁠, ⁠[<-⁠, and ⁠[[<-⁠ preserves the constraints by coercing and recycling all input items.

Usage

list_of(..., .ptype = NULL, .size = zap())

as_list_of(x, ...)

is_list_of(x)

## S3 method for class 'vctrs_list_of'
vec_ptype2(x, y, ..., x_arg = "", y_arg = "")

## S3 method for class 'vctrs_list_of'
vec_cast(x, to, ...)

Arguments

...	For list_of(), vectors to include in the list. For other methods, these dots must be empty.
.ptype	The type to restrict each list element to. One of: A prototype like integer() or double(). NULL, to infer the type from .... If no vector inputs are provided, an error is thrown. rlang::zap() to avoid placing any restrictions on the type.
.size	The size to restrict each list element to. One of: A scalar integer size. NULL, to infer the size from .... If no vector inputs are provided, an error is thrown. rlang::zap() to avoid placing any restrictions on the size.
x	For as_list_of(), a vector to be coerced to list_of. For is_list_of(), an object to test.
y, to	Arguments to vec_ptype2() and vec_cast().
x_arg, y_arg	Argument names for x and y. These are used in error messages to inform the user about the locations of incompatible types (see stop_incompatible_type()).

Examples

# Restrict the type, but not the size
x <- list_of(1:3, 5:6, 10:15)
x

if (requireNamespace("tibble", quietly = TRUE)) {
  # As a column in a tibble
  tibble::tibble(x = x)
}

# Coercion happens during assignment
x[1] <- list(4)
typeof(x[[1]])

try(x[1] <- list(4.5))

# Restrict the size, but not the type
x <- list_of(1, 2:3, .ptype = rlang::zap(), .size = 2)
x

# Recycling happens during assignment
x[1] <- list(4)
x

try(x[1] <- list(3:6))

# Restricting both size and type
x <- list_of(1L, 2:3, .ptype = integer(), .size = 2)
x

# Setting an element to `NULL`
x[2] <- list(NULL)
x

# Note that using `NULL` shortens the list, like a base R list
x[2] <- NULL
x

# Combining a list_of with a list results in a list
vec_c(list_of(1), list(2, "x"))

# Combining a list_of with another list_of tries to find a common element
# type and common element size, but will remove the constraint if that
# fails
x <- list_of(1, .ptype = double())
y <- list_of(c("a", "b"), .ptype = character(), .size = 2)
z <- list_of(c("c", "d", "e"), .ptype = character(), .size = 3)

# Falls back to a list
vec_c(x, y)

# Falls back to a `list_of<character>` with no size restriction
vec_c(y, z)

vctrs documentation built on Jan. 24, 2026, 1:07 a.m.