Wrap an object into a singleton list if not already a list

This function tests whether its first argument is a list according to the specified criterion; if not, puts it into a list of length 1.

Usage

enlist(x, test = c("inherits", "vector", "list"))

Arguments

x: an object to be wrapped.
test: how a string or a function to decide whether an object counts as a list; see Details.

Details

test can be one of the following

"inherits": use inherits(x, "list"). This will require the object to have class list and is generally the strictest (i.e., will wrap the most objects).
"list": use is.list(x). This will treat S3 objects based on lists as lists.
"vector": use is.vector(x). This will treat atomic vectors and expressions as lists.
a function with 1 argument: call as.logical(test(x)); if TRUE, the object is treated as a list; otherwise not.

Examples


data(mtcars)
stopifnot(
  # Atomic vectors don't inherit from lists.
  identical(enlist(1:3), list(1:3)),
  # Atomic vectors are not lists internally.
  identical(enlist(1:3, "list"), list(1:3)),
  # Atomic vectors are a type of R vector.
  identical(enlist(1:3, "vector"), 1:3),
  # Data frames don't inherit from lists.
  identical(enlist(mtcars), list(mtcars)),
  # Data frames are lists internally.
  identical(enlist(mtcars, "list"), mtcars),
  # Data frames are not considered R vectors.
  identical(enlist(mtcars, "vector"), list(mtcars))
)

# We treat something as a "list" if its first element is odd.
is.odd <- function(x) as.logical(x[1] %% 2)
stopifnot(
  # 1 is a list.
  identical(enlist(1, is.odd), 1),
  # 2 is not.
  identical(enlist(2, is.odd), list(2))
)