missing_arg
Generate or handle a missing argument
These functions help using the missing argument as a regular R object.
missing_arg()
generates a missing argument.is_missing()
is likebase::missing()
but also supports testing for missing arguments contained in other objects like lists.maybe_missing()
is useful to pass down an input that might be missing to another function. It avoids triggering an "argument is missing" error.
Usage
missing_arg()is_missing(x)
maybe_missing(x)
Arguments
- x
An object that might be the missing argument.
Other ways to reify the missing argument
base::quote(expr = )
is the canonical way to create a missing argument object.expr()
called without argument creates a missing argument.quo()
called without argument creates an empty quosure, i.e. a quosure containing the missing argument object.
Fragility of the missing argument object
The missing argument is an object that triggers an error if and
only if it is the result of evaluating a symbol. No error is
produced when a function call evaluates to the missing argument
object. This means that expressions like x[[1]] <- missing_arg()
are perfectly safe. Likewise, x[[1]]
is safe even if the result
is the missing object.
However, as soon as the missing argument is passed down between
functions through an argument, you're at risk of triggering a
missing error. This is because arguments are passed through
symbols. To work around this, is_missing()
and maybe_missing(x)
use a bit of magic to determine if the input is the missing
argument without triggering a missing error.
maybe_missing()
is particularly useful for prototyping
meta-programming algorithm in R. The missing argument is a likely
input when computing on the language because it is a standard
object in formals lists. While C functions are always allowed to
return the missing argument and pass it to other C functions, this
is not the case on the R side. If you're implementing your
meta-programming algorithm in R, use maybe_missing()
when an
input might be the missing argument object.
[[1]: R:[1 [[1]: R:[1
Life cycle
missing_arg()
andis_missing()
are stable.Like the rest of rlang,
maybe_missing()
is maturing.
Examples
# NOT RUN {
# The missing argument usually arises inside a function when the
# user omits an argument that does not have a default:
fn <- function(x) is_missing(x)
fn()
# Creating a missing argument can also be useful to generate calls
args <- list(1, missing_arg(), 3, missing_arg())
quo(fn(!!! args))
# Other ways to create that object include:
quote(expr = )
expr()
# It is perfectly valid to generate and assign the missing
# argument in a list.
x <- missing_arg()
l <- list(missing_arg())
# Just don't evaluate a symbol that contains the empty argument.
# Evaluating the object `x` that we created above would trigger an
# error.
# x # Not run
# On the other hand accessing a missing argument contained in a
# list does not trigger an error because subsetting is a function
# call:
l[[1]]
is.null(l[[1]])
# In case you really need to access a symbol that might contain the
# empty argument object, use maybe_missing():
maybe_missing(x)
is.null(maybe_missing(x))
is_missing(maybe_missing(x))
# Note that base::missing() only works on symbols and does not
# support complex expressions. For this reason the following lines
# would throw an error:
#> missing(missing_arg())
#> missing(l[[1]])
# while is_missing() will work as expected:
is_missing(missing_arg())
is_missing(l[[1]])
# }