R: Examples for 'rlang::missing

### ** Examples

# 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()

[1] TRUE

# Creating a missing argument can also be useful to generate calls
args <- list(1, missing_arg(), 3, missing_arg())
quo(fn(!!! args))

<quosure>
expr: ^fn(1, , 3, )
env:  0x55ccfe0c7d60

# 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]])

[1] FALSE

# 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))

[1] FALSE

is_missing(maybe_missing(x))

[1] TRUE

# 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())

[1] TRUE

is_missing(l[[1]])

[1] TRUE

Examples for 'rlang::missing_arg'

Generate or handle a missing argument