| poke_vars {tidyselect} | R Documentation |
Variables are made available to select helpers by registering them in a special placeholder.
scoped_vars() changes the current variables and sets up a
function exit hook that automatically restores the previous
variables once the current function returns.
with_vars() takes an expression to be evaluated in a variable
context.
poke_vars() changes the contents of the placeholder with a new
set of variables. It returns the previous variables invisibly and
it is your responsibility to restore them after you are
done. This is for expert use only.
peek_vars() returns the variables currently registered.
has_vars() returns TRUE if a variable context has been set,
FALSE otherwise.
poke_vars(vars) peek_vars() scoped_vars(vars, frame = caller_env()) with_vars(vars, expr) has_vars()
vars |
A character vector of variable names. |
frame |
The frame environment where the exit hook for restoring the old variables should be registered. |
expr |
An expression to be evaluated within the variable context. |
For poke_vars() and scoped_vars(), the old variables
invisibly. For peek_vars(), the variables currently
registered.
poke_vars(letters)
peek_vars()
# Now that the variables are registered, the helpers can figure out
# the positions of elements within the variable vector:
one_of(c("d", "z"))
# In a function be sure to restore the previous variables. An exit
# hook is the best way to do it:
fn <- function(vars) {
old <- poke_vars(vars)
on.exit(poke_vars(old))
one_of("d")
}
fn(letters)
fn(letters[3:5])
# The previous variables are still registered after fn() was
# called:
peek_vars()
# It is recommended to use the scoped variant as it restores the
# state automatically when the function returns:
fn <- function(vars) {
scoped_vars(vars)
starts_with("r")
}
fn(c("red", "blue", "rose"))
# The with_vars() helper makes it easy to pass an expression that
# should be evaluated in a variable context. Thanks to lazy
# evaluation, you can just pass the expression argument from your
# wrapper to with_vars():
fn <- function(expr) {
vars <- c("red", "blue", "rose")
with_vars(vars, expr)
}
fn(starts_with("r"))