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)
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.

peek_vars

```
poke_vars(letters)
peek_vars()
#> [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
#> [20] "t" "u" "v" "w" "x" "y" "z"
# Now that the variables are registered, the helpers can figure out
# the locations of elements within the variable vector:
all_of(c("d", "z"))
#> [1] "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))
all_of("d")
}
fn(letters)
#> [1] "d"
fn(letters[3:5])
#> [1] "d"
# The previous variables are still registered after fn() was
# called:
peek_vars()
#> [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
#> [20] "t" "u" "v" "w" "x" "y" "z"
# 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"))
#> [1] 1 3
# 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"))
#> [1] 1 3
```