A quosure is a type of quoted expression that includes a reference to the context where it was created. A quosure is thus guaranteed to evaluate in its original environment and can refer to local objects.
You can access the quosure components (its expression and its environment) with:
get_expr() and get_env(). These getters also support other
kinds of objects such as formulas.
quo_get_expr() and quo_get_env(). These getters only work
with quosures and throw an error with other types of input.
Test if an object is a quosure with is_quosure(). If you know an
object is a quosure, use the quo_ prefixed predicates to check
its contents, quo_is_missing(), quo_is_symbol(), etc.
is_quosure(x) quo_is_missing(quo) quo_is_symbol(quo, name = NULL) quo_is_call(quo, name = NULL, n = NULL, ns = NULL) quo_is_symbolic(quo) quo_is_null(quo) quo_get_expr(quo) quo_get_env(quo) quo_set_expr(quo, expr) quo_set_env(quo, env)
| x | An object to test. |
|---|---|
| quo | A quosure to test. |
| name | The name of the symbol or function call. If |
| n | An optional number of arguments that the call should match. |
| ns | The namespace of the call. If Can be a character vector of namespaces, in which case the call
has to match at least one of them, otherwise |
| expr | A new expression for the quosure. |
| env | A new environment for the quosure. |
A quosure usually does not carry environments for constant objects like strings or numbers. quo() and
enquo() only capture an environment for symbolic expressions. For instance, all of these return the
empty environment:
quo_get_env(quo("constant")) quo_get_env(quo(100)) quo_get_env(quo(NA))
On the other hand, quosures capture the environment of symbolic expressions, i.e. expressions whose meaning depends on the environment in which they are evaluated and what objects are defined there:
quo_get_env(quo(some_object)) quo_get_env(quo(some_function()))
When missing arguments are captured as quosures, either through
enquo() or quos(), they are returned as an empty quosure. These
quosures contain the missing argument and typically
have the empty environment as enclosure.
is_quosure() is stable.
quo_get_expr() and quo_get_env() are stable.
quo() for creating quosures by quotation; as_quosure()
and new_quosure() for constructing quosures manually.
quo <- quo(my_quosure) quo #> <quosure> #> expr: ^my_quosure #> env: 0x560eb184db08 # Access and set the components of a quosure: quo_get_expr(quo) #> my_quosure quo_get_env(quo) #> <environment: 0x560eb184db08> quo <- quo_set_expr(quo, quote(baz)) quo <- quo_set_env(quo, empty_env()) quo #> <quosure> #> expr: ^baz #> env: empty # Test wether an object is a quosure: is_quosure(quo) #> [1] TRUE # If it is a quosure, you can use the specialised type predicates # to check what is inside it: quo_is_symbol(quo) #> [1] TRUE quo_is_call(quo) #> [1] FALSE quo_is_null(quo) #> [1] FALSE # quo_is_missing() checks for a special kind of quosure, the one # that contains the missing argument: quo() #> <quosure> #> expr: ^ #> env: empty quo_is_missing(quo()) #> [1] TRUE fn <- function(arg) enquo(arg) fn() #> <quosure> #> expr: ^ #> env: empty quo_is_missing(fn()) #> [1] TRUE