A callable object is an object that can appear in the function position of a call (as opposed to argument position). This includes symbolic objects that evaluate to a function or literal functions embedded in the call.
Details
Note that strings may look like callable objects because
expressions of the form "list"() are valid R code. However,
that's only because the R parser transforms strings to symbols. It
is not legal to manually set language heads to strings.
Examples
# Symbolic objects and functions are callable:
is_callable(quote(foo))
#> [1] TRUE
is_callable(base::identity)
#> [1] TRUE
# node_poke_car() lets you modify calls without any checking:
lang <- quote(foo(10))
node_poke_car(lang, current_env())
# Use is_callable() to check an input object is safe to put as CAR:
obj <- base::identity
if (is_callable(obj)) {
lang <- node_poke_car(lang, obj)
} else {
abort("`obj` must be callable")
}
eval_bare(lang)
#> [1] 10