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.
is_callable(x)
x | An object to test. |
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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.
# 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