| fn_fmls {rlang} | R Documentation |
fn_fmls() returns a named list of formal arguments.
fn_fmls_names() returns the names of the arguments.
fn_fmls_syms() returns formals as a named list of symbols. This
is especially useful for forwarding arguments in constructed
calls.
fn_fmls(fn = caller_fn()) fn_fmls_names(fn = caller_fn()) fn_fmls_syms(fn = caller_fn()) fn_fmls(fn) <- value fn_fmls_names(fn) <- value
fn |
A function. It is lookep up in the calling frame if not supplied. |
value |
New formals or formals names for |
Unlike formals(), these helpers also work with primitive
functions. See is_function() for a discussion of primitive and
closure functions.
Note that the argument names are taken from the closures that are
created when passing the primitive to as_closure(). For instance,
while the arguments of the primitive operator + are labelled e1
and e2, fn_fmls_names() will return .x and .y. Note that
for many primitives the base R argument names are purely
placeholders since they don't perform regular argument matching.
E.g. this returns 5 instead of -5:
`-`(e2 = 10, 5)
To regularise the semantics of primitive functions, it is usually a good idea to coerce them to a closure first:
minus <- as_closure(`-`) minus(.y = 10, 5)
call_args() and call_args_names()
# Extract from current call:
fn <- function(a = 1, b = 2) fn_fmls()
fn()
# Works with primitive functions:
fn_fmls(base::switch)
# fn_fmls_syms() makes it easy to forward arguments:
call2("apply", !!! fn_fmls_syms(lapply))
# You can also change the formals:
fn_fmls(fn) <- list(A = 10, B = 20)
fn()
fn_fmls_names(fn) <- c("foo", "bar")
fn()