Extract arguments from a function

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


A function. It is lookep up in the calling frame if not supplied.


New formals or formals names for fn.


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)

See Also

call_args() and call_args_names()

  • fn_fmls
  • fn_fmls_names
  • fn_fmls_syms
  • fn_fmls<-
  • fn_fmls_names<-
library(rlang) # NOT RUN { # 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() # }
Documentation reproduced from package rlang, version 0.2.0, License: GPL-3

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