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

Arguments

fn	A function. It is lookep up in the calling frame if not supplied.
value	New formals or formals names for fn.

Details

Unlike formals(), these helpers throw an error with primitive functions instead of returning NULL.

See also

call_args() and call_args_names()

Examples

# Extract from current call:
fn <- function(a = 1, b = 2) fn_fmls()
fn()
#> $a
#> [1] 1
#> 
#> $b
#> [1] 2
#> 

# fn_fmls_syms() makes it easy to forward arguments:
call2("apply", !!! fn_fmls_syms(lapply))
#> apply(X = X, FUN = FUN, ...)

# You can also change the formals:
fn_fmls(fn) <- list(A = 10, B = 20)
fn()
#> $A
#> [1] 10
#> 
#> $B
#> [1] 20
#> 

fn_fmls_names(fn) <- c("foo", "bar")
fn()
#> $foo
#> [1] 10
#> 
#> $bar
#> [1] 20
#>