Question

How can I get the formals (arguments) from a call object? formals() only seems to work with functions.

Answer 1

Well, a call does not have formals, only actual arguments… The difference being that a function like foo <- function(x, y, ..., z=42) can be called with actual arguments like foo(42, bar=13).

…But getting the arguments can be done like this:

a <- call('foo', a=42, 13)
as.list(a)[-1]
#$a
#[1] 42
#
#[[2]]
#[1] 13

…on the other hand, you can usually (not always) find the actual function and find the formals for it:

a <- quote(which(letters=='g'))
formals(match.fun(a[[1]]))
#$x
#
#$arr.ind
#[1] FALSE
#
#$useNames
#[1] TRUE

Here you’d need to add some error handling if the function can’t be found (as with the call to foo above)…