There is formals, but don't think it will show for all functions. It does for ls, but not for str 
> formals(ls)
$name
$pos
-1L
$envir
as.environment(pos)
$all.names
[1] FALSE
$pattern
$sorted
[1] TRUE
>
If I may ask, why can't you get the default arguments values from documentation? How exactly do you want to use these programmatically?