On 12/12/2008 1:06 PM, hadley wickham wrote:
Oh yes, that's a good point. But wouldn't the following do the job?
"&.selector" <- function(a, b) {
function(n) a(n) & b(n)
}
or
"&.selector" <- function(a, b) {
function(n) intersect(a(n), b(n))
}
depending on whether selectors return logical or numeric vectors.
Writing functions for | and ! would be similarly easy. Or am I
missing something?
No, I think those definitions would be fine, but I'd be concerned about
speed issues if we start messing with primitives.
Speed or expressiveness: pick one? ;) People could always use the
regular subsetting mechanisms if they want the best speed - any
changes to support selectors wouldn't affect the speed of the other
methods of subsetting, would they?
While we're at it, we might as well do the same sort of thing for :, and
define a selector named end, and then 3:end would give a selector from 3 to
the end, which brings us back to the original question. So it's not nearly
as intrusive as I thought it would be.
3:end() do you mean? Or do you mean extending seq so that it uses
unevaluted input?
My end would be the output of your end(). If there are no args and no
local context, I don't see the need for it to be a function call. It
would just be defined as something like
end <- structure( function(n) c(rep(FALSE, n-1), TRUE), class="selector")
I'm not sure what the definition of : should be if one of the args is a
selector.
Duncan
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