with vs. attach

I want a function to evaluate one argument
in the environment of a data.frame supplied
as another argument.  "attach" works for
this, but "with" does not.  Is there a way
to make "with" work?  I'd rather not attach
the data.frame.


With the following two functions "eval.w.attach"
works but "eval.w.with" fails:


dat <- data.frame(a=1:2)
eval.w.attach <- function(x, dat){
   attach(dat)
   X <- x
   detach()
   X
}

eval.w.with <- function(x, dat){
   with(dat, x)
}

eval.w.attach(a/2, dat) # returns c(.5, 1)

eval.w.with(a/2, dat) # Error ... 'a' not found


Thanks, Spencer Graves