On Sun, Nov 9, 2014 at 10:38 AM, Noufal Ibrahim KV <nou...@nibrahim.net.in> wrote:
> > Okay, I've been struggling through the proglang course on coursera and > this thing came up > > val x = 2; > fun f y = x + y; > > The second line creates a function that adds `x` to it's argument. Since > ML is statically scoped, this is really a function that adds 2 to its > argument. Even if I later create a new binding for x later like so > > val x = 10; > f (3); > > I will still get back 5 (i.e. 2 + 3) instead of 13 (10 + 3). This makes > sense to me as an example of lexical scoping. The bindings are at the > time of definition rather than invocation. > > With python, it's different. The claim is that it has lexical scoping > but. > > x = 2 > def f(y): return x + y > > x = 10 > f(3) > > The answer is, distressingly, 13. The explanation was that although > Python has lexical scoping, that closure "close over variables rather > than values". Meaning that the variables (and not their values) at > definition time are stored in the closure. This is how elisp does it > which claims to be, by default, dynamically scoped. > > (setq m 5) > (defun test (x) > (+ x m)) > > (test 3) ; -> 8 > (setq m 15) > (test 3) ; -> 18 > > > So, my question is, how different is this from dynamic scoping (like in > elisp) where the values are actually picked up from the execution > (rather than definition) environment. This business of "closes over > variables rather than values" sounds like a cop out. > > Comments? > There are cases where both lexical scope and dynamic scope produce same results. The example that you gave falls into that category. Try the following example: (setq x 5) (defun getx () x) (getx) (defun f (x) (getx)) (f 10) (setq x 4) (f 10) The function getx takes the values of x that is closest to in the stack, not the one that is lexically closest. Another example to demonstrate that. (defun g (x) (h x)) (defun h (y) (getx)) (g 3) With lexical scoping a can be resolved at the time of declaring the function it self (as in name x in getx is alias to x defined in the top-level), where as with dynamic scoping it can only be determined at the run time. Because of this lexical scoping is usually faster. Anand _______________________________________________ BangPypers mailing list BangPypers@python.org https://mail.python.org/mailman/listinfo/bangpypers
