On Sun, Nov 9, 2014 at 1:50 PM, Abhishek L <abhishek.lekshma...@gmail.com> wrote:
> I'll try to explain a bit, have to admit though, even my understanding > is not all that clear, and all is open for further discussion. > > Noufal Ibrahim KV writes: > > > 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. > > > Here, technically ML's val bindings are actually immutable. When you > actually do a val x = 10; you're creating a new binding for x *shadowing* > the previous binding. Something like > > fun f y = x + y; > > creates a closure with f, and the free variable x.The closure locks > the function with the environment the function was defined in, locking x > to 2, as variables are always looked up in the dynamic environment they > are defined in before they are called, x is bound to 2. Also ML will not > allow you to define a function with x not previously defined, (ie no > forward lookups) > > > 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". > > Here x was a mutable variable, doing a similiar ML construct, ie > > val x = ref 2 > fun f y = !x + 2 > > f 10 ; evals to 12 > x := 10 > f 10 ; evals to 20 > > So this becomes a problem of closures over mutable variables? ie every > closure looks up the value of same variable, x which gets mutated > around. WDYT? > > PS I'm not sure whether this mail is going to hit twice in the list, > had replied some 30 mins ago, but the mail still failed to appear in the > list > Unless attachments are present, it should reach in time . > -- > Regards > Abhishek > _______________________________________________ > BangPypers mailing list > BangPypers@python.org > https://mail.python.org/mailman/listinfo/bangpypers > -- *Thanks & Regardskracekumar"Talk is cheap, show me the code" -- Linus Torvaldshttp://kracekumar.com <http://kracekumar.com>* _______________________________________________ BangPypers mailing list BangPypers@python.org https://mail.python.org/mailman/listinfo/bangpypers
