Objects vs closures
people.csail.mit.edu
people.csail.mit.edu
Situational Cycle: a move sequence that starts and ends at the same position such that at the end of that cycle it is the same player's turn as it was at the start. If cycles are allowed without restrictions on them, it is possible for a game to go on indefinitely. [1]
The 'ko' situation is the most common cycle that occurs in the game of go.
Ko: a situation where two alternating single stone captures would repeat the original board position. The alternating captures could repeat indefinitely, preventing the game from ending. The ko rule resolves the situation. [2]
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http://en.wikipedia.org/wiki/Classical_electromagnetism_and_... may be interesting. Especially the section "Relationship between electricity and magnetism".
http://en.wikipedia.org/wiki/Classical_electromagnetism_and_...
The main thing is to consider how charge densities change as we switch reference frames. Because the lengths contract when we switch to a reference frame where things are in motion, we see higher charge densities (same charge per less length).
A simple demonstration: Say we have a charge neutral, current carrying wire. We model it as a bunch of positive charges staying still, and some negative ones moving. The positive and neutral balance, but there is still a current because only the negatives are moving. Now imagine we switch to a different reference frame, one where the negative charges aren't moving, but the positive one's are, we are flying along parallel to the current. Now the positive charges contract relativisticly, so the density of positive charges is now greater than the negatives, and hence the wire appears to be carrying a net charge.
Now, you can arrange your wire in any number of ways to produce a more interesting field. For example you can arrange it in a loop (or several loops). Since, once again the magnetic field is perpendicular to the wire, at the center of the loop it will be perpendicular to the plane of the loop. This way you can make an approximation of a constant field along an axis (just in that small space at the center of the loop/coil).
The fun part is that the inverse relationship also works: a changing magnetic field produces an electric field. Thus, taking a coil and moving a magnet inside of it sufficiently fast produces an electric field, which compels the electric charges in the coil to move producing an AC current. The change in the magnetic field is called the magnetic flux. This process is described by Faraday's law: http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction#Th...
Lastly, Maxwell combined these two equations with two other fundamental equations of electrodynamics (namely, Gauss's law and Gauss's law for magnetism) with some minor corrections for some special cases: http://en.wikipedia.org/wiki/Maxwell%27s_equations. In physics laws and such are very often named not after the first person to discover something, but after the last. From these equations you can derive that light is nothing more than an electromagnetic wave (which of course then goes into the whole duality of light).
I've often wished that C# let me implement interfaces with anonymous classes. Since it doesn't, I get around it by creating AdHoc classes for interfaces that I use a lot.
For instance, if I have an interface Foo defined as:
interface Foo
{
string Bar(int x);
}
class AdHocFoo : Foo
{
Func<int, string> _bar;
public AdHocFoo(Func<int, string> bar)
{
_bar = bar;
}
public string Bar(int x)
{
return _bar(x);
}
}
I don't really believe this to be efficient. The linked text by Kiselyov implements the dispatch via Scheme's (case ...) expression. Efficient dynamic dispatch means one indirection through a vtable, so one load instruction more compared to a normal function call. Which compilers for functional languages can perform this optimization?
Since we are talking about Scheme here, we could compare the dispatch to dynamic languages like Python or Ruby, where the dispatch means looking up a string in a hashmap. I'm willing to believe that Scheme's case can keep up with that.
(defprotocol Foo
(bar [this])
(baz [this]))
(defn make-foo [a]
(reify Foo
(bar [this] a)
(baz [this] a)))
(dotimes [_ 10]
(let [x (make-foo 'x)]
(time
(dotimes [_ 1e8]
(bar x)
(baz x))))
(of course, Clojure prefers values+behaviors, aka objects, without mucking it up with state)
For comparison, looping 1e8 times with two calls to empty functions in a static language takes ~639ms, gives me ~3ns per call. So, you can see why my first suspicion was that the JVM was doing something like just inlining the methods and avoiding the call altogether. Considering the differences in our reported numbers, you may have a newer JVM than me, and if it is beating simple CALL instructions, it must be inlining them or avoiding some of the looping.
(define (error . args) #f)
(define (class x y z)
(lambda (d)
(case d ('x x) ('y y) ('z z) (else (error "Invalid slot ~a" d)))))
(define obj (class 'this 'is-a 'test))
(display (obj (read)))
(newline)
Note that I was unable to get the compiler to produce dispatch code at all without dispatching on the result of READ.