Priorities and signs

This was almost going to be titled 'signs are hard'. I hat a problem that a simple timer queue implementation did not quite work. Problem: Some timers seemed to vanish from the java.util.PriorityQueue, or were in the wrong order. I tried, for lack of other ideas to change the sign of the compare method, to see if I got that wrong. Letting the handler thread wait for and pick the latest timer is not a good idea. But alas, that wasn't it, and there goes the title.

Google ("java PriorityQueue remove"): Somewhat unexpectedly, the first result is not the documentation of the queue but rather exactly the report for the bug that is biting me: remove does remove the first element it finds that compares equal to its argument, not that exact object: Voila: wrong element removed; that timer never fires, I'm not getting #ticks.

Good. So now I know what's wrong and can finally implement a strategy to do fewer removes in the first place. Many timers here serve as a timeout and leave the queue not at the front but by remove. And the timeout is constant. So if a timer is set to a later time I can just leave it in the queue and reposition it when it finally appears up front.

Also there is a decision to be made whether I leave the workaround in. It's fixed somewhere in 1.6 (b51?), and I can't rely on the JVM always being that new.

cvs to svn via git

It actually works. The problem: I wanted to take the history of a partial cvs repository into svn. Directly using the tools was out for two points: First, one needs access to the svn repository itself (at least I think so and didn't bother to check because of) second I wanted to patch the paths of the java packages contained therein. Apparently eclipse doesn't quite want to deal with company_name.com, so it needs to converted to companyname.com even though the company only owns the former domain.

Anyway, I did not figure out yet how to use git-cvsimport and git-svn both on the same repository sensibly; and I think git-svn rewriting of the commits doesn't quite make that feasible. But the problem already starts with getting both histories to have a common ancestor. Unfortunately git does not have a null commit as the universal base.

Ok, final approach: Create project base directories (the ttb) in svn, do an git svn clone on that. Separately, use git-cvsimport to get the history from cvs into git. (Caveat: The approach does only handle a single linear history well.) Use git format-patch --root commit to get a series of patches, run those through sed -e s:company_name.com:companyname.com:g (which patches directory names as well as imports and package declarations). Then apply (git-am) the resulting commits in the git-svn repository, and git-svn dcommit them. Done.

Except that my link to the svn repository isn't exacly fast at the moment (roundtrip at about a second), and the 60 commits took two hours. svn does not seem to like slow links; a simple tag operation (svn cp trunk tag/some) on a small thingy took me more than a minute over GPRS once.

Here's the complete commands:

mkdir myproj-cvs
cd myproj-cvs
git-cvsimport -p x -v -k -a -d :pserver:krey@localhost:/opt/cvs mystuff/myproj
git-format-patch -o out --root `cat .git/refs/heads/master`
mkdir mod
cd out
for i in *.patch; do sed -e s:company_name:companyname:g $i >../mod/$i; done
cd ../..       
svn mkdir http://localhost:4080/repos/mystuff/myproj -m 'base dir'
svn mkdir http://localhost:4080/repos/mystuff/myproj/trunk -m 'trunk dir'
git-svn clone -s http://localhost:4080/repos/mystuff/myproj
cd myproj
git am ~/myproj-cvs/mod/*
git svn dcommit

Inband signalling is evil

In telephone inband signalling is, or rather, was to control circuit setup and teardown by tone signals within the speech band that is also transmitted from speaker to listener. Had the unfortunate effect that you use a specific whistle on the phone and lose the connection. Soon got exploited in more creative ways.

Anyway, in C++'s std::string often the same thing happens. When in C, you have a char*, and you can null it to mark the no value case. Not so in C++. You need a special value, and usually this is the empty string "". Fine and dandy as long as you can be sure that that value will never actually occur. Once this happens all the code will already be riddled with if (val != "") and you never find them all. Bad luck, just like with Y2K.

Neither do you have Maybe, the Haskell way of avoiding null pointer exceptions completely.

Dr. No vs. anonymous functions

You may remember the first James Bond movie where the hero gets into contaminated areas and thus gets the decontamination treat. Now, this doesn't just happen but is shown in quite some details. As if to show off 'hey, look, we know all this stuff' instead of simply assuming it.

Now there is a similar thing with the syntax of anonymous functions. In some languages it is like function (x) return 2 * x; end while others say the same as \x{2 * x}. The shorter syntax stand less in the way and makes it possible to write, say, list.filter (\x{x.city == quest}).map (\x{x.zipcode}) in one line, while the former notation will cause massive keyword clutter hiding the actual operations.

There is a case in that the longer syntax may have been judiciously chosen to make clear that the use of anonymous functions (plus closures) is not encouraged in this language. I don't assume that it is a case of show-off, as in Cobol which needs to make clear that it can DISPLAY or COMPUTE something, as if this wasn't to be expected of a computer language. But the question remains, whether, in any specific language the form of anonymous functions wasn't just chosen because of its similarity to named functions. In named functions this is no big deal as their opening has a line for itself anyway. Anonymous functions need to be unobtrusive to be really useful, while the more verbose forms make many uses unwieldy. The question is whether the language designers really wanted to discourage that use or whether they didn't think quite that far in that direction, not having actively used functional style themselves.

The other ternary operator

There is an interesting stain on OO languages like C++ and Java, namely with member function invocation. Normally, complex expression can be taken apart, and the intermediate value stored in a variable. int a = 2 * (3 +1); can equally be written int helper = 3 +1; int a = 2 * helper;. However, this breaks with obj.meth (). When you try instead to say helper = obj.meth; helper ();, you will run into a problem: What is the type of helper? It should be a function pointer/reference. Java does not even have functions; in C++ you are equally at a loss for a proper type. (I'm not going into that actually the parameter types aka signature form a part of the method name.)

Indeed the compilers simply go to treat the combination a.b(c) as a single operator. In Java the method and member name spaces are actually separate; a method can have the same name as a member. (Does that make it a Java-2?)

The only language that makes this explicit is Lua. Member invocation is obj:meth(arg), which is the same as obj.meth(obj,arg) (except that obj is only evaluated once).

I don't know of any language that does it properly. Even though a member function exists as code only once, logically there is a distinct function for each object. The member selection needs to bind the function code address and the object pointer together, into something that is quite similar to a closure.

Custom syntax: Macros and closures

I'm lots interested in getting custom syntax for this and that where I get annoyed by C++ or especially Java simply not letting me doing any. For example, in Java you need to write all these EJB-style accessors yourself. In ruby you just say attr_reader :attr1, :attr2, and when reading other people's code you can be sure that there isn't a typo that let getAttr1 return attr2 by accident.

Just as I was embracing the idea that macros are absolutely necessary for much stuff,
ruby came along and brought block arguments. And tada, you can do map and everything without macros just as regular function. Some guys even ported the idea to java. So I became unsure whether macros are really necessary. Ok, basically I still believe that because (programmatic) macros can do anything; I think they are the best way to integrate custom code generators.

And then came along Raganwald and wanted something like ph = find_whatever()&&.author&&.phone: Call find_whatever, and if it returns something, get the author, and if there is one, get his phone number. Otherwise the whole expression is nil.

Now it is amazing that you can get ruby to do something like that at all, even though it cheats on two points: It does not use the intended syntax (obj&&.name) but obj.andand.name (note the conspicious second dot), and it does not actually take the shortcut but instead goes through the rest with a dummy object.

But he does not just to avoid the helper variables, but also to get a left-to-right reading. You just can't do that in lisp, for instance. (Ok, it's going to be the inverse.)

But why this hit me: While I have somes idea how I would bring macros into language with syntax, I have no idea yet how I would make &&. definable. The problems here:

• When you get to the &&. you are already in the middle of the expression the macro would need to replace, so there would be a need to allow operators be a kind of macro


• Member selection is not a binary, but a postfix operator! While it looks like one, the right hand side is not an expression, but a compile-time constant. (In a sense there are many different operators, one for each possible member name.)

I guess I need to think about that some more...how to enable user-defined operators not only as functions, but also as macros.

Burned by override: Limits of refactoring by compiler

In OO programming (here: java) there is the difference between redefining a method in a subclass, and adding a new method in a subclass. It also is quite easy to accidentally add one instead of override one by having a subtly different method signature.

Then there is 'refactoring by compiler' as I call it. Just do the intended interface change at one place and be guided by the compiler errors to all the places that needs adatptions to that. In sufficiently static languages that does actually work (and it is a bit hard to get a program that compiles but does not work).

Anyway in this case I changed, compiled, fixed, and the program did not work in very strange ways. Well, it was the case from above: I started my refactoring by changing the signature of a overridden method, thereby making it into a new one that never got invoked. The macro did no longer think it was one, it went downhill.

Good thing scala requires to declare whether a method is an override. Java does too, but my personal java-fu is from times too old to actively use @override (or to sprinkle code with finals, for that matter).

Back to coding...