Sunday, April 6, 2008

Approximating other people and dynamic scoping

I have a theory (or rather a hypothesis) that you can approximate how other people react, think, do, etc, in a given situation by asking yourself: what would I have done in the same situation.

Yeah, I know its sound pretty stupid... because we're all different, right? But for small things like "It such nice weather. I really like an icecream. I wonder if I have to stand in a long queue to buy one" it work fairly well. In this example, I probably would have to wait a while to get an icecream, because if I want an icecream other people will as well.

To get to the point, when applying this "theory" on my latest micro-project I realized that having to call the done() method to close a dynamic scope annoy a lot of people. Why? Because it annoys me. Here are a few reasons for that:
  • it's an implementation detail that is irrelevant to the service the Scope class provides;
  • in some sense it exposes implementation;
  • it's a detail that is easy to forget;
  • forgetting to call done() will not break your code in all cases, thus, doing so is a hard-to-find bug.
In summary, the Scope class sucks. Let's make suck a bit less.

Instead of explicitly pushing and poping objects to the Stack<Object> that Scope contains I'm using the call stack of the current thread. That is, when the Scope.of method is called it looks at the current call stack and finds where a new scope was created. This makes the done() method redundant and fixes the problems with the Scope class that annoyed me.

Searching through the call-stack is heavier on the CPU, but it's easier on the programmer - a trade-off I'm willing to make. There is also a bit of memory overhead because Scope now contains a map that holds object that otherwise would not exist at all or be possible to garbage collect. Again, a trade-off I'm willing to make.

To conclude, with the new version of Scope it's now possible to do
new Scope(someObject) { {
} };

instead of
new Scope(someObject) { {
} }.done();

which is a Good Thing.

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