Monday, March 11, 2013

Scheme in 5000 lines of C part 3: Testing, random thoughts on closures.

I haven't made as much progress on the Scheme recently as I would have liked. Work's been busy, the house has needed some fixing, and I'm still chewing over how I want to implement closures.

Since I want to make *some* progress I rewrote the testing framework. Before I just had some sample strings embedded in main that I read-evaled-printed, and looked at the output to see if the changes I made had the desired effect.

I felt some (a lot of) shame because (semi-)robust testing is supposed to be my day job. So I moved most of that away into a test harness that's pretty stupid/simple but actually performs validation. Funny that.

There's a directory, tests/, that contains the test cases. One per file. Each file has the input that is fed to read-eval-print, the output of which is captured, and then compared to the expected value.

I'm still not quite sure how I'm going to handle closures though. Variable capture I think is going to be fairly easy. I'm not quite sure how I'm going to store references to inner functions though. For example, this function:

(define closure-test
  (lambda (foo)
    (lambda ()
      (set! foo (+ 1 foo))

The outer function (closure-test) will need to hold some sort of managed reference to the inner function so that when closure-test is called it can create a new function object and a new environment, but I'm not quite sure if I want to bake the reference into the bytecode, or do something else. Maybe re-write function objects to include a table of contained functions? That might work.

Follow on to part 4: Scheme in 5000 lines of C part 4: (Un)necessary refactorings
View the source on GitHub:

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