Closures

Wednesday, November 17, 2010
I decided to take a break from the low-level work of debugging and garbage collection, and work on some language features, which is a much more pleasant activity. It only took a couple of days to get closures working. Here's one of the unit tests, for example:

def testClosureCounter {
  var a = 0;
  let f = fn -> int {
    return a++;
  };

  Debug.assertEq(0, f());
  Debug.assertEq(1, f());
  Debug.assertEq(2, f());
}

The mechanism behind closures is fairly efficient: The closure environment is a subclass of Object that has a member slot for each captured variable. For 'let' variables (which cannot be changed after the initial assignment), the slot contains just a copy of the variable. For 'var' variables, the slot is a reference to a shared closure cell, whose type is tart.core.MutableRef[T], so that both the inner and outer functions can modify the same variable.

The closure itself is passed around as a pair of pointers - the pointer to the executable code, and the pointer to the environment. Note that closures are type-compatible with bound methods - that is, anywhere you can put a bound method, you can also put a closure, assuming that it takes the same argument types.

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