disjoint closure environments

[toshok]

the following function:

function foo() {
  var a = [1,2,3,4];
  var b = 1;
  var c = 2;

  var new_a = a.map (function (el) { return el * b; }

  return function () {
    return c;
  }
}

will create a single closure environment holding both b (closed over by the function passed to map) and c (closed over by the returned function.)

In this totally disjoint case, ejs could split the closure environments, but I need to investigate if the overhead of splitting the closure makes sense for small environments (I'm guessing no for this particular case, particularly since the closed over variables are never set to anything other than primitives..)

Also, what happens if the sets are not totally disjoint? Take the following:

function foo() {
  var a = [1,2,3,4];
  var b = 1;
  var c = 2;
  var d = 4;

  var new_a = a.map (function (el) { return el * b + d; }

  return function () {
    return c + d;
  }
}

so now both functions close over d as well.

we could make 3 environments in this case;

env_0 = { d: 4 }
env_1 = { env: env_0, b: 1 }
env_2 = { env: env_0, c: 2 }

accesses to d get replaced with something different from within each function:

1. env_1.env.d in the function passed to map
2. env_2.env.d in the returned function.

This also has a cost in the increased indirection to get to the d binding, so need some heuristic based on the number of accesses to d to determine if we should just create 1 environment.