Closures snapshot values

Here’s a bug JavaScript shipped for years: build a few functions inside a loop, each meant to remember the loop counter, and they all end up seeing the final value instead. JS had to add per-iteration let to patch it.

It can’t happen here, because an Ascent closure captures by value — it snapshots the names it uses at the moment it’s created:

mut fns: List<Fn() -> Int> = [];
for i in 0..3 {
  fns = fns.append(fn(): Int => i);
};
[fns[0](), fns[1](), fns[2]()]

Three functions, each built in a different turn of the loop. Call them afterward and you get [0, 1, 2] — the value i held when each one was created — not [3, 3, 3]. The closure remembers a value, not a live link back to the slot.

(The rare time you actually want a closure to track later changes, that’s Ref<T> — a value that holds a shared slot. Much later.)