Lykn

WeakMap and WeakSet

Weak collections hold weak references — when no other reference to the key (WeakMap) or value (WeakSet) exists, it can be garbage collected even though it’s still in the collection.

WeakMap

Object keys that don’t prevent garbage collection:

(bind cache (new WeakMap))

(func process-with-cache
  :args (:object item)
  :returns :any
  :body
  (if (cache:has item)
    (cache:get item)
    (do
      (bind result (expensive-computation item))
      (cache:set item result)
      result)))

When item is garbage collected elsewhere, the cache entry disappears automatically. No memory leak. No manual cleanup.

Use cases: caching computed results keyed by object identity, storing private data associated with DOM elements, associating metadata with objects you don’t own.

WeakSet

Tracking objects without preventing garbage collection:

(bind processed (new WeakSet))

(func process-once
  :args (:object item)
  :body
  (if (not (processed:has item))
    (do
      (processed:add item)
      (do-work item))))

Constraints

• Keys (WeakMap) / values (WeakSet) must be objects or symbols — not primitives
• Not iterable — no for-of, no .size, no .keys()
• Not enumerable — you can’t list what’s in them

These constraints exist because weak references interact with the garbage collector, which operates non-deterministically. If you could enumerate a WeakMap’s keys, the result would depend on when GC last ran — a recipe for non-reproducible bugs.