Collections in Functional Lykn

Maps and Sets are mutable — .set(), .add(), .delete() all mutate in place. This sits oddly with surface Lykn’s immutability preference.

The Pragmatic Approach

Local mutation is fine — if a Map lives entirely inside a function and nobody else sees it, mutating it is safe:

(func build-index
  :args (:array items)
  :returns :any
  :body
  (bind index (new Map))
  (for-of item items
    (index:set item:id item))
  index)

Shared State: Use `cell`

For Maps/Sets that live beyond function scope:

(bind cache (cell (new Map)))

;; Update via swap! — mutation is marked with !
(swap! cache (fn (:any m) (do (m:set key value) m)))

The ! on swap! marks the mutation point. The Map itself mutates inside the swap function, but the cell boundary makes it visible and controlled.

This is one of the places where Lykn’s immutability ideal meets JavaScript’s mutable reality. Copying a Map on every update is possible ((new Map existing-map)) but expensive for large collections. The pragmatic answer: use cell for the ownership boundary, mutate the collection inside swap!, and accept that JavaScript’s built-in collections are mutable by design.

Keyboard shortcuts

Lykn