PuduLangMediator.Utils.Erasure

type: module path: "@root/src/PuduLangMediator/Utils/Erasure.pudu" fidelity: Active grammar: "[[grammar/pudu]]" depth_score: 0.9 depth_status: DEEP tags: [module, leaf, backbone] aliases: [PuduLangMediator.Utils.Erasure]

Purpose

Values carried without naming their type, read back type-safely. A Packed value is a closure that writes the value into the Slot it was packed through; unpack runs it against a slot and answers what arrived, so only the slot it was packed through ever reads it back.

Interface

Signatures

export type Packed = fn() -> ()

export type Slot[T] = { lock: Sync.Mutex, held: Sync.Cell[Option[T]] }

export fn slot[T]() -> Slot[T]

export fn pack[T](target: &Slot[T], value: T) -> Packed

export fn unpack[T](source: &Slot[T], packed: &Packed) -> Option[T]

Linkage

• Requires: standard library only.
• Consumed by: [[src/PuduLangMediator/Catalog]], [[src/PuduLangMediator/Compose]], [[src/PuduLangMediator/Context]], [[src/PuduLangMediator/Mediator]], [[src/PuduLangMediator/Message]], [[src/PuduLangMediator/Notification]], [[src/PuduLangMediator/Request]], [[src/PuduLangMediator/Stream]].

Algorithm

1. pack(slot, value) captures the slot's cell and the value; running the closure stores Some(value) in that cell.
2. unpack(slot, packed) takes the slot's lock, clears the cell, runs the closure, and swaps the cell back to None, answering what it held.
3. A value packed through another slot writes another cell, so the reading slot still holds None.

Negative Logic (Prohibited Paths)

• No cast exists anywhere: the type a value reads back as is the type its slot was made for.
• The cell is never read outside its lock, so two concurrent reads through one slot cannot see each other's value.

Edge Cases

• A packed value reads back any number of times.
• Slots are runtime values; a const cannot hold one, which is why kinds and context keys are made once at startup.

Depth

DEPTH 0.9 (DEEP). Tested by the suite mirroring this module under test/.

Grill Log

• Q: Why closures and cells instead of a dynamic value and a downcast? A: Pudu has no runtime type test or cast; dynamic Trait widens but never narrows. A slot gives every erased value exactly one typed reader, which is type-safe by construction. _Rejected:_ serializing messages to text (loses types and costs a round trip); one closed sum of every message (closes the set of kinds).
• Q: Why a lock per slot? A: Unpacking writes and then reads one shared cell; two threads unpacking through the same slot would otherwise read each other's value. The critical section is two cell operations. _Rejected:_ a fresh cell per unpack (the packed closure already captured the slot's cell).

Referenced by

[[CHANGELOG]] · [[architecture/_MOC]] · [[decisions/ADR-0002-typed-kinds]] · [[domain/Erasure]] · [[grammar/pudu]] · [[src/PuduLangMediator/Catalog]] · [[src/PuduLangMediator/Compose]] · [[src/PuduLangMediator/Context]] · [[src/PuduLangMediator/Mediator]] · [[src/PuduLangMediator/Message]] · [[src/PuduLangMediator/Notification]] · [[src/PuduLangMediator/Request]] · [[src/PuduLangMediator/Stream]] · [[src/PuduLangMediator/Utils/_MOC]]