Fibers and spawn

Informative guide. This Book page explains Beskid but does not define the standard. For normative requirements, use the Beskid Standard.

Fiber<i32> worker = spawn DoWork(42);

spawn schedules a cooperative fiber with a typed handle, not “fire a thread.”

Types

Every spawn expression must type-check to Fiber<T> where T is the entry callable’s return type (Concurrency.Fiber in corelib_concurrency). You do not get T directly from spawn; use Join.

Normative feature: Fibers and spawn.

Handle and cancellation

The handle exposes OnCancelled as an event on the child fiber handle, not on the entry callable. Cancellation flows: Cancel → observe OnCancelled → Join / channel errors per the decisions record.

The handle is move-only. Join and Detach consume it, while Cancel only requests cancellation and leaves the handle usable.

stateDiagram-v2
  accTitle: Fiber handle lifecycle
  accDescr: A spawned fiber runs and can be cancelled. It ends as completed, cancelled, panicked or stack overflow and is then consumed by Join, or the handle is consumed early by Detach.
  [*] --> Running: spawn
  Running --> Running: Cancel requests cancellation
  Running --> Completed: returns a value
  Running --> Cancelled: cancellation observed
  Running --> Panicked: child panics
  Running --> StackOverflow: stack limit exceeded
  Running --> Detached: Detach
  Completed --> Joined: Join returns Ok
  Cancelled --> Joined: Join returns Err Cancelled
  Panicked --> Joined: Join returns Err Panicked
  StackOverflow --> Joined: Join returns Err StackOverflow
  Joined --> [*]
  Detached --> [*]

Text equivalent: spawn starts a running fiber. Cancel may be called repeatedly while it runs. The fiber ends as completed, cancelled, panicked or stack overflow, and Join consumes the handle with the matching Result. Detach consumes the handle without joining and waives the shutdown join.

Semantic rules (cheat sheet)

Rule	Consequence
Stack references must not escape spawn	StackReferenceEscapesSpawn diagnostic
Detach waives shutdown join	Otherwise runtime joins non-detached children when main returns
Cross-fiber payload	Channels only, not mutex-as-mailbox

Lowering

beskid_codegen emits fiber_spawn with environment captures rooted for GC. Runtime details: Fiber scheduler and stacks.

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