Scope Exit

A scope is both a placement node in the scope tree and a result boundary. Scope exit is the result that boundary reports after its owned work has converged.

A child scope is also the root of a subtree. It may contain an entry process, processes created by spawn(...), and further child scopes. Work inside that subtree can still have local results. When routine code waits for that scope, it waits for the work in the subtree to converge, then reads the result reported by the scope.

Exit Belongs to the Boundary

import { sleep } from "@shajara/host";
import { branch, spawn } from "@shajara/host/primitives";

function* publishListing() {
  return yield* branch(function* listingScope() {
    yield* spawn(function* writeSearchIndex() {
      yield* sleep(5);

      throw new Error("index failed");
    });

    yield* sleep(20);

    return "published";
  }); // Throws ScopeError.
}

writeSearchIndex is a process inside listingScope, and it throws the original error. publishListing is not waiting on that process future. It is waiting on listingScope, the boundary created by branch(...). The visible result is therefore that scope’s failed exit. The relevant part of the thrown ScopeError has this shape:

{
  kind: "scope",
  cause: {
    kind: "external",
    raw: new Error("index failed"),
  },
}

The original error remains attached inside the cause, but the boundary seen by the caller is the scope.

For a normal exit, the same reading applies: after all owned work converges, the scope reports the entry value. For cancellation, the same boundary reports cancellation after the work in that subtree has been canceled. The useful distinction is not the list of outcomes; it is the boundary being observed.

Scope exit is therefore different from waiting on a future or using a channel endpoint. Waiting on a future reads that future’s settlement. Sending to or receiving from a channel reads that channel’s state. Observing a scope reads the boundary itself: the waiting routine receives the scope’s exit, not every result inside it.