Failure in gen — yield* fail and the E channel

Effect v3.12.0

the typed error channel was introduced in The typed error channel ↗

In plain TypeScript, throw ends a function early and leaves nothing behind in the signature. Inside gen, yield* Effect.fail short-circuits the block the same way, so the lines below it never run when the id is empty. The difference is the type. The failure does not vanish, it lands in E as MissingId.

Now add a second failure. A different branch yields NotFound, and here is the payoff of writing errors in gen. You never declared a union anywhere. The compiler walks every branch the block can reach and builds E for you, so forgetting to handle a case later becomes a type error instead of a surprise in production, with the type widening on its own to MissingId or NotFound.

import { Data, Effect } from "effect";

class MissingId extends Data.TaggedError("MissingId")<{}> {}

// yield* Effect.fail short-circuits the block the way throw leaves a function —
// but the failure is typed, so it lands in E instead of vanishing.
const lookup = (id: string) =>
  Effect.gen(function* () {
    if (id === "") return yield* Effect.fail(new MissingId());
    const name = yield* Effect.succeed("Ada");
    return name;
  });
//    ^? (id: string) => Effect.Effect<string, MissingId, never>

// States are executed, so inspect the failure with runSyncExit, never a throw.
console.log(Effect.runSyncExit(lookup(""))); // Exit Failure: MissingId

❚❚ beat