Distinct failures with Data.TaggedError

Effect v3.12.0

Effect will let you fail with any value, even a plain Error. That holds up until you have more than one kind of failure. With a bare Error the only thing telling them apart is the message text, so you compare strings by hand. That is fragile. Reword the message once and the branch quietly stops matching.

Data.TaggedError fixes the identity problem. Each failure becomes its own class with a literal name and whatever typed fields it needs, plus value equality you did not have to write. Now you branch on the tag, something the compiler can read, instead of a sentence you have to keep in sync. That tag is also what catchTag will match in the next lesson.

import { Effect } from "effect";

// You can fail with any value — but a bare Error is hard to tell apart later.
const lookup = (id: string): Effect.Effect<string, Error> =>
  id === "" ? Effect.fail(new Error("missing id")) : Effect.succeed("Ada");

// To branch on which failure happened you sniff the message string. Fragile.
const program = lookup("").pipe(
  Effect.catchAll((err) =>
    Effect.succeed(err.message === "missing id" ? "no id" : "other"),
  ),
);

console.log(Effect.runSync(program)); // no id

❚❚ beat