The typed error channel — try/catch vs Effect

Effect v3.12.0

A thrown error is the one failure TypeScript will not type for you. Watch how it hides. The signature promises a plain result and says nothing about failure, so the call reads as safe. Then it throws. You catch the error and the compiler hands you unknown, so now you are guessing at runtime about a failure the type never admitted.

Effect gives that failure a place to live, the E in Effect of A, E, R. You wrap the risky call in Effect.try and name the failure as it leaves. Now the type says the quiet part out loud. It returns a port, or it fails with a ParseError, and the caller can see that before the code ever runs.

One typed failure is useful. The real point is what happens when you compose. Chain the parse onto a range check and Effect tracks both outcomes for you. The channel becomes the union of every way the chain can fail, inferred, with nothing dropped. Add ten more steps and the type still lists every failure until the day you decide to handle it.

// Plain TypeScript: a thrown error has no type. `catch` hands you `unknown`,
// and nothing in the signature warned you a failure was even possible.
function parseConfig(json: string): { port: number } {
  return JSON.parse(json) as { port: number }; // may throw — invisible in the type
}

try {
  const config = parseConfig("{ not json");
  console.log(config.port);
} catch (err) {
  // err is `unknown`; the compiler can't tell you what went wrong.
  console.log("parse failed");
}

❚❚ beat