Why Effect & the Effect<A, E, R> type

Effect v3.12.0

Here is a function every TypeScript developer has written, and it is quietly lying to you. It signals failure by throwing, and the instant it does, that failure vanishes from the type. The signature still promises a User and nothing else, so the caller never sees it coming.

Effect's entire pitch starts right here: put the failure back into the return type, where you cannot miss it. The compiler reads it back as Effect of User, MissingId, never, spelling out success, failure, and no requirements, all three channels in plain sight. Read it aloud: getUser gives you a User, or it fails with a MissingId. Failing is now an ordinary value you return, not a surprise thrown behind the type's back.

interface User {
  id: string;
  name: string;
}

// Failure is signalled by throwing — so it vanishes from the type.
function getUser(id: string): User {
  if (id === "") throw new Error("missing id");
  return { id, name: "Ada" };
}

const user = getUser("1");
//    ^? User

❚❚ beat