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Either.md

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Either

Either<L, R> represents a value that can be one of two possibilities:

  • Left<L> → failure / error / exceptional case
  • Right<R> → success / valid result

It is commonly used to model computations that may fail with a reason.

Motivation

Unlike Maybe, which only represents absence, Either preserves why something failed.

Either<Error, number>
Either<string, Config>

This makes failure explicit, typed, and composable.

Creating Values

const ok = right(42);
const err = left("invalid input");

Pattern Matching

value.match(
  err => console.error(err),
  res => console.log(res)
);

Both branches must be handled explicitly.

Mapping

right(10).map(x => x * 2); // Right(20)
left("err").map(x => x);  // Left("err")

If the mapping function returns an Either, it is flattened automatically.

Flattening

right(right(10)).flatten(); // Right(10)
left(left("e")).flatten();  // Left("e")

Nested Either values are collapsed while preserving all error types.

Async Interop

promiseFlaten

await right(Promise.resolve(5)).promiseFlaten(); // Right(5)
await left(Promise.resolve("err")).promiseFlaten(); // Left("err")

Runtime Guards

if (isEither(value)) {
  value.isRight();
}

Utilities

EithertUtils.all

EithertUtils.all(a, b, c)(
  () => "at least one failed",
  (ra, rb, rc) => ra.value + rb.value + rc.value
);

Runs the success branch only if all are Right.

Laws

  • Mapping preserves Left
  • Right identity: right(x).map(f) ≡ right(f(x))
  • Left propagation is guaranteed

When to Use

Use Either when:

  • Failure is expected and meaningful
  • You want typed error channels
  • Exceptions are undesirable

Prefer Result when modeling API-style success/failure semantics.