zena:result

Result's storable counterpart: the same two arms as a sealed reference type, for when a result must outlive return position — an array of per-item outcomes, a field holding a deferred answer. Ok carries what the inline form's true arm does, Err the false arm's payload.

Reach for the inline Result<T, E> first: it allocates nothing. This type is for the positions inline tuples cannot occupy, and costs one allocation per outcome.

A value outcome.

An error outcome.

Result<T, E> — a value that is either an ok payload or an error payload.

Unlike Option, this is an inline multi-value union, not a heap type: it compiles to WebAssembly multi-value returns, so returning an ok or an error costs no allocation. That is why it is a type alias rather than a class hierarchy — see docs/design/result-option.md.

There are three lanes, not two, so that each lane is type-homogeneous: the discriminant, the ok payload, and the error payload. The unused payload in each arm is a hole (_).

import { Result } from 'zena:result';

let checkedSub = (a: i32, b: i32): Result<i32, String> => {
  if (b > a) {
    return (false, _, "underflow");
  }
  return (true, a - b, _);
};

if (let (true, v, _) = checkedSub(10, 4)) {
  // v is 6
}

Inline tuples are return-position-only, so Result<T, E> may name a function's return type but cannot be stored in a variable, field, or collection. To keep a result around, store it as an Outcome below, or match it and store the payload.