Declares an invariant on the success value: the value passes through when predicate holds, otherwise the Task rejects with the declared failure built from the value — which is what makes it the one-line spelling of the HTTP envelope check:

import { define, is } from '../result/failure.ts';
const PageFailed = define('PageFailed', (d: { status: number }) => `page failed: HTTP ${d.status}`);
const respond = (status: number) => Task(() => new Response(null, { status }));

const checked = await respond(404).ensure((res) => res.ok, (res) => PageFailed({ status: res.status })).result();
is(checked) && checked.code; // 'PageFailed' — declared, typed, one line
await respond(200).ensure((res) => res.ok, (res) => PageFailed({ status: res.status })).map((r) => r.status); // 200