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FxDart 101 › Раздел 13 · Typed errors › Either × pipelines

Either × pipelines

Typed errors fused with FxDart's lazy, concurrency-aware pipelines — the part neither Arrow nor any Dart FP library has.

Lecture

A chain of Either values gets Either-aware terminals: rights() and lefts() keep one side, separated() splits both at once (same record shape as partition), and sequence() is all-or-nothing — every success collected into one list, failing fast on the first Left. Because the pipeline is lazy, fail-fast is literal: sequence() stops pulling at the first failure, so later elements are never computed.

The fail-slow twin is mapOrAccumulate(transform) on any fx()/async chain: validate every element, keep every failure. On async chains it takes concurrency: n and rides the same concurrent(n) back-channel as the rest of FxDart — n elements in flight, results in order, and each element runs in its own scope, so a failure in one can never leak into a sibling.

All of these are eager by design, which also makes them the sanctioned escape from the laziness × raise hazard: never return a lazy pipeline from a raise block — return one of these results instead.

Demo 1 · rights, lefts & separated

Demo 2 · sequence — fail-fast, literally

Demo 3 · concurrent fail-slow validation

Demo 4 · flattenOrAccumulate & the async extracts

When you already have the Eithers, the fail-slow terminal used to be spelled mapOrAccumulate((r, v) => r.bind(v)) — an identity bind. flattenOrAccumulate() (Arrow's name) is that terminal directly: every success, or every failure as a Nel. It completes the trio — separated() keeps both sides, sequence() fails fast, flattenOrAccumulate() fails slow. And the async chain now carries the whole extract family (rights / lefts / separated / sequence / flattenOrAccumulate), so an async validation feeds a counts badge in one terminal.

Try it yourself

Exercise: sum what parsed, report what didn't.