Function fallback

Answers with something rather than failing, once everything else has been tried: stale data, an empty list, a degraded result.

It belongs outermost, where the failure it sees is the one the caller would otherwise have caught. It is a TypedPolicy rather than a Policy because it produces a result, and a result has a type, so it wraps a stack rather than sitting in one:

const rates = await fallback(() => cached)(policy(readRates))();
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The handler receives the failure, so "fall back only for this" is a decision you make with the error in hand rather than one this package makes for you.