map, flatMap, combineLatest — what do they do?

Middle PRO

The essence

• map — synchronously transforms every value: Publisher<A> → Publisher<B>. Adds no errors; for a throwing transform there is tryMap.
• flatMap — for every value creates a new Publisher and merges their outputs into one stream (e.g. id → network request). The inner publishers run in parallel; maxPublishers: limits their number. An error from any inner publisher terminates the whole stream — handle it inside (catch, replaceError).
• combineLatest — joins several publishers: on every new value from any of them it emits a tuple of the latest values of all. It emits nothing until every source has produced at least one value. Typical for form validation (email + password → button enabled).