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RxDart vs FxDart › #2 · Running balance from a deposit feed

Running balance from a deposit feed

Toss-up

• scan

Requirement

An account opens at zero and receives seven movements — deposits positive, withdrawals negative. Print the balance after each movement, one line per step. The data is in the code; both versions must print the lines shown under Expected output.

Balance: 250
Balance: 170
Balance: 290
Balance: 250
Balance: 750
Balance: 430
Balance: 520

Side by side

Why they differ

They barely do. Running state is a fold with its intermediate steps exposed, and both libraries call that fold scan — RxDart as a stream transformer, FxDart as a lazy operator ported from the same Rx lineage. One accumulation per movement, in order, on both sides.

The visible differences are cadence details, not model differences. RxDart's scan takes a seed and emits one value per event (its accumulator also receives an index); FxDart's seeded scan follows FxTS and yields the seed itself first, so the panel uses the unseeded scan1 — for a balance that opens at zero, each partial sum is the balance, and the two cadences line up exactly. Beyond that, the only residue is delivery: the stream version collects through an async main, the pull version is one synchronous chain. A fair tie — both sides say the requirement with a single operator.