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RxDart vs FxDart › #11 · Stock level after each move

Stock level after each move

Toss-up

• fx
• scan
• map

Requirement

A warehouse ledger for one SKU lists signed moves — positive receipts, negative shipments — starting from an opening stock of 20. Print the opening level, then each move with the level after it, flagging any negative level as a backorder. The moves are in the code; both versions must print the lines shown under Expected output.

start: 20
+40: 60
-25: 35
-50: -15 (backorder)
+30: 15
-20: -5 (backorder)
-10: -15 (backorder)
+45: 30

Side by side

Why they differ

Running state is scan in both dialects — FxDart's is the FxTS port of the same Rx idea, so the fold itself is identical: an accumulator record carrying the move's label and the level after it. The one visible seam is the seed. FxDart's scan emits the seed as its first value, so the opening start: 20 line falls out of the chain for free. RxDart's scan starts emitting at the first fold, so the opening level has to be replayed with startWith — one extra operator, not a hardship.

Past the seed, the two pipelines are the same three words, and the flagging map reads equally well on either side — the pull version merely stays synchronous because the ledger is already in memory, while the stream version awaits its own delivery. Running state over an ordered sequence is home turf for both models: a tie, with the seed-emission difference as the only trivia worth remembering when porting between them.