RxDart vs FxDart › #10 · Number the checklist

Number the checklist

FxDart wins

• fx
• zipWithIndex
• map

Requirement

Turn six setup steps into a numbered checklist — 1. Unbox the sensor kit and so on, one line per step, numbering from one. The data is in the code; both versions must print the lines shown under Expected output.

1. Unbox the sensor kit
2. Charge the base station
3. Pair the remote
4. Mount the wall bracket
5. Run the self-test
6. Register the warranty

Side by side

Why they differ

Neither core Stream nor RxDart has an indexed map. The closest thing RxDart offers is scan, whose accumulator happens to receive an index as its third argument — so the rx spelling shown numbers the steps by riding a fold: a seed you must invent ('') and an accumulated value you immediately ignore. The alternatives are no cleaner in kind — a mutable counter the mapper closes over, or zipping against Rx.range — every route smuggles the index in from outside, because no operator carries it. It works, and it still reads like a workaround.

FxDart says the thing directly: zipWithIndex pairs each element with its position, and a plain map formats the pair. This is not a push-vs-pull gap so much as a vocabulary gap — an indexed pairing operator is trivially expressible in either model, Rx just never grew one — but the reader of each panel feels it: one side states "element with its index", the other encodes it in an accumulator's spare parameter. Verdict: FxDart.