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RxDart vs FxDart › #12 · Weekly totals from a daily series

Weekly totals from a daily series

FxDart wins

• fx
• chunk
• sumBy
• map
• zipWithIndex

Requirement

Three weeks of daily spend (August 1–21, stored in cents) roll up into one line per week: week n: $total, with the total converted to dollars at two decimal places. The 21 amounts are in the code; both versions must print the three lines shown under Expected output.

week 1: $83.50
week 2: $89.95
week 3: $91.75

Side by side

Why they differ

The windowing itself is a wash: bufferCount(7) and chunk(7) are the identical idiom for the same fixed window, and both would emit a short trailing window if 21 didn't divide evenly. The job splits on numbering the windows. RxDart has no indexed operator, so the idiomatic move is to draft scan as a counter — an accumulator record whose only role is to carry week + 1 alongside the buffer. It works, but the fold is a bystander wearing a state operator's clothes.

The pull side has a purpose-built word: zipWithIndex pairs each chunk with its position lazily, no accumulator in sight, and sumBy folds each week's cents into dollars in the same breath. That is the recurring tier-2 pattern — both models window finite data fine, but the pull vocabulary is wider exactly where bookkeeping (indexes, keys, partial aggregates) meets the window. One repurposed operator versus one intended one: the verdict goes to FxDart.