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Dart vs FxDart › #18 · Spending inside a date window

Spending inside a date window

Toss-up

• dropWhile
• takeWhile
• sumBy

Requirement

A ledger export is already sorted by date. Total the spending between 2026-07-08 and 2026-07-21 inclusive — without scanning the whole list: skip entries before the window, take entries while still inside it, and sum what remains. The data is in the code below; both versions must print the line shown under Expected output.

Spent 2026-07-08 to 2026-07-21: $104.04

Side by side

Why they differ

Barely. Dart ships skipWhile and takeWhile on every Iterable, and both are lazy — the plain-Dart version exploits the sort order exactly the way the FxDart one does, and reads just as well. The only real difference is the last step: sumBy names the intent, where fold spells out the seed and the combine. That is a one-word win, not a structural one — call it a tie. If your codebase already chains with fx, use it here for consistency; if not, plain Dart is fine.