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RxDart vs FxDart › #4 · Total the valid even amounts

Total the valid even amounts

FxDart wins

• fx
• compact
• filter
• sum

Requirement

A statement import produced a list of parsed amounts where two lines failed to parse (null). Drop the failures, keep the even amounts, and print their total. The data is in the code; both versions must print the line shown under Expected output.

Even total: 202

Side by side

Why they differ

The pipelines are almost word-for-word the same — whereNotNull → where → fold against compact → filter → sum. What differs is everything around them. The RxDart side must lift a plain list into a Stream, go async, and await a fold, because a stream only yields values over turns of the event loop — even when every value is already sitting in memory. The FxDart side stays a synchronous expression: pull values, sum, done.

That is the recurring theme of this Part: for data that is finite and already here, a stream adds a delivery mechanism the problem never asked for. RxDart's operator vocabulary is good — whereNotNull is exactly compact — but the model underneath charges an async tax on every fixed-data task. Here the entire answer is one number, so the ceremony — the lift, the async main, the awaited fold — is the whole difference between the two programs; that is what carries the verdict on this page, where later pairs with the same residue settle for a tie. The verdict flips in Part 4, where values genuinely arrive over time and that same machinery becomes the point.