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Dart vs FxDart › #29 · Monthly category report, sorted by spend

Monthly category report, sorted by spend

FxDart wins

• filter
• groupBy
• map
• sortBy
• join
• foldByOrSkip

Requirement

From a ledger that spills over from June into July 2026, build the July spending report: keep only July transactions, total each category, and print one line per category — biggest spend first. The data is in the code below; both versions must print the lines shown under Expected output.

Food: $74.60
Bills: $60.34
Fun: $23.25
Transport: $14.15

Side by side

Why they differ

Native Dart has no groupBy, so the loop does the grouping and the totalling at once inside a mutable map — compact, but the four requirements (July only, per category, totalled, ranked) are tangled into one body. The FxDart chain keeps them as four visible steps: filter the month, groupBy category, map each group to its total, sortBy descending — and join formats the report. Adding a requirement (say, a minimum total) is one more chain step; in the loop it is another branch inside an already-busy body.

Two FxDart spellings

The benchmark below carries a third bar, which only one other page does. The chain above is the one to write: filter and foldBy as two named steps, each answering one question. What it cannot do is inline its own predicate. filter is a lazy stage, so it keeps that predicate in an iterator field, and the AOT compiler cannot see through a field — every transaction pays a real indirect call whose body never fuses into the loop. foldBy does not have that problem; it is strict, so its callbacks are parameters and get inlined.

foldByOrSkip, shown above main in the FxDart panel, moves the test into the key: a null key skips the row, so one callback both selects and buckets, and it is a parameter of a body small enough to inline. Over 1,000,000 transactions that is the difference between the second and third bars; the first is the hand-written loop.

Write the chain by default — two unrelated questions read better as two steps. Reach for foldByOrSkip when the pipeline is hot and a profile says that predicate is the cost.