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Phase 7.5. List aggregations + arena_query

FieldValue
MEPMEP-54
StatusLANDED
Started2026-05-28 (GMT+7)
Landed2026-05-28 (GMT+7)
Tracking PR#22567
Commitcb2410a13d

Gate​

17 fixtures: 4 math (sum_int, list_contains_int, list_contains_str, list_contains_bool), 5 type-cast (list_max_int, list_max_string, list_min_int, list_min_float, min_max_combined), 8 arena (arena_int_filter, arena_bool_filter, arena_float_select, arena_str_select, arena_nested_query, arena_order_take, arena_large_result, arena_join_inner). All 193 transpiler3/go fixtures green.

Lowering decisions​

The four list reductions lower to inline helpers because Go 1.22's stdlib lacks a generic min/max/sum over slices that preserves the rune-aware behaviour needed for strings:

  • ListSumExpr -> mochiListSumI64 or mochiListSumF64 depending on the element type. The accumulator type pin matches the element type (int64, float64) so summation is bit-exact relative to a sequential for _, v := range xs { acc += v }.
  • ListMinExpr / ListMaxExpr -> mochiListMin{I64,F64,Str} / mochiListMax{I64,F64,Str}. String variants compare lex-by-byte (matching the C runtime's strcmp).
  • ListContainsExpr -> slices.Contains for the v in xs predicate; the slices import is registered on demand. No custom helper because the stdlib version is type-parameterised and handles all four scalar element types.

NumCastExpr lowers int(x) for float -> int truncation to int64(x). The truncate-toward-zero semantics match both Mochi and Go.

StrUpperExpr / StrLowerExpr lower to strings.ToUpper(s) / strings.ToLower(s). These are pulled in by arena_str_select even though they don't strictly belong to the aggregation surface; bundling them here avoided a separate Phase 7.5.1 just for strings.

Files changed​

FilePurpose
transpiler3/go/lower/expr.goListSumExpr, ListMinExpr, ListMaxExpr, ListContainsExpr, NumCastExpr, StrUpperExpr, StrLowerExpr
transpiler3/go/lower/lower.gomochiListSumI64, mochiListSumF64, mochiListMin{I64,F64,Str}, mochiListMax{I64,F64,Str} helper texts
tests/transpiler3/go/fixtures/sum_int/, list_*/, arena_*/, min_max_*/17 fixtures

Test set​

  • 17 TestPhase1Hello/sum_int, list_contains_*, list_max_*, list_min_*, min_max_combined, arena_* subtests.

Closeout notes​

Phase 7.5 was scoped to "what arena_query needs" rather than "every list aggregation in MEP-54 §X" because the arena_query fixture suite is the regression goalpost from the JVM baseline. Hitting all 8 arena fixtures in one phase was a forcing function: each one transitively requires a different aggregation, and shipping them piecemeal would have hidden inter-aggregation bugs (e.g., min and max sharing a string-comparison subtle).