Phase 17. Math (abs, floor, ceil) and map helpers (len, keys, values)

Field	Value
MEP	MEP-56 §Phases
Status	LANDED
Started	2026-05-29 12:01 (GMT+7)
Landed	2026-05-29 12:01 (GMT+7)
Tracking issue	none
Tracking PR	#22510
Commit	d4ab0240e7

Gate

TestPhase17MathMap in transpiler3/ruby/build/phase17_test.go: four subtests (math_abs_int, math_abs_float, math_floor_ceil, map_len). Each subtest compiles a Mochi program (e.g. abs(-7) must print 7, floor(3.7) must print 3, ceil(3.7) must print 4) and runs the emitted .rb under the resolved Ruby toolchain with -I mochi-runtime/lib, then diffs stdout. The two abs subtests cover both the int (abs_i64) and float (abs_f64) IR signatures so the math dispatch is exercised on each path.

Lowering decisions

Math is a single dispatch table on aotir.MathCallExpr.Func; map helpers are direct Hash method calls (transpiler3/ruby/lower/lower.go lines 1022 to 1053):

• aotir.MathCallExpr with Func == "abs_i64" or "abs_f64" to MethodCall{Method: "abs"} (lines 1022 to 1029). Ruby Integer#abs and Float#abs are both C-level and return the same primitive type, so the int / float split survives the lowering without a cast.
• aotir.MathCallExpr with Func == "floor" to MethodCall{Method: "floor"} (lines 1030 to 1031). Ruby Float#floor returns Integer, matching Mochi's int result type.
• aotir.MathCallExpr with Func == "ceil" to MethodCall{Method: "ceil"} (lines 1032 to 1033). Same Float -> Integer return as floor.
• An unrecognised Func returns nil, fmt.Errorf("ruby lower: unknown math func %q", e.Func) (lines 1034 to 1035), so future math additions (e.g. sqrt) fail loudly instead of silently producing wrong Ruby.
• aotir.MapLenExpr to MethodCall{Method: "size"} (lines 1036 to 1041); Hash#size is the canonical name (length is an alias but size matches the set / array choice).
• aotir.MapKeysExpr to MethodCall{Method: "keys"} (lines 1042 to 1047). Hash#keys returns Array<K> in insertion order, matching Mochi's keys(map) ordering guarantee since Phase 3.5 settled on insertion-ordered Hash.
• aotir.MapValuesExpr to MethodCall{Method: "values"} (lines 1048 to 1053).

The shared MethodCall shape lets all six builtins render through the same rtree.MethodCall.RubyExprString() formatter, so the emitted Ruby is consistently receiver.method with no per-call special casing.

Files changed

File	Purpose
transpiler3/ruby/lower/lower.go	MathCallExpr dispatch (lines 1022 to 1035) and three Map*Expr cases (lines 1036 to 1053)
transpiler3/ruby/build/phase17_test.go	TestPhase17MathMap with 4 subtests

Test set

• TestPhase17MathMap/math_abs_int, math_abs_float, math_floor_ceil, map_len.

Closeout notes

Phase 17 landed on CRuby 4.0 (Homebrew). The MathCallExpr.Func dispatch is intentionally a switch with an explicit error default so a future Mochi-side math addition that forgets to update the Ruby lowerer surfaces as a build error, not a silent miscompile. Hash#size (not length) is the consistent name used across the set / map / omap pages so a reader skimming phase-16 and phase-17 together sees one idiom. keys and values come through the same MethodCall node as len to keep render shapes uniform; insertion order falls out of Ruby's Hash semantics for free.