purescript-lua · Unisay · Jul 27, 2026 · Jul 27, 2026 · Jul 27, 2026 · Jul 27, 2026
diff --git a/bench/goldens/fnew_Bench.RecordFold.txt b/bench/goldens/fnew_Bench.RecordFold.txt
@@ -0,0 +1,16 @@
+chunk: Bench.RecordFold.lua
+runtime: LuaJIT 2.1.1741730670
+main-chunk FNEW: 5
+function-body FNEW: 9
+total FNEW: 14
+prototypes: 15
+function-body FNEW sites:
+  Bench.RecordFold.lua:10
+  Bench.RecordFold.lua:9
+  Bench.RecordFold.lua:20
+  Bench.RecordFold.lua:19
+  Bench.RecordFold.lua:44
+  Bench.RecordFold.lua:43
+  Bench.RecordFold.lua:42
+  Bench.RecordFold.lua:51
+  Bench.RecordFold.lua:50
diff --git a/bench/goldens/tnew_Bench.RecordFold.txt b/bench/goldens/tnew_Bench.RecordFold.txt
@@ -0,0 +1,8 @@
+chunk: Bench.RecordFold.lua
+runtime: LuaJIT 2.1.1741730670
+main-chunk TNEW+TDUP: 4
+function-body TNEW+TDUP: 1
+total TNEW+TDUP: 5
+prototypes: 15
+function-body TNEW+TDUP sites:
+  Bench.RecordFold.lua:26 TNEW
diff --git a/bench/goldens/trace_record_fold.txt b/bench/goldens/trace_record_fold.txt
@@ -0,0 +1,15 @@
+spec: record_fold
+runtime: LuaJIT 2.1.1741730670
+workload: n=100000 reps=4 result=15000350000
+aborts (distinct site -- reason):
+  Bench.RecordFold.lua:19 -- NYI: bytecode FNEW
+  Bench.RecordFold.lua:20 -- NYI: bytecode FNEW
+  Bench.RecordFold.lua:28 -- inner loop in root trace
+  Bench.RecordFold.lua:50 -- NYI: bytecode FNEW
+  Bench.RecordFold.lua:51 -- NYI: bytecode FNEW
+bytecode end state (J*=compiled, I*=blacklisted):
+  Bench.RecordFold.lua:17 IFORL
+  Bench.RecordFold.lua:28 JLOOP
+  Bench.RecordFold.lua:49 IFUNCF
+  Bench.RecordFold.lua:50 JFUNCF
+counts: aborts=5 compiled=2 blacklisted=2
diff --git a/bench/macro/record_fold.lua b/bench/macro/record_fold.lua
@@ -0,0 +1,16 @@
+-- A fold whose step builds a record, updates it, and reads the result
+-- back field-wise -- never passing either table on whole. Unpacked
+-- (issue #240) the step is plain arithmetic; boxed, every element
+-- allocates the literal plus the update's runtime copy.
+return {
+  artifact = "Bench.RecordFold",
+  n = 100000,
+  drive = function(mod, n)
+    return mod.run(n)
+  end,
+  ideal = function(n)
+    local acc = 0
+    for i = 1, n do acc = acc + i + (i + 1) * 2 end
+    return acc
+  end,
+}
diff --git a/changelog.d/20260727_180000_unisay_unpack_fieldwise_records.md b/changelog.d/20260727_180000_unisay_unpack_fieldwise_records.md
@@ -0,0 +1,33 @@
+### Added
+
+- Let-bound records read only field-wise unpack to scalars (#240). A
+  record literal or record update whose binder never flows anywhere as
+  a whole value exists only to be projected or updated again, yet it
+  still allocated its table — and every update paid a runtime copy on
+  top. Such a binding now explodes into per-field bindings: reads
+  become the field values, an update over a known literal is rebuilt
+  as a single literal, and a chained update coalesces onto its base
+  record with the patch lists merged. The fold reaches across sibling
+  `let` bindings, so the defaults pattern dissolves whole. A fold step
+  previously compiled to
+
+  ```lua
+  local r_S_0 = { lo = i_S_0, hi = i_S_0 + 1 }
+  local s_S_0 = PSLUA_object_update(r_S_0, { hi = r_S_0.hi * 2 })
+  return acc_S_1 + s_S_0.lo + s_S_0.hi
+  ```
+
+  now becomes allocation-free arithmetic:
+
+  ```lua
+  return acc_S_1 + i_S_0 + (i_S_0 + 1) * 2
+  ```
+
+  4.6× faster under LuaJIT and 5.3× under PUC Lua 5.1 on the
+  `Bench.RecordFold` macrobenchmark. Locally-built dictionary records
+  collapse the same way — `Golden.LongWriterBind` drops its Writer
+  dictionary tower and 57 lines of Lua. A record used as a whole value
+  anywhere — returned, passed on, compared — keeps its allocation, the
+  soundness boundary `Golden.ScalarReplacement.Test` pins from both
+  sides (see `propagateKnownObjectThroughLet` in
+  `Language.PureScript.Backend.IR.Optimizer`).