erlang: lists flatmap/2 + filtermap/2 + mapfoldl/3 + search/2 (862/862)

Higher-order list ops in lib/erlang/lists-ext.sx. filtermap honours
true/false/{true,V}; mapfoldl returns {Mapped, Acc}; search returns
{value,E}|false. lists_ext suite 83 -> 91.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/erlang/lists-ext.sx

@@ -484,6 +484,80 @@
   er-bif-lists-droplast
   (fn (vs) (er-ext-droplast (er-bif-arg1 vs "lists:droplast"))))
 
+;; ── more higher-order (flatmap / filtermap / mapfoldl / search) ───
+(define
+  er-ext-flatmap
+  (fn (f lst)
+    (cond
+      (er-nil? lst) (er-mk-nil)
+      (er-cons? lst)
+        (er-list-append
+          (er-apply-fun f (list (get lst :head)))
+          (er-ext-flatmap f (get lst :tail)))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-flatmap
+  (fn (vs) (er-ext-flatmap (nth vs 0) (nth vs 1))))
+
+(define
+  er-ext-atom-true?
+  (fn (v) (and (er-atom? v) (= (get v :name) "true"))))
+
+(define
+  er-ext-filtermap
+  (fn (f lst)
+    (cond
+      (er-nil? lst) (er-mk-nil)
+      (er-cons? lst)
+        (let ((r (er-apply-fun f (list (get lst :head)))))
+          (cond
+            (er-ext-atom-true? r)
+              (er-mk-cons (get lst :head) (er-ext-filtermap f (get lst :tail)))
+            (and
+              (er-tuple? r)
+              (= (len (get r :elements)) 2)
+              (er-ext-atom-true? (nth (get r :elements) 0)))
+              (er-mk-cons (nth (get r :elements) 1) (er-ext-filtermap f (get lst :tail)))
+            :else (er-ext-filtermap f (get lst :tail))))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-filtermap
+  (fn (vs) (er-ext-filtermap (nth vs 0) (nth vs 1))))
+
+(define
+  er-ext-mapfoldl
+  (fn (f acc lst mapped)
+    (cond
+      (er-nil? lst)
+        (er-mk-tuple (list (er-list-reverse-iter mapped (er-mk-nil)) acc))
+      (er-cons? lst)
+        (let ((r (er-apply-fun f (list (get lst :head) acc))))
+          (let ((es (get r :elements)))
+            (er-ext-mapfoldl f (nth es 1) (get lst :tail)
+              (er-mk-cons (nth es 0) mapped))))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-mapfoldl
+  (fn (vs) (er-ext-mapfoldl (nth vs 0) (nth vs 1) (nth vs 2) (er-mk-nil))))
+
+(define
+  er-ext-search
+  (fn (pred lst)
+    (cond
+      (er-nil? lst) (er-mk-atom "false")
+      (er-cons? lst)
+        (if (er-truthy? (er-apply-fun pred (list (get lst :head))))
+          (er-mk-tuple (list (er-mk-atom "value") (get lst :head)))
+          (er-ext-search pred (get lst :tail)))
+      :else (er-mk-atom "false"))))
+
+(define
+  er-bif-lists-search
+  (fn (vs) (er-ext-search (nth vs 0) (nth vs 1))))
+
 ;; ── register ──────────────────────────────────────────────────────
 ;; Hook into er-register-builtin-bifs! rather than registering once:
 ;; the registry can be reset + rebuilt mid-run (tests/runtime.sx does
@@ -516,7 +590,11 @@
     (er-register-pure-bif! "lists" "sublist" 3 er-bif-lists-sublist)
     (er-register-pure-bif! "lists" "nthtail" 2 er-bif-lists-nthtail)
     (er-register-pure-bif! "lists" "split" 2 er-bif-lists-split)
-    (er-register-pure-bif! "lists" "droplast" 1 er-bif-lists-droplast)))
+    (er-register-pure-bif! "lists" "droplast" 1 er-bif-lists-droplast)
+    (er-register-pure-bif! "lists" "flatmap" 2 er-bif-lists-flatmap)
+    (er-register-pure-bif! "lists" "filtermap" 2 er-bif-lists-filtermap)
+    (er-register-pure-bif! "lists" "mapfoldl" 3 er-bif-lists-mapfoldl)
+    (er-register-pure-bif! "lists" "search" 2 er-bif-lists-search)))
 
 (define er-ext-prev-register-builtins er-register-builtin-bifs!)
 (define er-register-builtin-bifs!

lib/erlang/scoreboard.json

@@ -1,7 +1,7 @@
 {
   "language": "erlang",
-  "total_pass": 854,
-  "total": 854,
+  "total_pass": 862,
+  "total": 862,
   "suites": [
     {"name":"tokenize","pass":62,"total":62,"status":"ok"},
     {"name":"parse","pass":52,"total":52,"status":"ok"},
@@ -15,6 +15,6 @@
     {"name":"ffi","pass":37,"total":37,"status":"ok"},
     {"name":"vm","pass":78,"total":78,"status":"ok"},
     {"name":"send_after","pass":10,"total":10,"status":"ok"},
-    {"name":"lists_ext","pass":83,"total":83,"status":"ok"}
+    {"name":"lists_ext","pass":91,"total":91,"status":"ok"}
   ]
 }

lib/erlang/scoreboard.md

@@ -1,6 +1,6 @@
 # Erlang-on-SX Scoreboard
 
-**Total: 854 / 854 tests passing**
+**Total: 862 / 862 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -16,7 +16,7 @@
 | ✅ | ffi | 37 | 37 |
 | ✅ | vm | 78 | 78 |
 | ✅ | send_after | 10 | 10 |
-| ✅ | lists_ext | 83 | 83 |
+| ✅ | lists_ext | 91 | 91 |
 
 
 Generated by `lib/erlang/conformance.sh`.

lib/erlang/tests/lists_ext.sx

@@ -310,3 +310,34 @@
 
 (er-lx-test "droplast single"
   (er-lx-nm "lists:droplast([9]) =:= []") "true")
+
+;; ── lists:flatmap/2 ───────────────────────────────────────────────
+(er-lx-test "flatmap duplicates"
+  (er-lx-nm "lists:flatmap(fun(X) -> [X,X] end, [1,2]) =:= [1,1,2,2]") "true")
+
+(er-lx-test "flatmap empty"
+  (er-lx-nm "lists:flatmap(fun(X) -> [X] end, []) =:= []") "true")
+
+;; ── lists:filtermap/2 ─────────────────────────────────────────────
+(er-lx-test "filtermap transform"
+  (er-lx-nm
+    "lists:filtermap(fun(X) -> case X rem 2 of 0 -> {true, X*10}; _ -> false end end, [1,2,3,4]) =:= [20,40]")
+  "true")
+
+(er-lx-test "filtermap bool keep"
+  (er-lx-nm "lists:filtermap(fun(X) -> X > 2 end, [1,2,3,4]) =:= [3,4]") "true")
+
+;; ── lists:mapfoldl/3 ──────────────────────────────────────────────
+(er-lx-test "mapfoldl map+acc"
+  (er-lx-nm
+    "lists:mapfoldl(fun(X,A) -> {X*2, A+X} end, 0, [1,2,3]) =:= {[2,4,6],6}") "true")
+
+(er-lx-test "mapfoldl empty"
+  (er-lx-nm "lists:mapfoldl(fun(X,A) -> {X,A} end, 5, []) =:= {[],5}") "true")
+
+;; ── lists:search/2 ────────────────────────────────────────────────
+(er-lx-test "search hit"
+  (er-lx-nm "lists:search(fun(X) -> X > 2 end, [1,2,3,4]) =:= {value,3}") "true")
+
+(er-lx-test "search miss"
+  (er-lx-nm "lists:search(fun(X) -> X > 9 end, [1,2,3])") "false")

plans/erlang-on-sx.md

@@ -159,6 +159,8 @@ The Phase 9 opcodes are registered, tested, and bridged SX↔OCaml, but inert: n
 
 _Newest first._
 
+- **2026-06-30 stdlib hardening — `lists` flatmap/filtermap/mapfoldl/search** — Added `flatmap/2`, `filtermap/2` (`true` keep / `false` drop / `{true, V}` transform), `mapfoldl/3` (returns `{MappedList, AccFinal}`), `search/2` (`{value, E}` | `false`) to `lib/erlang/lists-ext.sx`. `lists_ext` suite 83→**91** (+8). Conformance **854 → 862/862**. The `lists` module is now broadly covered (sort/usort/keylists/fold/partition/while/flatten/min/max/zip/slicing/flatmap/filtermap/mapfoldl/search on top of the originals). loops/erlang only.
+
 - **2026-06-30 stdlib hardening — `lists` slicing** — Added `sublist/2`, `sublist/3`, `nthtail/2`, `split/2`, `droplast/1` to `lib/erlang/lists-ext.sx`. `sublist` is lenient (clamps to list length); `nthtail/2` and `split/2` are strict (`badarg` when the list is shorter than N, matching the stdlib); `droplast/1` raises on `[]`. `lists_ext` suite 70→**83** (+13). Conformance **841 → 854/854**. loops/erlang only.
 
 - **2026-06-30 stdlib hardening — `lists` zip family** — Added `zip/2`, `zipwith/3`, `unzip/1` to `lib/erlang/lists-ext.sx`. Length mismatch (zip/zipwith) and malformed/non-pair elements (unzip) raise `badarg` (port equivalent of Erlang's `function_clause`). `lists_ext` suite 62→**70** (+8, incl. a zip/unzip roundtrip). Conformance **833 → 841/841**. loops/erlang only.