erlang: lists sublist/2,3 + nthtail/2 + split/2 + droplast/1 (854/854)

Slicing family in lib/erlang/lists-ext.sx. sublist lenient; nthtail
and split strict (badarg when list shorter than N); droplast raises
on []. lists_ext suite 70 -> 83.

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

lib/erlang/lists-ext.sx

@@ -422,6 +422,68 @@
   (fn (vs)
     (er-ext-unzip (er-bif-arg1 vs "lists:unzip") (er-mk-nil) (er-mk-nil))))
 
+;; ── slicing (sublist / nthtail / split / droplast) ────────────────
+(define
+  er-ext-sublist2
+  (fn (lst n)
+    (cond
+      (or (<= n 0) (er-nil? lst)) (er-mk-nil)
+      (er-cons? lst)
+        (er-mk-cons (get lst :head) (er-ext-sublist2 (get lst :tail) (- n 1)))
+      :else (er-mk-nil))))
+
+;; lenient drop (used by sublist/3); never raises
+(define
+  er-ext-drop-cons
+  (fn (lst n)
+    (cond
+      (or (<= n 0) (er-nil? lst)) lst
+      (er-cons? lst) (er-ext-drop-cons (get lst :tail) (- n 1))
+      :else lst)))
+
+;; strict drop (used by nthtail/2 + split/2); raises if list too short
+(define
+  er-ext-nthtail
+  (fn (n lst)
+    (cond
+      (<= n 0) lst
+      (er-cons? lst) (er-ext-nthtail (- n 1) (get lst :tail))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-sublist
+  (fn (vs)
+    (cond
+      (= (len vs) 2) (er-ext-sublist2 (nth vs 0) (nth vs 1))
+      (= (len vs) 3)
+        (er-ext-sublist2
+          (er-ext-drop-cons (nth vs 0) (- (nth vs 1) 1))
+          (nth vs 2))
+      :else (error "Erlang: lists:sublist: wrong arity"))))
+
+(define
+  er-bif-lists-nthtail
+  (fn (vs) (er-ext-nthtail (nth vs 0) (nth vs 1))))
+
+(define
+  er-bif-lists-split
+  (fn (vs)
+    (let ((n (nth vs 0)) (lst (nth vs 1)))
+      (er-mk-tuple
+        (list (er-ext-sublist2 lst n) (er-ext-nthtail n lst))))))
+
+(define
+  er-ext-droplast
+  (fn (lst)
+    (cond
+      (and (er-cons? lst) (er-nil? (get lst :tail))) (er-mk-nil)
+      (er-cons? lst) (er-mk-cons (get lst :head) (er-ext-droplast (get lst :tail)))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-droplast
+  (fn (vs) (er-ext-droplast (er-bif-arg1 vs "lists:droplast"))))
+
 ;; ── register ──────────────────────────────────────────────────────
 ;; Hook into er-register-builtin-bifs! rather than registering once:
 ;; the registry can be reset + rebuilt mid-run (tests/runtime.sx does
@@ -449,7 +511,12 @@
     (er-register-pure-bif! "lists" "min" 1 er-bif-lists-min)
     (er-register-pure-bif! "lists" "zip" 2 er-bif-lists-zip)
     (er-register-pure-bif! "lists" "zipwith" 3 er-bif-lists-zipwith)
-    (er-register-pure-bif! "lists" "unzip" 1 er-bif-lists-unzip)))
+    (er-register-pure-bif! "lists" "unzip" 1 er-bif-lists-unzip)
+    (er-register-pure-bif! "lists" "sublist" 2 er-bif-lists-sublist)
+    (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)))
 
 (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": 841,
-  "total": 841,
+  "total_pass": 854,
+  "total": 854,
   "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":70,"total":70,"status":"ok"}
+    {"name":"lists_ext","pass":83,"total":83,"status":"ok"}
   ]
 }

lib/erlang/scoreboard.md

@@ -1,6 +1,6 @@
 # Erlang-on-SX Scoreboard
 
-**Total: 841 / 841 tests passing**
+**Total: 854 / 854 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -16,7 +16,7 @@
 | ✅ | ffi | 37 | 37 |
 | ✅ | vm | 78 | 78 |
 | ✅ | send_after | 10 | 10 |
-| ✅ | lists_ext | 70 | 70 |
+| ✅ | lists_ext | 83 | 83 |
 
 
 Generated by `lib/erlang/conformance.sh`.

lib/erlang/tests/lists_ext.sx

@@ -267,3 +267,46 @@
 
 (er-lx-test "zip/unzip roundtrip"
   (er-lx-nm "lists:unzip(lists:zip([1,2],[3,4])) =:= {[1,2],[3,4]}") "true")
+
+;; ── lists:sublist/2,3 ─────────────────────────────────────────────
+(er-lx-test "sublist/2 first n"
+  (er-lx-nm "lists:sublist([1,2,3,4,5],3) =:= [1,2,3]") "true")
+
+(er-lx-test "sublist/2 over length"
+  (er-lx-nm "lists:sublist([1,2],5) =:= [1,2]") "true")
+
+(er-lx-test "sublist/2 zero"
+  (er-lx-nm "lists:sublist([1,2,3],0) =:= []") "true")
+
+(er-lx-test "sublist/3 mid"
+  (er-lx-nm "lists:sublist([1,2,3,4,5],2,3) =:= [2,3,4]") "true")
+
+(er-lx-test "sublist/3 to end"
+  (er-lx-nm "lists:sublist([1,2,3],2,10) =:= [2,3]") "true")
+
+;; ── lists:nthtail/2 ───────────────────────────────────────────────
+(er-lx-test "nthtail mid"
+  (er-lx-nm "lists:nthtail(2,[1,2,3,4]) =:= [3,4]") "true")
+
+(er-lx-test "nthtail zero"
+  (er-lx-nm "lists:nthtail(0,[1,2]) =:= [1,2]") "true")
+
+(er-lx-test "nthtail full"
+  (er-lx-nm "lists:nthtail(3,[1,2,3]) =:= []") "true")
+
+;; ── lists:split/2 ─────────────────────────────────────────────────
+(er-lx-test "split mid"
+  (er-lx-nm "lists:split(2,[1,2,3,4,5]) =:= {[1,2],[3,4,5]}") "true")
+
+(er-lx-test "split zero"
+  (er-lx-nm "lists:split(0,[1,2]) =:= {[],[1,2]}") "true")
+
+(er-lx-test "split full"
+  (er-lx-nm "lists:split(3,[1,2,3]) =:= {[1,2,3],[]}") "true")
+
+;; ── lists:droplast/1 ──────────────────────────────────────────────
+(er-lx-test "droplast"
+  (er-lx-nm "lists:droplast([1,2,3]) =:= [1,2]") "true")
+
+(er-lx-test "droplast single"
+  (er-lx-nm "lists:droplast([9]) =:= []") "true")

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` 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.
 
 - **2026-06-30 stdlib hardening — `lists` flatten/max/min** — Added `flatten/1` (deep recursive flatten via `er-list-append`), `max/1`, `min/1` (full term order via `er-ext-lt?`, `badarg` on empty) to `lib/erlang/lists-ext.sx`. Gotcha caught: `er-ext-lt?` returns a raw SX boolean, so the extreme-finder uses it directly in `if` rather than wrapping in `er-truthy?` (which only recognises Erlang bool atoms, not SX booleans — the first cut wrapped it and silently never updated the running best). `lists_ext` suite 52→**62** (+10). Conformance **823 → 833/833**. loops/erlang only.