erlang: list comprehensions (+12 tests)

lib/erlang/parser-expr.sx

@@ -281,12 +281,56 @@
   (fn
     (st)
     (er-expect! st "punct" "[")
-    (if
+    (cond
       (er-is? st "punct" "]")
       (do (er-advance! st) {:type "nil"})
-      (let
-        ((elems (list (er-parse-expr-prec st 0))))
-        (er-parse-list-tail st elems)))))
+      :else (let
+        ((first (er-parse-expr-prec st 0)))
+        (cond
+          (er-is? st "punct" "||") (er-parse-list-comp st first)
+          :else (er-parse-list-tail st (list first)))))))
+
+(define
+  er-parse-list-comp
+  (fn
+    (st head)
+    (er-advance! st)
+    (let
+      ((quals (list (er-parse-lc-qualifier st))))
+      (er-parse-list-comp-tail st head quals))))
+
+(define
+  er-parse-list-comp-tail
+  (fn
+    (st head quals)
+    (cond
+      (er-is? st "punct" ",")
+      (do
+        (er-advance! st)
+        (append! quals (er-parse-lc-qualifier st))
+        (er-parse-list-comp-tail st head quals))
+      (er-is? st "punct" "]")
+      (do (er-advance! st) {:head head :qualifiers quals :type "lc"})
+      :else (error
+        (str
+          "Erlang parse: expected ',' or ']' in list comprehension, got '"
+          (er-cur-value st)
+          "'")))))
+
+(define
+  er-parse-lc-qualifier
+  (fn
+    (st)
+    (let
+      ((e (er-parse-expr-prec st 0)))
+      (cond
+        (er-is? st "punct" "<-")
+        (do
+          (er-advance! st)
+          (let
+            ((source (er-parse-expr-prec st 0)))
+            {:kind "gen" :pattern e :source source}))
+        :else {:kind "filter" :expr e}))))
 
 (define
   er-parse-list-tail

lib/erlang/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "language": "erlang",
-  "total_pass": 444,
-  "total": 444,
+  "total_pass": 456,
+  "total": 456,
   "suites": [
     {"name":"tokenize","pass":62,"total":62,"status":"ok"},
     {"name":"parse","pass":52,"total":52,"status":"ok"},
-    {"name":"eval","pass":260,"total":260,"status":"ok"},
+    {"name":"eval","pass":272,"total":272,"status":"ok"},
     {"name":"runtime","pass":39,"total":39,"status":"ok"},
     {"name":"ring","pass":4,"total":4,"status":"ok"},
     {"name":"ping-pong","pass":4,"total":4,"status":"ok"},

lib/erlang/scoreboard.md

@@ -1,12 +1,12 @@
 # Erlang-on-SX Scoreboard
 
-**Total: 444 / 444 tests passing**
+**Total: 456 / 456 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | tokenize | 62 | 62 |
 | ✅ | parse | 52 | 52 |
-| ✅ | eval | 260 | 260 |
+| ✅ | eval | 272 | 272 |
 | ✅ | runtime | 39 | 39 |
 | ✅ | ring | 4 | 4 |
 | ✅ | ping-pong | 4 | 4 |

lib/erlang/tests/eval.sx

@@ -902,6 +902,41 @@
       (nm (if (er-atom? reason) reason (nth (get reason :elements) 0)))))
   "badarg")
 
+;; ── list comprehensions ───────────────────────────────────────
+(er-eval-test "lc map double"
+  (ev "hd([X * 2 || X <- [1, 2, 3]])") 2)
+(er-eval-test "lc map sum"
+  (ev "lists:foldl(fun (X, Acc) -> X + Acc end, 0, [X * 2 || X <- [1, 2, 3]])")
+  12)
+(er-eval-test "lc length"
+  (ev "length([X || X <- [1, 2, 3, 4, 5]])") 5)
+(er-eval-test "lc filter sum"
+  (ev "lists:foldl(fun (X, Acc) -> X + Acc end, 0, [X || X <- [1, 2, 3, 4, 5], X rem 2 =:= 0])")
+  6)
+(er-eval-test "lc filter only"
+  (ev "length([X || X <- [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], X > 5])")
+  5)
+(er-eval-test "lc empty source"
+  (get (ev "[X || X <- []]") :tag) "nil")
+(er-eval-test "lc all filtered"
+  (get (ev "[X || X <- [1, 2, 3], X > 100]") :tag) "nil")
+(er-eval-test "lc cartesian length"
+  (ev "length([{X, Y} || X <- [1, 2, 3], Y <- [a, b]])")
+  6)
+(er-eval-test "lc pattern match"
+  (ev "lists:foldl(fun (X, Acc) -> X + Acc end, 0, [V || {ok, V} <- [{ok, 1}, {error, x}, {ok, 2}, {ok, 3}]])")
+  6)
+(er-eval-test "lc nested generators"
+  (ev "length([{X, Y} || X <- [1, 2, 3], Y <- [10, 20, 30], X + Y > 12])")
+  7)
+(er-eval-test "lc squares"
+  (ev "lists:foldl(fun (X, Acc) -> X + Acc end, 0, [X*X || X <- [1, 2, 3, 4, 5]])")
+  55)
+;; First {ok, X} tuple: head of [{ok,a}, {ok,b}] is {ok, a}.
+(er-eval-test "lc tuple capture"
+  (nm (nth (get (get (ev "[{ok, X} || X <- [a, b]]") :head) :elements) 0))
+  "ok")
+
 (define
   er-eval-test-summary
   (str "eval " er-eval-test-pass "/" er-eval-test-count))

lib/erlang/transpile.sx

@@ -123,6 +123,7 @@
         (= ty "send") (er-eval-send node env)
         (= ty "receive") (er-eval-receive node env)
         (= ty "try") (er-eval-try node env)
+        (= ty "lc") (er-eval-lc node env)
         (= ty "match") (er-eval-match node env)
         :else (error (str "Erlang eval: unsupported node type '" ty "'"))))))
 
@@ -1281,3 +1282,84 @@
           (do
             (er-env-restore! env snap)
             (er-eval-of-clauses clauses subject env (+ i 1))))))))
+
+;; ── list comprehensions ─────────────────────────────────────────
+;; `[E || Pat <- Source, FilterExpr, ...]`. Walk qualifiers in order:
+;; generators iterate their source list and bind the pattern (with
+;; env snapshot/restore so each iteration starts from the same
+;; baseline); filters skip when falsy. At the end of the qualifier
+;; chain, evaluate `head` and append to the accumulator. Build the
+;; final cons chain in O(n) with a single right-fold.
+(define
+  er-eval-lc
+  (fn
+    (node env)
+    (let
+      ((acc (list)))
+      (er-lc-walk (get node :qualifiers) 0 (get node :head) env acc)
+      (er-list-from-sx-list acc))))
+
+(define
+  er-lc-walk
+  (fn
+    (quals i head env acc)
+    (if
+      (>= i (len quals))
+      (append! acc (er-eval-expr head env))
+      (let
+        ((q (nth quals i)))
+        (cond
+          (= (get q :kind) "gen")
+          (let
+            ((src (er-eval-expr (get q :source) env)))
+            (er-lc-iter-gen
+              src
+              (get q :pattern)
+              quals
+              i
+              head
+              env
+              acc))
+          (= (get q :kind) "filter")
+          (when
+            (er-truthy? (er-eval-expr (get q :expr) env))
+            (er-lc-walk quals (+ i 1) head env acc))
+          :else (error "Erlang LC: unknown qualifier"))))))
+
+(define
+  er-lc-iter-gen
+  (fn
+    (src pat quals i head env acc)
+    (cond
+      (er-nil? src) nil
+      (er-cons? src)
+      (let
+        ((snap (er-env-copy env)))
+        (when
+          (er-match! pat (get src :head) env)
+          (er-lc-walk quals (+ i 1) head env acc))
+        (er-env-restore! env snap)
+        (er-lc-iter-gen
+          (get src :tail)
+          pat
+          quals
+          i
+          head
+          env
+          acc))
+      :else (error "Erlang LC: generator source is not a list"))))
+
+(define
+  er-list-from-sx-list
+  (fn
+    (xs)
+    (let
+      ((acc (list (er-mk-nil))))
+      (for-each
+        (fn
+          (i)
+          (let
+            ((j (- (- (len xs) 1) i)))
+            (set-nth! acc 0 (er-mk-cons (nth xs j) (nth acc 0)))))
+        (range 0 (len xs)))
+      (nth acc 0))))