erlang: list comprehensions (+12 tests)

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))))