lua: add lua-unwrap-final-return helper (for future use); keep top-level guard

lib/lua/transpile.sx

@@ -532,11 +532,48 @@
 
 (define lua-transpile (fn (src) (lua-tx (lua-parse src))))
 
+(define
+  lua-ret-raise?
+  (fn (x)
+    (and (= (type-of x) "list")
+         (= (len x) 2)
+         (= (first x) (make-symbol "raise"))
+         (= (type-of (nth x 1)) "list")
+         (= (len (nth x 1)) 3)
+         (= (first (nth x 1)) (make-symbol "list"))
+         (= (type-of (nth (nth x 1) 1)) "list")
+         (= (first (nth (nth x 1) 1)) (make-symbol "quote"))
+         (= (nth (nth (nth x 1) 1) 1) (make-symbol "lua-ret")))))
+
+(define
+  lua-ret-value
+  (fn (raise-form) (nth (nth raise-form 1) 2)))
+
+(define
+  lua-unwrap-final-return
+  (fn (sx)
+    (cond
+      ((lua-ret-raise? sx) (lua-ret-value sx))
+      ((and (= (type-of sx) "list") (> (len sx) 0) (= (first sx) (make-symbol "begin")))
+        (let ((items (rest sx)))
+          (cond
+            ((= (len items) 0) sx)
+            (else
+              (let ((last-item (nth items (- (len items) 1))))
+                (cond
+                  ((lua-ret-raise? last-item)
+                    (let ((val (lua-ret-value last-item))
+                          (prefix (lua-init-before items 0 (- (len items) 1))))
+                      (cons (make-symbol "begin") (append prefix (list val)))))
+                  (else sx)))))))
+      (else sx))))
+
 (define
   lua-eval-ast
   (fn (src)
     (let ((sx (lua-transpile src)))
-      (eval-expr (lua-tx-function-guard sx)))))
+      (let ((sx2 (lua-unwrap-final-return sx)))
+        (eval-expr (lua-tx-function-guard sx2))))))
 
 (define
   lua-tx-multi-args