make gosub a continuation

beautiful-racket/br/demo/basic/expander.rkt

@@ -13,8 +13,8 @@
     (remove-duplicates (map syntax->datum (filter (λ(s) (syntax-property s 'id)) (syntax-flatten stx))) eq?)))
 
 (define-macro (basic-module-begin (basic-program PROGRAM-LINE ...))
-  (with-pattern
-   ([(UNIQUE-ID ...) (map (compose1 syntax-local-introduce (λ(id) (datum->syntax #f id)))
+  (with-pattern ([(UNIQUE-ID ...)
+                  (map (compose1 syntax-local-introduce (λ(id) (datum->syntax #f id)))
                        (gather-unique-ids #'(PROGRAM-LINE ...)))])
     #'(#%module-begin
        (define UNIQUE-ID 0) ...
@@ -63,23 +63,15 @@
 
 (define return-stack empty)
 
-(define (do-gosub this-line where)
-  (if (or (empty? return-stack)
-          (not (= this-line (car return-stack))))
-      (begin
-        (set! return-stack (cons this-line return-stack))
-        (basic:goto where))
-      ;; if (= number (car return-stack))
-      ;; then we reached this line by `return`, which means the end of a gosub
-      (set! return-stack (cdr return-stack))))
+(define (basic:gosub where)
+  (let/cc return-k
+    (set! return-stack (cons return-k return-stack))
+    (basic:goto where)))
 
 (struct $line (number thunk) #:transparent)
-(define-macro line
-  [(_ NUMBER (statement "gosub" WHERE))
-   #'($line NUMBER (λ () (do-gosub NUMBER WHERE)))]
-  [(_ NUMBER . STATEMENTS)
+(define-macro (line NUMBER . STATEMENTS)
    #'($line NUMBER (λ () (with-handlers ([end-line-signal? (λ _ #f)])
-                           . STATEMENTS)))])
+                           . STATEMENTS))))
 
 (define-macro statement
   [(statement ID "=" EXPR) #'(set! ID EXPR)]
@@ -157,7 +149,10 @@
 
 (define (basic:goto where) where)
 
-(define (basic:return) (car return-stack))
+(define (basic:return)
+  (define return-k (car return-stack))
+  (set! return-stack (cdr return-stack))
+  (return-k #f))
 
 (define (basic:stop) (basic:end))
 (define (basic:end) (raise-end-program-signal))

beautiful-racket/br/demo/basic/gosub.bas

@@ -1,9 +1,9 @@
 #lang br/demo/basic
 10 GOSUB 50
-15 PRINT "BOOM"
+15 PRINT "2 of 3"
 17 GOSUB 30
 20 END
-30 PRINT "YAY"
+30 PRINT "3 of 3"
 40 RETURN
-50 PRINT "50"
+50 PRINT "1 of 3"
 55 RETURN