more lenient pass macro

quad2/attr.rkt

@@ -39,8 +39,7 @@
                     ;; otherwise treat return value as new attr value
                     [new-av (hash-set! attrs ak new-av)]))
              (set-add! attrs-seen attrs))
-           (loop (quad-elems q)))))
-  qs)
+           (loop (quad-elems q))))))
 
 (define-pass (upgrade-attr-keys qs)
   ;; convert attr keys from symbols to attr struct types

quad2/pipeline.rkt

@@ -40,9 +40,15 @@
                   (when (current-use-preconditions?)
                     (unless (PRECOND-PROC ARG)
                       (raise-argument-error 'PASS-NAME (format "~a (as precondition)" 'PRECOND-PROC) ARG)))
-                  (define res (let () EXPRS ...))
-                  (when (current-use-postconditions?)
-                  (unless (POSTCOND-PROC res)
-                    (raise-argument-error 'PASS-NAME (format "~a (as postcondition)" 'POSTCOND-PROC) res)))
-                  res))
+                  ;; a pass can be functional or mutational.
+                  ;; if it returns void, assume mutational
+                  ;; and return the input item.
+                  (define res (match (let () EXPRS ...)
+                                [(? void?) ARG]
+                                [val val]))
+                  (begin0
+                    res
+                    (when (current-use-postconditions?)
+                      (unless (POSTCOND-PROC res)
+                        (raise-argument-error 'PASS-NAME (format "~a (as postcondition)" 'POSTCOND-PROC) res))))))
             'PASS-NAME))))]))