Conditional assertion

syndicate/syntax-classes.rkt

@@ -2,13 +2,18 @@
 ;;; SPDX-License-Identifier: LGPL-3.0-or-later
 ;;; SPDX-FileCopyrightText: Copyright © 2021 Tony Garnock-Jones <tonyg@leastfixedpoint.com>
 
-(provide (for-syntax <name>
+(provide (for-syntax <when>
+                     <name>
                      <daemon?>))
 
 (require (for-syntax racket/base))
 (require (for-syntax syntax/parse))
 
 (begin-for-syntax
+  (define-splicing-syntax-class <when>
+    (pattern (~seq #:when E))
+    (pattern (~seq) #:attr E #'#t))
+
   (define-splicing-syntax-class <name>
     (pattern (~seq #:name N))
     (pattern (~seq) #:attr N #''?))

syndicate/syntax.rkt

@@ -154,9 +154,15 @@
                      item-stx])))))]))
 
 (define-event-expander assert
-  (syntax-rules ()
-    [(_ expr)
-     (turn-assert/dataflow! this-turn this-target (action () (->preserve expr)))]))
+  (lambda (stx)
+    (syntax-parse stx
+      [(_ condition:<when> expr)
+       #`(turn-assert/dataflow! this-turn
+                                this-target
+                                (action ()
+                                  (if condition.E
+                                      (->preserve expr)
+                                      (void))))])))
 
 (define-event-expander stop-when
   (syntax-rules ()