Pattern quasiquotation (!!!)

syndicate/pattern.rkt

@@ -136,6 +136,8 @@
       [(quasiquote p) #`(Pattern-DLit (DLit 'p))]
       [(quote p) #`(Pattern-DLit (DLit 'p))]
 
+      [(unquote p) #'p]
+
       [(ctor piece ...)
        (struct-info? (id-value #'ctor))
        (let-values (((label arity) (struct-label-and-arity #'ctor (length (syntax->list #'(piece ...))))))
@@ -165,6 +167,9 @@
        (dollar-id? #'id)
        #`(Pattern-DBind (DBind '#,(undollar #'id) (Pattern-DDiscard (DDiscard))))]
 
+      [($ (unquote bp) p)
+       #`(Pattern-DBind (DBind bp #,(analyse-pattern #'p)))]
+
       [($ id p)
        #`(Pattern-DBind (DBind 'id #,(analyse-pattern #'p)))]
 
@@ -247,10 +252,20 @@
 
       [other #'other])))
 
-(define-syntax (:pattern stx)
-  (syntax-case stx ()
-    [(_ pat-stx)
-     (analyse-pattern #'pat-stx)]))
+(define-pattern-expander :pattern
+  (lambda (stx)
+    (syntax-case stx ()
+      [(_ pat-stx)
+       ;; This effectively quasiquotes the pattern; a naked unquote in
+       ;; the pattern returns to the "outer" context. This is useful
+       ;; for observing observers:
+       ;;
+       ;;   (Observe (:pattern (some-pattern ...)) _)
+       (analyse-pattern #'pat-stx)]))
+  (lambda (stx)
+    (syntax-case stx ()
+      [(_ pat-stx)
+       (analyse-pattern #'pat-stx)])))
 
 ;; (provide :bindings)
 ;; (define-syntax (:bindings stx)