Added try-order-point< and try-order-point>

main.rkt

@@ -24,6 +24,8 @@
          ~order-point
          order-point<
          order-point>
+         try-order-point<
+         try-order-point>
          ~mixin
          ~post-check
          ~post-fail
@@ -38,4 +40,5 @@
          aggregate-global-counter
          (for-template define-syntax/parse+simple)
          define/syntax-parse+simple
-         try-attribute)
+         try-attribute
+         if-attribute)

private/no-order.rkt

@@ -28,7 +28,8 @@
                      phc-toolkit/untyped
                      racket/list
                      racket/pretty)
-         "parameters.rkt")
+         "parameters.rkt"
+         "try-attribute.rkt")
 
 (provide define-eh-alternative-mixin
          ~seq-no-order
@@ -36,6 +37,8 @@
          ~order-point
          order-point<
          order-point>
+         try-order-point<
+         try-order-point>
          (expander-out eh-mixin))
 
 (define-expander-type eh-mixin)
@@ -160,3 +163,9 @@
 (define-syntax-rule (order-point> a b)
   (and (attribute a) (attribute b)
        (> (attribute a) (attribute b))))
+
+(define-syntax-rule (try-order-point< a b)
+  (if-attribute a (if-attribute b (order-point< a b) #f) #f))
+
+(define-syntax-rule (try-order-point> a b)
+  (if-attribute a (if-attribute b (order-point> a b) #f) #f))

scribblings/extensible-parser-specifications.scrbl

@@ -136,21 +136,61 @@
          #:grammar
          [(a #,tribute-name)
           (b #,tribute-name)]]{
-                          
  Returns @racket[#t] when the first element matched by
  @racket[(~order-point a #,ntax-pattern ...)] occurs before the first element
  matched by @racket[(~order-point b #,ntax-pattern ...)]. Otherwise, returns
- @racket[#f].}
+ @racket[#f].
+
+ This operation does not fail if @racket[a] or @racket[b] are bound to
+ @racket[#f] (i.e. their corresponding @racket[_syntax-pattern ...] did not
+ match). Instead, in both cases, it returns @racket[#f].}
 
 @defform[(order-point> a b)
          #:grammar
          [(a #,tribute-name)
           (b #,tribute-name)]]{
-                          
  Returns @racket[#t] when the first element matched by
  @racket[(~order-point a #,ntax-pattern ...)] occurs after the first element
  matched by @racket[(~order-point b #,ntax-pattern ...)]. Otherwise, returns
- @racket[#f].}
+ @racket[#f].
+
+ This operation does not fail if @racket[a] or @racket[b] are bound to
+ @racket[#f] (i.e. their corresponding @racket[_syntax-pattern ...] did not
+ match). Instead, in both cases, it returns @racket[#f].}
+
+@defform[(try-order-point< a b)
+         #:grammar
+         [(a #,tribute-name)
+          (b #,tribute-name)]]{
+
+ Like @racket[order-point<], except that it does not fail if @racket[a] or
+ @racket[b] are not attributes, or if they are bound to @racket[#f]. Instead, in
+ all those cases, it returns @racket[#f].
+
+ It can be used as follows:
+
+ @racketblock[
+ (~post-fail "a must appear after b"
+             #:when (try-order-point< a b))]
+
+ The same caveats as for @racket[try-attribute] apply.}
+
+@defform[(try-order-point> a b)
+         #:grammar
+         [(a #,tribute-name)
+          (b #,tribute-name)]]{
+
+ Like @racket[order-point>], except that it does not fail if @racket[a] or
+ @racket[b] are not attributes, or if they are bound to @racket[#f]. Instead, in
+ all those cases, it returns @racket[#f].
+
+ It can be used as follows:
+
+ @racketblock[
+ (~post-fail "a must appear before b"
+             #:when (try-order-point> a b))]
+
+ The same caveats as for @racket[try-attribute] apply.}
 
 @defform[(~mixin #,-alternative-mixin)]{