From 16290d0fcc36f1f0d084c4d4987524c80886506b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Georges=20Dup=C3=A9ron?= <georges.duperon@gmail.com>
Date: Mon, 19 Sep 2016 21:45:20 +0200
Subject: [PATCH] Added if-attribute

---
 private/try-attribute.rkt                         | 15 +++++++++++----
 .../extensible-parser-specifications.scrbl        |  6 ++++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/private/try-attribute.rkt b/private/try-attribute.rkt
index 81c0277..cdc3ad9 100644
--- a/private/try-attribute.rkt
+++ b/private/try-attribute.rkt
@@ -2,11 +2,18 @@
 
 (require (for-syntax racket/base))
 
-(provide try-attribute)
+(provide try-attribute if-attribute)
+
+(define-syntax (if-attribute stx)
+  (syntax-case stx ()
+    [(_ name if-branch else-branch)
+     (if (syntax-pattern-variable? (syntax-local-value #'name (λ () #f)))
+         #'if-branch
+         #'else-branch)]))
 
 (define-syntax (try-attribute stx)
   (syntax-case stx ()
+    [(_ name fallback)
+     #'(if-attribute (attribute name) fallback)]
     [(_ name)
-     (if (syntax-pattern-variable? (syntax-local-value #'name (λ () #f)))
-         #'(attribute name)
-         #'#f)]))
\ No newline at end of file
+     #'(if-attribute (attribute name) #f)]))
\ No newline at end of file
diff --git a/scribblings/extensible-parser-specifications.scrbl b/scribblings/extensible-parser-specifications.scrbl
index 5008599..b41d32b 100644
--- a/scribblings/extensible-parser-specifications.scrbl
+++ b/scribblings/extensible-parser-specifications.scrbl
@@ -352,6 +352,12 @@ appear in the matched syntax (aside from issues related to greediness).
  declared, while using the operation's neutral element to not alter the final
  result.}
 
+@defform[(if-attribute #,tribute-name if-branch else-branch)]{
+ This macro expands to @racket[if-branch] if @racket[#,tribute-name] is bound as
+ a syntax pattern variable, and to @racket[else-branch] otherwise.
+
+ The same caveats as for @racket[try-attribute] apply.}
+
 @defform[(~named-seq #,tribute-name #,ntax-pattern ...)]{
  Equivalent to @racket[{~seq #,ntax-pattern ...}], but also binds the
  @racket[#,tribute-name] to the whole sequence. If the sequence appears inside