commit 5289907fd901e7164054555e538f10e5e5640a95
parent 9d7614099111918bb7007a3d5b8d1bf2a1e6cd6e
Author: Greg Hendershott <greghendershott@gmail.com>
Date: Mon, 12 Nov 2012 23:15:00 -0500
Add section about with-syntax*
Also point out gotcha re mysterious error message if
we forget to require racket/syntax.
Diffstat:
| M | main.rkt | | | 57 | ++++++++++++++++++++++++++++++++++++++++++++++++++++----- |
1 file changed, 52 insertions(+), 5 deletions(-)
diff --git a/main.rkt b/main.rkt
@@ -839,9 +839,9 @@ Instead we want the datum in the syntax objects, such as the symbols
And now it works!
-@subsubsection{@racket[with-syntax]}
+Next, some shortcuts.
-Now for two shortcuts.
+@subsubsection{@racket[with-syntax]}
Instead of an additional, nested @racket[syntax-case], we could use
@racket[with-syntax]@margin-note*{Another name for
@@ -869,12 +869,59 @@ Whether you use an additional @racket[syntax-case] or use
pattern variable. Don't let the terminology and structure make it seem
mysterious.
+@subsubsection{@racket[with-syntax*]}
+
+We may recall that @racket[let] doesn't let us use a definition in a
+subsequent clause:
+
+@i[
+(let ([a 0]
+ [b a])
+ (values a b))
+]
+
+We could nest @racket[let]s:
+
+@i[
+(let ([a 0])
+ (let ([b a])
+ (values a b)))
+]
+
+Or we could use @racket[let*]:
+
+@i[
+(let* ([a 0]
+ [b 0])
+ (values a b))
+]
+
+Similarly there is a @racket[with-syntax*] variation of
+@racket[with-syntax]:
+
+@i[
+(require (for-syntax racket/syntax))
+(define-syntax (foo stx)
+ (syntax-case stx ()
+ [(_ a)
+ (with-syntax* ([b #'a]
+ [c #'b])
+ #'c)]))
+]
+
+One gotcha is that @racket[with-syntax*] isn't provided by
+@racket[racket/base]. We must @racket[(require (for-syntax
+racket/syntax))]. Otherwise we may get a rather bewildering error
+message:
+
+@italic{@tt{...: ellipses not allowed as an expression in: ...}}.
+
+
@subsubsection{@racket[format-id]}
-Also, there is a utility function in @racket[racket/syntax] called
+There is a utility function in @racket[racket/syntax] called
@racket[format-id] that lets us format identifier names more
-succinctly. As we've learned, we need to @racket[require] the module
-using @racket[for-syntax], since we need it at compile time:
+succinctly than what we did above:
@i[
(require (for-syntax racket/syntax))
