add define-once

* module/ice-9/boot-9.scm (define-once): New syntax.

* doc/ref/api-binding.texi (Top Level):
* NEWS: Add notes about define-once.

NEWS

@@ -10,6 +10,11 @@ latest prerelease, and a full NEWS corresponding to 1.8 -> 2.0.
 
 Changes since the 1.9.15 prerelease:
 
+** New syntax: define-once
+
+`define-once' is like Lisp's `defvar': it creates a toplevel binding,
+but only if one does not exist already.
+
 ** Improved exactness handling for complex number parsing
 
 When parsing non-real complex numbers, exactness specifiers are now

doc/ref/api-binding.texi

@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010
+@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2011
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -88,6 +88,23 @@ longer visible.
 Attention: Scheme definitions inside local binding constructs
 (@pxref{Local Bindings}) act differently (@pxref{Internal Definitions}).
 
+Many people end up in a development style of adding and changing
+definitions at runtime, building out their program without restarting
+it.  (You can do this using @code{reload-module}, the @code{reload} REPL
+command, the @code{load} procedure, or even just pasting code into a
+REPL.)  If you are one of these people, you will find that sometimes you
+there are some variables that you @emph{don't} want to redefine all the
+time.  For these, use @code{define-once}.
+
+@fnindex defvar
+@deffn {Scheme Syntax} define-once name value
+Create a top level variable named @var{name} with value @var{value}, but
+only if @var{name} is not already bound in the current module.
+@end deffn
+
+Old Lispers probably know @code{define-once} under its Lisp name,
+@code{defvar}.
+
 
 @node Local Bindings
 @subsection Local Variable Bindings

module/ice-9/boot-9.scm

@@ -473,6 +473,11 @@ If there is no handler at all, Guile prints an error and then exits."
        (with-syntax ((s (datum->syntax x (syntax-source x))))
          #''s)))))
 
+(define-syntax define-once
+  (syntax-rules ()
+    ((_ sym val)
+     (define sym
+       (if (module-locally-bound? (current-module) 'sym) sym val)))))