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-rw-r--r--doc/ref/api-languages.texi22
-rw-r--r--libguile/boolean.c13
2 files changed, 31 insertions, 4 deletions
diff --git a/doc/ref/api-languages.texi b/doc/ref/api-languages.texi
index 839e6eae2..763e79859 100644
--- a/doc/ref/api-languages.texi
+++ b/doc/ref/api-languages.texi
@@ -1,7 +1,7 @@
@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
-@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2010
-@c Free Software Foundation, Inc.
+@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2010, 2016,
+@c 2017, 2018 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.
@node Other Languages
@@ -108,6 +108,24 @@ code to maintain their current semantics. @code{nil}, which in Elisp
would just be written and read as @code{nil}, in Scheme has the external
representation @code{#nil}.
+In Elisp code, @code{#nil}, @code{#f}, and @code{'()} behave like
+@code{nil}, in the sense that they are all interpreted as @code{nil} by
+Elisp @code{if}, @code{cond}, @code{when}, @code{not}, @code{null}, etc.
+To test whether Elisp would interpret an object as @code{nil} from
+within Scheme code, use @code{nil?}:
+
+@deffn {Scheme Procedure} nil? obj
+Return @code{#t} if @var{obj} would be interpreted as @code{nil} by
+Emacs Lisp code, else return @code{#f}.
+
+@lisp
+(nil? #nil) @result{} #t
+(nil? #f) @result{} #t
+(nil? '()) @result{} #t
+(nil? 3) @result{} #f
+@end lisp
+@end deffn
+
This decision to have @code{nil} as a low-level distinct value
facilitates interoperability between the two languages. Guile has chosen
to have Scheme deal with @code{nil} as follows:
diff --git a/libguile/boolean.c b/libguile/boolean.c
index f8c773839..635c14905 100644
--- a/libguile/boolean.c
+++ b/libguile/boolean.c
@@ -1,4 +1,5 @@
-/* Copyright (C) 1995, 1996, 2000, 2001, 2006, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
+/* Copyright (C) 1995, 1996, 2000, 2001, 2006, 2008-2011, 2018
+ * Free Software Foundation, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public License
@@ -64,7 +65,15 @@ SCM_DEFINE (scm_not, "not", 1, 0, 0,
SCM_DEFINE (scm_nil_p, "nil?", 1, 0, 0,
(SCM x),
- "Return @code{#t} iff @var{x} is nil, else return @code{#f}.")
+ "Return @code{#t} if @var{x} would be interpreted as @code{nil}\n"
+ "by Emacs Lisp code, else return @code{#f}.\n"
+ "\n"
+ "@example\n"
+ "(nil? #nil) @result{} #t\n"
+ "(nil? #f) @result{} #t\n"
+ "(nil? '()) @result{} #t\n"
+ "(nil? 3) @result{} #f\n"
+ "@end example")
#define FUNC_NAME s_scm_nil_p
{
return scm_from_bool (scm_is_lisp_false (x));