Clarify doc of scan-lists.

* doc/lispref/syntax.texi (Motion via Parsing): Doc fix for scan-lists.

* src/syntax.c (Fscan_lists): Doc fix.

Fixes: debbugs:10833

4 files changed, 36 insertions, 23 deletions

diff --git a/doc/lispref/ChangeLog b/doc/lispref/ChangeLog
index ab01544c453..85e78abdc32 100644
--- a/doc/lispref/ChangeLog
+++ b/doc/lispref/ChangeLog
@@ -1,3 +1,7 @@
+2012-02-17  Chong Yidong  <cyd@gnu.org>
+
+        * syntax.texi (Motion via Parsing): Doc fix for scan-lists.
+
 2012-02-17  Glenn Morris  <rgm@gnu.org>
 
         * hooks.texi (Standard Hooks): Fix cross-ref to Emacs manual.
diff --git a/doc/lispref/syntax.texi b/doc/lispref/syntax.texi
index dbfa4ca9045..e2fb3238642 100644
--- a/doc/lispref/syntax.texi
+++ b/doc/lispref/syntax.texi
@@ -646,25 +646,25 @@ result, Emacs treats them as four consecutive empty string constants.
 based on parsing expressions.
 
 @defun scan-lists from count depth
-This function scans forward @var{count} balanced parenthetical groupings
-from position @var{from}.  It returns the position where the scan stops.
-If @var{count} is negative, the scan moves backwards.
-
-If @var{depth} is nonzero, assume that the starting point is already
-@var{depth} parentheses deep.  This function counts out @var{count}
-number of points where the parenthesis depth goes back to zero, then
-stops.  Thus, a positive value for @var{depth} has the effect of
-moving out @var{depth} levels of parenthesis, whereas a negative
-@var{depth} has the effect of moving deeper by @var{-depth} levels of
-parenthesis.
+This function scans forward @var{count} balanced parenthetical
+groupings from position @var{from}.  It returns the position where the
+scan stops.  If @var{count} is negative, the scan moves backwards.
+
+If @var{depth} is nonzero, treat the starting position as being
+@var{depth} parentheses deep.  The scanner moves forward or backward
+through the buffer until the depth changes to zero @var{count} times.
+Hence, a positive value for @var{depth} has the effect of moving out
+@var{depth} levels of parenthesis from the starting position, while a
+negative @var{depth} has the effect of moving deeper by @var{-depth}
+levels of parenthesis.
 
 Scanning ignores comments if @code{parse-sexp-ignore-comments} is
 non-@code{nil}.
 
-If the scan reaches the beginning or end of the buffer (or its
-accessible portion), and the depth is not zero, an error is signaled.
-If the depth is zero but the count is not used up, @code{nil} is
-returned.
+If the scan reaches the beginning or end of the accessible part of the
+buffer before it has scanned over @var{count} parenthetical groupings,
+the return value is @code{nil} if the depth at that point is zero; if
+the depth is non-zero, a @code{scan-error} error is signaled.
 @end defun
 
 @defun scan-sexps from count
diff --git a/src/ChangeLog b/src/ChangeLog
index 12b4fa97c33..7ad28b0473d 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,7 @@
+2012-02-17  Chong Yidong  <cyd@gnu.org>
+
+        * syntax.c (Fscan_lists): Doc fix (Bug#10833).
+
 2012-02-15  Paul Eggert  <eggert@cs.ucla.edu>
 
         * image.c (MAX_IMAGE_SIZE): Increase from 6.0 to 10.0; see
diff --git a/src/syntax.c b/src/syntax.c
index bb473a52b3a..645a65f13e6 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -2844,18 +2844,23 @@ scan_lists (register EMACS_INT from, EMACS_INT count, EMACS_INT depth, int sexpf
 
 DEFUN ("scan-lists", Fscan_lists, Sscan_lists, 3, 3, 0,
        doc: /* Scan from character number FROM by COUNT lists.
-Returns the character number of the position thus found.
+Scan forward if COUNT is positive, backward if COUNT is negative.
+Return the character number of the position thus found.
+
+A \"list", in this context, refers to a balanced parenthetical
+grouping, as determined by the syntax table.
 
-If DEPTH is nonzero, paren depth begins counting from that value,
-only places where the depth in parentheses becomes zero
-are candidates for stopping; COUNT such places are counted.
-Thus, a positive value for DEPTH means go out levels.
+If DEPTH is nonzero, treat that as the nesting depth of the starting
+point (i.e. the starting point is DEPTH parentheses deep).  This
+function scans over parentheses until the depth goes to zero COUNT
+times.  Hence, positive DEPTH moves out that number of levels of
+paretheses, while negative DEPTH moves to a deeper level.
 
 Comments are ignored if `parse-sexp-ignore-comments' is non-nil.
 
-If the beginning or end of (the accessible part of) the buffer is reached
-and the depth is wrong, an error is signaled.
-If the depth is right but the count is not used up, nil is returned.  */)
+If we reach the beginning or end of the accessible part of the buffer
+before we have scanned over COUNT lists, return nil if the depth at
+that point is zero, and signal a error if the depth is nonzero.  */)
   (Lisp_Object from, Lisp_Object count, Lisp_Object depth)
 {
   CHECK_NUMBER (from);