Re: Enhancement request: enabling Variant in C parsers

[Akim Demaille]

Hi!

> Le 19 août 2018 à 18:21, Rici Lake <address@hidden> a écrit :
> 
> Vaguely on this topic, since you seem to be working on a new release:

Yup.

> On Sun, Aug 19, 2018, 10:23 Akim Demaille <address@hidden> wrote:
> 
>> 
>> Note that you don’t have anything to do to reset the parser, all the
>> problem is actually the scanner.  The parser object holds no state,
>> everything is local to yyparse.
>> 
> 
> That's not the case for a push parser (which unfortunately is not an option
> in C++ or Java.)

I agree, push parsers are really nice.  It’s too bad history
has put so much focus on pull.

If you want to submit push parsers for C++/Java, be sure to
expect enthousiasme on my side :)

> As it happens, that's my preferred API, and at some point
> I needed to convince myself that it was unnecessary to reset the parser
> state "object".
> 
> Since this fact is not documented, it is presumably unspecified whether any
> guarantee will be maintained. I think it's a useful guarantee, and since it
> is already implemented it might as well be documented, by adding something
> like the following paragraph to the end of section 4.2:
> 
>> After yypush_parse returns a status other than YYPUSH_MORE, the parser
> instance yyps may be reused for a new parse.
> 
> I guess a similar guarantee could be provided in the documentation for
> yypull_parse (without mention of YYPUSH_MORE), since that is just a thin
> wrapper around yypush_parse.

See below.

> ---
> 
> The fact that the parser state is reusable even after an error simplifies
> reuse. For example, a calculator application which parses each input line
> as an expression can just keep reusing the same pstate even if an input was
> invalid. (The scanner will need to be reset but that's not bison's problem.)

Too good to be left in the mailing list archive.  WDYT?


commit 7ff6b7bbcab88ce667d97ae37171a9cfc9ec8d66
Author: Akim Demaille <address@hidden>
Date:   Sun Aug 19 20:53:59 2018 +0200

    doc: clarify that the push parser object can be reused
    
    Suggested by Rici Lake.
    https://lists.gnu.org/archive/html/bug-bison/2018-08/msg00033.html
    
    * doc/bison.texi: Complete description of the first node in the main
    @menu.
    (Push Decl): Remove the 'experimental' warnings about push parser.
    Clarify that the push parser object can be reused in several parses.

diff --git a/doc/bison.texi b/doc/bison.texi
index b463588b..9eff2886 100644
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -86,8 +86,8 @@ Cover art by Etienne Suvasa.
 @end ifnottex
 
 @menu
-* Introduction::
-* Conditions::
+* Introduction::        What GNU Bison is.
+* Conditions::          Conditions for using Bison and its output.
 * Copying::             The GNU General Public License says
                           how you can copy and share Bison.
 
@@ -5327,9 +5327,6 @@ valid grammar.
 @cindex push parser
 @findex %define api.push-pull
 
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
-
 A pull parser is called once and it takes control until all its input
 is completely parsed.  A push parser, on the other hand, is called
 each time a new token is made available.
@@ -5907,8 +5904,6 @@ the @code{full} value was introduced in Bison 2.7
 
 @item Purpose: Request a pull parser, a push parser, or both.
 @xref{Push Decl, ,A Push Parser}.
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
 
 @item Accepted Values: @code{pull}, @code{push}, @code{both}
 
@@ -6588,27 +6583,29 @@ int  yyparse (int *randomness);
 @section The Push Parser Function @code{yypush_parse}
 @findex yypush_parse
 
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
-
 You call the function @code{yypush_parse} to parse a single token.  This
 function is available if either the @samp{%define api.push-pull push} or
 @samp{%define api.push-pull both} declaration is used.
 @xref{Push Decl, ,A Push Parser}.
 
 @deftypefun int yypush_parse (yypstate address@hidden)
-The value returned by @code{yypush_parse} is the same as for yyparse with
-the following exception: it returns @code{YYPUSH_MORE} if more input is
+The value returned by @code{yypush_parse} is the same as for @code{yyparse}
+with the following exception: it returns @code{YYPUSH_MORE} if more input is
 required to finish parsing the grammar.
+
+After @code{yypush_parse} returns a status other than @code{YYPUSH_MORE},
+the parser instance @code{yyps} may be reused for a new parse.
 @end deftypefun
 
+The fact that the parser state is reusable even after an error simplifies
+reuse.  For example, a calculator application which parses each input line
+as an expression can just keep reusing the same @code{yyps} even if an input
+was invalid.
+
 @node Pull Parser Function
 @section The Pull Parser Function @code{yypull_parse}
 @findex yypull_parse
 
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
-
 You call the function @code{yypull_parse} to parse the rest of the input
 stream.  This function is available if the @samp{%define api.push-pull both}
 declaration is used.
@@ -6616,15 +6613,14 @@ declaration is used.
 
 @deftypefun int yypull_parse (yypstate address@hidden)
 The value returned by @code{yypull_parse} is the same as for @code{yyparse}.
+
+The parser instance @code{yyps} may be reused for new parses.
 @end deftypefun
 
 @node Parser Create Function
 @section The Parser Create Function @code{yystate_new}
 @findex yypstate_new
 
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
-
 You call the function @code{yypstate_new} to create a new parser instance.
 This function is available if either the @samp{%define api.push-pull push} or
 @samp{%define api.push-pull both} declaration is used.
@@ -6641,9 +6637,6 @@ allocated.
 @section The Parser Delete Function @code{yystate_delete}
 @findex yypstate_delete
 
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
-
 You call the function @code{yypstate_delete} to delete a parser instance.
 function is available if either the @samp{%define api.push-pull push} or
 @samp{%define api.push-pull both} declaration is used.
@@ -13187,8 +13180,6 @@ The function to delete a parser instance, produced by 
Bison in push mode;
 call this function to delete the memory associated with a parser.
 @xref{Parser Delete Function, ,The Parser Delete Function
 @code{yypstate_delete}}.  Does nothing when called with a null pointer.
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
 @end deffn
 
 @deffn {Function} yypstate_new
@@ -13196,8 +13187,6 @@ The function to create a parser instance, produced by 
Bison in push mode;
 call this function to create a new parser.
 @xref{Parser Create Function, ,The Parser Create Function
 @code{yypstate_new}}.
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
 @end deffn
 
 @deffn {Function} yypull_parse
@@ -13205,16 +13194,12 @@ The parser function produced by Bison in push mode; 
call this function to
 parse the rest of the input stream.
 @xref{Pull Parser Function, ,The Pull Parser Function
 @code{yypull_parse}}.
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
 @end deffn
 
 @deffn {Function} yypush_parse
 The parser function produced by Bison in push mode; call this function to
 parse a single token.  @xref{Push Parser Function, ,The Push Parser Function
 @code{yypush_parse}}.
-(The current push parsing interface is experimental and may evolve.
-More user feedback will help to stabilize it.)
 @end deffn
 
 @deffn {Macro} YYRECOVERING