This is wisent.info, produced by makeinfo version 4.3 from wisent.texi.

This manual documents the Wisent parser generator.

   Copyright (C) 2001, 2002, 2003, 2004 David Ponce

   Some texts are borrowed or adapted from the manual of Bison version
1.35.  The text in section entitled "Understanding the automaton" is
adapted from the section "Understanding Your Parser" in the manual of
Bison version 1.49.

   Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999,
2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.1 or any later version published by the Free Software
     Foundation; with the Invariant Sections being list their titles,
     with the Front-Cover Texts being list, and with the Back-Cover
     Texts being list.  A copy of the license is included in the
     section entitled "GNU Free Documentation License".
   
INFO-DIR-SECTION Emacs
START-INFO-DIR-ENTRY
* Semantic Wisent parser development: (wisent).
END-INFO-DIR-ENTRY

   This file documents Application Development with Semantic.
_Infrastructure for parser based text analysis in Emacs_

   Copyright (C) 2001, 2002, 2003, 2004 Eric M. Ludlam, David Ponce,
and Richard Y. Kim


File: wisent.info,  Node: Top,  Next: Wisent Overview,  Up: (dir)

Wisent Parser Development
*************************

Wisent (the European Bison ;-) is an Emacs Lisp implementation of the
GNU Compiler Compiler Bison.

This manual describes how to use Wisent to develop grammars for
programming languages, and how to use grammars to parse language source
in Emacs buffers.

It also describes how Wisent is used with the semantic tool set
described in the *Note Semantic Manual: (semantic)Top.

* Menu:

* Wisent Overview::
* Wisent Grammar::
* Wisent Parsing::
* Wisent Semantic::
* GNU Free Documentation License::
* Index::


File: wisent.info,  Node: Wisent Overview,  Next: Wisent Grammar,  Prev: Top,  Up: Top

Wisent Overview
***************

"Wisent" (the European Bison) is an implementation in Emacs Lisp of the
GNU Compiler Compiler Bison. Its code is a port of the C code of GNU
Bison 1.28 & 1.31.

For more details on the basic concepts for understanding Wisent, it is
worthwhile to read the *Note Bison Manual: Top.

Wisent can generate compilers compatible with the semantic tool set.
See the *Note Semantic Manual: (semantic)Top.

It benefits from these Bison features:

   * It uses a fast but not so space-efficient encoding for the parse
     tables, described in Corbett's PhD thesis from Berkeley:

          `Static Semantics in Compiler Error Recovery'
          June 1985, Report No. UCB/CSD 85/251.

   * For generating the lookahead sets, Wisent uses the well-known
     technique of F. DeRemer and A. Pennello they described in:

          `Efficient Construction of LALR(1) Lookahead Sets'
          October 1982, ACM TOPLS Vol 4 No 4.

   * Wisent resolves shift/reduce conflicts using operator precedence
     and associativity.

   * Parser error recovery is accomplished using rules which match the
     special token `error'.

Nevertheless there are some fundamental differences between Bison and
Wisent.

   * Wisent is intended to be used in Emacs.  It reads and produces
     Emacs Lisp data structures.  All the additional code used in
     grammars is Emacs Lisp code.

   * Contrary to Bison, Wisent does not generate a parser which combines
     Emacs Lisp code and grammar constructs.  They exist separately.
     Wisent reads the grammar from a Lisp data structure and then
     generates grammar constructs as tables.  Afterward, the derived
     tables can be included and byte-compiled in separate Emacs Lisp
     files, and be used at a later time by the Wisent's parser engine.

   * Wisent allows multiple start nonterminals and allows a call to the
     parsing function to be made for a particular start nonterminal.
     For example, this is particularly useful to parse a region of an
     Emacs buffer.  semantic heavily depends on the availability of
     this feature.


File: wisent.info,  Node: Wisent Grammar,  Next: Wisent Parsing,  Prev: Wisent Overview,  Up: Top

Wisent Grammar
**************

In order for Wisent to parse a language, it must be described by a
"context-free grammar".  That is a grammar specified as rules that can
be applied regardless of context.  For more information, see *Note
Language and Grammar: (bison)Language and Grammar, in the Bison manual.

The formal grammar is formulated using "terminal" and "nonterminal"
items.  Terminals can be Emacs Lisp symbols or characters, and
nonterminals are symbols only.

Terminals (also known as "tokens") represent the lexical elements of
the language like numbers, strings, etc..

For example `PLUS' can represent the operator `+'.

Nonterminal symbols are described by rules:

     RESULT == COMPONENTS...

`RESULT' is a nonterminal that this rule describes and `COMPONENTS' are
various terminals and nonterminals that are put together by this rule.

For example, this rule:

     exp == exp PLUS exp

Says that two groupings of type `exp', with a `PLUS' token in between,
can be combined into a larger grouping of type `exp'.

* Menu:

* Grammar format::
* Example::
* Compiling a grammar::
* Conflicts::


File: wisent.info,  Node: Grammar format,  Next: Example,  Prev: Wisent Grammar,  Up: Wisent Grammar

Grammar format
==============

To be acceptable by Wisent a context-free grammars must respect a
particular format.  That is, must be represented as an Emacs Lisp list
of the form:

`(TERMINALS ASSOCS . NON-TERMINALS)'

TERMINALS
     Is the list of terminal symbols used in the grammar.

ASSOCS
     Specify the associativity of TERMINALS.  It is `nil' when there is
     no associativity defined, or an alist of
     `(ASSOC-TYPE . ASSOC-VALUE)' elements.

     ASSOC-TYPE must be one of the `default-prec', `nonassoc', `left'
     or `right' symbols.  When ASSOC-TYPE is `default-prec',
     ASSOC-VALUE must be `nil' or `t' (the default).  Otherwise it is a
     list of tokens which must have been previously declared in
     TERMINALS.

     For details, see *Note Contextual Precedence: (bison)Contextual
     Precedence, in the Bison manual.

NON-TERMINALS
     Is the list of nonterminal definitions.  Each definition has the
     form:

     `(NONTERM . RULES)'

     Where NONTERM is the nonterminal symbol defined and RULES the list
     of rules that describe this nonterminal.  Each rule is a list:

     `(COMPONENTS [PRECEDENCE] [ACTION])'

     Where:

    COMPONENTS
          Is a list of various terminals and nonterminals that are put
          together by this rule.

          For example,

               (exp ((exp ?+ exp))          ;; exp: exp '+' exp
                    )                       ;;    ;

          Says that two groupings of type `exp', with a `+' token in
          between, can be combined into a larger grouping of type `exp'.

          By convention, a nonterminal symbol should be in lower case,
          such as `exp', `stmt' or `declaration'.  Terminal symbols
          should be upper case to distinguish them from nonterminals:
          for example, `INTEGER', `IDENTIFIER', `IF' or `RETURN'.  A
          terminal symbol that represents a particular keyword in the
          language is conventionally the same as that keyword converted
          to upper case.  The terminal symbol `error' is reserved for
          error recovery.

          Scattered among the components can be "middle-rule" actions.
          Usually only ACTION is provided (*note action::).

          If COMPONENTS in a rule is `nil', it means that the rule can
          match the empty string.  For example, here is how to define a
          comma-separated sequence of zero or more `exp' groupings:

               (expseq  (nil)               ;; expseq: ;; empty
                        ((expseq1))         ;;       | expseq1
                        )                   ;;       ;
               
               (expseq1 ((exp))             ;; expseq1: exp
                        ((expseq1 ?, exp))  ;;        | expseq1 ',' exp
                        )                   ;;        ;

    PRECEDENCE
          Assign the rule the precedence of the given terminal item,
          overriding the precedence that would be deduced for it, that
          is the one of the last terminal in it.  Notice that only
          terminals declared in ASSOCS have a precedence level.  The
          altered rule precedence then affects how conflicts involving
          that rule are resolved.

          PRECEDENCE is an optional vector of one terminal item.

          Here is how PRECEDENCE solves the problem of unary minus.
          First, declare a precedence for a fictitious terminal symbol
          named `UMINUS'.  There are no tokens of this type, but the
          symbol serves to stand for its precedence:

               ...
               ((default-prec t) ;; This is the default
                (left '+' '-')
                (left '*')
                (left UMINUS))

          Now the precedence of `UMINUS' can be used in specific rules:

               (exp    ...                  ;; exp:    ...
                        ((exp ?- exp))      ;;         | exp '-' exp
                       ...                  ;;         ...
                        ((?- exp) [UMINUS]) ;;         | '-' exp %prec UMINUS
                       ...                  ;;         ...
                       )                    ;;         ;

          If you forget to append `[UMINUS]' to the rule for unary
          minus, Wisent silently assumes that minus has its usual
          precedence.  This kind of problem can be tricky to debug,
          since one typically discovers the mistake only by testing the
          code.

          Using `(default-prec nil)' declaration makes it easier to
          discover this kind of problem systematically.  It causes
          rules that lack a PRECEDENCE modifier to have no precedence,
          even if the last terminal symbol mentioned in their
          components has a declared precedence.

          If `(default-prec nil)' is in effect, you must specify
          PRECEDENCE for all rules that participate in precedence
          conflict resolution.  Then you will see any shift/reduce
          conflict until you tell Wisent how to resolve it, either by
          changing your grammar or by adding an explicit precedence.
          This will probably add declarations to the grammar, but it
          helps to protect against incorrect rule precedences.

          The effect of `(default-prec nil)' can be reversed by giving
          `(default-prec t)', which is the default.

          For more details, see *Note Contextual Precedence:
          (bison)Contextual Precedence, in the Bison manual.

          It is important to understand that ASSOCS declarations defines
          associativity but also assign a precedence level to
          terminals.  All terminals declared in the same `left',
          `right' or `nonassoc' association get the same precedence
          level.  The precedence level is increased at each new
          association.

          On the other hand, PRECEDENCE explicitly assign the precedence
          level of the given terminal to a rule.

    ACTION
          An action is an optional Emacs Lisp function call, like this:

          `(identity $1)'

          The result of an action determines the semantic value of a
          rule.

          From an implementation standpoint, the function call will be
          embedded in a lambda expression, and several useful local
          variables will be defined:

         `$N'
               Where N is a positive integer.  Like in Bison, the value
               of `$N' is the semantic value of the Nth element of
               COMPONENTS, starting from 1.  It can be of any Lisp data
               type.

         `$regionN'
               Where N is a positive integer.  For each `$N' variable
               defined there is a corresponding `$regionN' variable.
               Its value is a pair `(START-POS .  END-POS)' that
               represent the start and end positions (in the lexical
               input stream) of the `$N' value.  It can be `nil' when
               the component positions are not available, like for an
               empty string component for example.

         `$region'
               Its value is the leftmost and rightmost positions of
               input data matched by all COMPONENTS in the rule.  This
               is a pair `(LEFTMOST-POS .  RIGHTMOST-POS)'.  It can be
               `nil' when components positions are not available.

         `$nterm'
               This variable is initialized with the nonterminal symbol
               (NONTERM) the rule belongs to.  It could be useful to
               improve error reporting or debugging.  It is also used
               to automatically provide incremental re-parse entry
               points for semantic tags (*note Wisent Semantic::).

         `$action'
               The value of `$action' is the symbolic name of the
               current semantic action (*note Debugging actions::).

          When an action is not specified a default value is supplied,
          it is `(identity $1)'.  This means that the default semantic
          value of a rule is the value of its first component.
          Excepted for a rule matching the empty string, for which the
          default action is to return `nil'.


File: wisent.info,  Node: Example,  Next: Compiling a grammar,  Prev: Grammar format,  Up: Wisent Grammar

Example
=======

Here is an example to parse simple infix arithmetic expressions.  See
*Note Infix Calc: (bison)Infix Calc, in the Bison manual for details.

     '(
       ;; Terminals
       (NUM)
     
       ;; Terminal associativity & precedence
       ((nonassoc ?=)
        (left ?- ?+)
        (left ?* ?/)
        (left NEG)
        (right ?^))
     
       ;; Rules
       (input
        ((line))
        ((input line)
         (format "%s %s" $1 $2))
        )
     
       (line
        ((?;)
         (progn ";"))
        ((exp ?;)
         (format "%s;" $1))
        ((error ?;)
         (progn "Error;")))
        )
     
       (exp
        ((NUM)
         (string-to-number $1))
        ((exp ?= exp)
         (= $1 $3))
        ((exp ?+ exp)
         (+ $1 $3))
        ((exp ?- exp)
         (- $1 $3))
        ((exp ?* exp)
         (* $1 $3))
        ((exp ?/ exp)
         (/ $1 $3))
        ((?- exp) [NEG]
         (- $2))
        ((exp ?^ exp)
         (expt $1 $3))
        ((?\( exp ?\))
         (progn $2))
        )
       )

In the bison-like "WY" format (*note Wisent Semantic::) the grammar
looks like this:

     %token <number> NUM
     
     %nonassoc '=' ;; comparison
     %left '-' '+'
     %left '*' '/'
     %left NEG     ;; negation--unary minus
     %right '^'    ;; exponentiation
     
     %%
     
     input:
         line
       | input line
         (format "%s %s" $1 $2)
       ;
     
     line:
         ';'
         {";"}
       | exp ';'
         (format "%s;" $1)
       | error ';'
         {"Error;"}
       ;
     
     exp:
         NUM
         (string-to-number $1)
       | exp '=' exp
         (= $1 $3)
       | exp '+' exp
         (+ $1 $3)
       | exp '-' exp
         (- $1 $3)
       | exp '*' exp
         (* $1 $3)
       | exp '/' exp
         (/ $1 $3)
       | '-' exp %prec NEG
         (- $2)
       | exp '^' exp
         (expt $1 $3)
       | '(' exp ')'
         {$2}
       ;
     
     %%


File: wisent.info,  Node: Compiling a grammar,  Next: Conflicts,  Prev: Example,  Up: Wisent Grammar

Compiling a grammar
===================

After providing a context-free grammar in a suitable format, it must be
translated into a set of tables (an "automaton") that will be used to
drive the parser.  Like Bison, Wisent translates grammars that must be
"LALR(1)".

A grammar is LALR(1) if it is possible to tell how to parse any portion
of an input string with just a single token of look-ahead: the
"look-ahead token".  See *Note Language and Grammar: (bison)Language
and Grammar, in the Bison manual for more information.

Grammar translation (compilation) is achieved by the function:

 - Function: wisent-compile-grammar grammar &optional start-list
     Compile GRAMMAR and return an LALR(1) automaton.

     Optional argument START-LIST is a list of start symbols
     (nonterminals).  If `nil' the first nonterminal defined in the
     grammar is the default start symbol.  If START-LIST contains only
     one element, it defines the start symbol.  If START-LIST contains
     more than one element, all are defined as potential start symbols,
     unless `wisent-single-start-flag' is non-`nil'.  In that case the
     first element of START-LIST defines the start symbol and others
     are ignored.

     The LALR(1) automaton is a vector of the form:

     `[ACTIONS GOTOS STARTS FUNCTIONS]'

    ACTIONS
          A state/token matrix telling the parser what to do at every
          state based on the current look-ahead token.  That is shift,
          reduce, accept or error.  See also *Note Wisent Parsing::.

    GOTOS
          A state/nonterminal matrix telling the parser the next state
          to go to after reducing with each rule.

    STARTS
          An alist which maps the allowed start symbols (nonterminals)
          to lexical tokens that will be first shifted into the parser
          stack.

    FUNCTIONS
          An obarray of semantic action symbols.  A semantic action is
          actually an Emacs Lisp function (lambda expression).


File: wisent.info,  Node: Conflicts,  Prev: Compiling a grammar,  Up: Wisent Grammar

Conflicts
=========

Normally, a grammar should produce an automaton where at each state the
parser has only one action to do (*note Wisent Parsing::).

In certain cases, a grammar can produce an automaton where, at some
states, there are more than one action possible.  A such grammar is
"ambiguous", and generated "conflicts".

The parser can't be driven by an automaton which isn't completely
"deterministic", that is which contains conflicts.  It is necessary to
resolve the conflicts to eliminate them.  Wisent resolves conflicts
like Bison does.

There are two sorts of conflicts:

"shift/reduce conflicts"
     When either a shift or a reduction would be valid at the same
     state.

     Such conflicts are resolved by choosing to shift, unless otherwise
     directed by operator precedence declarations.  See *Note
     Shift/Reduce: (bison)Shift/Reduce, in the Bison manual for more
     information.

"reduce/reduce conflicts"
     That occurs if there are two or more rules that apply to the same
     sequence of input.  This usually indicates a serious error in the
     grammar.

     Such conflicts are resolved by choosing to use the rule that
     appears first in the grammar, but it is very risky to rely on
     this.  Every reduce/reduce conflict must be studied and usually
     eliminated.  See *Note Reduce/Reduce: (bison)Reduce/Reduce, in the
     Bison manual for more information.

* Menu:

* Grammar Debugging::
* Understanding the automaton::


File: wisent.info,  Node: Grammar Debugging,  Next: Understanding the automaton,  Up: Conflicts

Grammar debugging
-----------------

To help writing a new grammar, `wisent-compile-grammar' can produce a
verbose report containing a detailed description of the grammar and
parser (equivalent to what Bison reports with the `--verbose' option).

To enable the verbose report you can set to non-`nil' the variable:

 - Option: wisent-verbose-flag
     non-`nil' means to report verbose information on generated parser.

Or interactively use the command:

 - Command: wisent-toggle-verbose-flag
     Toggle whether to report verbose information on generated parser.

The verbose report is printed in the temporary buffer `*wisent-log*'
when running interactively, or in file `wisent.output' when running in
batch mode.  Different reports are separated from each other by a line
like this:

     *** Wisent SOURCE-FILE - 2002-06-27 17:33

where SOURCE-FILE is the name of the Emacs Lisp file from which the
grammar was read.  See *Note Understanding the automaton::, for details
on the verbose report.

*Please Note*
     To help debugging the grammar compiler itself, you can set this
     variable to print the content of some internal data structures:

      - Variable: wisent-debug-flag
          non-`nil' means enable some debug stuff.


File: wisent.info,  Node: Understanding the automaton,  Prev: Grammar Debugging,  Up: Conflicts

Understanding the automaton
---------------------------

This section (took from the manual of Bison 1.49) describes how to use
the verbose report printed by `wisent-compile-grammar' to understand
the generated automaton, to tune or fix a grammar.

We will use the following example:

     (let ((wisent-verbose-flag t)) ;; Print a verbose report!
       (wisent-compile-grammar
        '((NUM STR)                          ; %token NUM STR
     
          ((left ?+ ?-)                      ; %left '+' '-';
           (left ?*))                        ; %left '*'
     
          (exp                               ; exp:
           ((exp ?+ exp))                    ;    exp '+' exp
           ((exp ?- exp))                    ;  | exp '-' exp
           ((exp ?* exp))                    ;  | exp '*' exp
           ((exp ?/ exp))                    ;  | exp '/' exp
           ((NUM))                           ;  | NUM
           )                                 ;  ;
     
          (useless                           ; useless:
           ((STR))                           ;    STR
           )                                 ;  ;
          )
        'nil)                                ; no %start declarations
       )

When evaluating the above expression, grammar compilation first issues
the following two clear messages:

     Grammar contains 1 useless nonterminals and 1 useless rules
     Grammar contains 7 shift/reduce conflicts

The `*wisent-log*' buffer details things!

The first section reports conflicts that were solved using precedence
and/or associativity:

     Conflict in state 7 between rule 1 and token '+' resolved as reduce.
     Conflict in state 7 between rule 1 and token '-' resolved as reduce.
     Conflict in state 7 between rule 1 and token '*' resolved as shift.
     Conflict in state 8 between rule 2 and token '+' resolved as reduce.
     Conflict in state 8 between rule 2 and token '-' resolved as reduce.
     Conflict in state 8 between rule 2 and token '*' resolved as shift.
     Conflict in state 9 between rule 3 and token '+' resolved as reduce.
     Conflict in state 9 between rule 3 and token '-' resolved as reduce.
     Conflict in state 9 between rule 3 and token '*' resolved as reduce.

The next section reports useless tokens, nonterminal and rules (note
that useless tokens might be used by the scanner):

     Useless nonterminals:
     
        useless
     
     
     Terminals which are not used:
     
        STR
     
     
     Useless rules:
     
     #6     useless: STR;

The next section lists states that still have conflicts:

     State 7 contains 1 shift/reduce conflict.
     State 8 contains 1 shift/reduce conflict.
     State 9 contains 1 shift/reduce conflict.
     State 10 contains 4 shift/reduce conflicts.

The next section reproduces the grammar used:

     Grammar
     
       Number, Rule
       1       exp -> exp '+' exp
       2       exp -> exp '-' exp
       3       exp -> exp '*' exp
       4       exp -> exp '/' exp
       5       exp -> NUM

And reports the uses of the symbols:

     Terminals, with rules where they appear
     
     $EOI (-1)
     error (1)
     NUM (2) 5
     STR (3) 6
     '+' (4) 1
     '-' (5) 2
     '*' (6) 3
     '/' (7) 4
     
     
     Nonterminals, with rules where they appear
     
     exp (8)
         on left: 1 2 3 4 5, on right: 1 2 3 4

The report then details the automaton itself, describing each state
with it set of "items", also known as "pointed rules".  Each item is a
production rule together with a point (marked by `.') that the input
cursor.

     state 0
     
         NUM	shift, and go to state 1
     
         exp	go to state 2

State 0 corresponds to being at the very beginning of the parsing, in
the initial rule, right before the start symbol (`exp').  When the
parser returns to this state right after having reduced a rule that
produced an `exp', it jumps to state 2.  If there is no such transition
on a nonterminal symbol, and the lookahead is a `NUM', then this token
is shifted on the parse stack, and the control flow jumps to state 1.
Any other lookahead triggers a parse error.

In the state 1...

     state 1
     
         exp  ->  NUM .   (rule 5)
     
         $default	reduce using rule 5 (exp)

the rule 5, `exp: NUM;', is completed.  Whatever the lookahead
(`$default'), the parser will reduce it.  If it was coming from state
0, then, after this reduction it will return to state 0, and will jump
to state 2 (`exp: go to state 2').

     state 2
     
         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)
     
         $EOI	shift, and go to state 11
         '+'	shift, and go to state 3
         '-'	shift, and go to state 4
         '*'	shift, and go to state 5
         '/'	shift, and go to state 6

In state 2, the automaton can only shift a symbol.  For instance,
because of the item `exp -> exp . '+' exp', if the lookahead if `+', it
will be shifted on the parse stack, and the automaton control will jump
to state 3, corresponding to the item `exp -> exp . '+' exp':

     state 3
     
         exp  ->  exp '+' . exp   (rule 1)
     
         NUM	shift, and go to state 1
     
         exp	go to state 7

Since there is no default action, any other token than those listed
above will trigger a parse error.

The interpretation of states 4 to 6 is straightforward:

     state 4
     
         exp  ->  exp '-' . exp   (rule 2)
     
         NUM	shift, and go to state 1
     
         exp	go to state 8
     
     
     
     state 5
     
         exp  ->  exp '*' . exp   (rule 3)
     
         NUM	shift, and go to state 1
     
         exp	go to state 9
     
     
     
     state 6
     
         exp  ->  exp '/' . exp   (rule 4)
     
         NUM	shift, and go to state 1
     
         exp	go to state 10

As was announced in beginning of the report, `State 7 contains 1
shift/reduce conflict.':

     state 7
     
         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp '+' exp .   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)
     
         '*'	shift, and go to state 5
         '/'	shift, and go to state 6
     
         '/'	[reduce using rule 1 (exp)]
         $default	reduce using rule 1 (exp)

Indeed, there are two actions associated to the lookahead `/': either
shifting (and going to state 6), or reducing rule 1.  The conflict
means that either the grammar is ambiguous, or the parser lacks
information to make the right decision.  Indeed the grammar is
ambiguous, as, since we did not specify the precedence of `/', the
sentence `NUM + NUM / NUM' can be parsed as `NUM + (NUM / NUM)', which
corresponds to shifting `/', or as `(NUM + NUM) / NUM', which
corresponds to reducing rule 1.

Because in LALR(1) parsing a single decision can be made, Wisent
arbitrarily chose to disable the reduction, see *Note Conflicts::.
Discarded actions are reported in between square brackets.

Note that all the previous states had a single possible action: either
shifting the next token and going to the corresponding state, or
reducing a single rule.  In the other cases, i.e., when shifting _and_
reducing is possible or when _several_ reductions are possible, the
lookahead is required to select the action.  State 7 is one such state:
if the lookahead is `*' or `/' then the action is shifting, otherwise
the action is reducing rule 1.  In other words, the first two items,
corresponding to rule 1, are not eligible when the lookahead is `*',
since we specified that `*' has higher precedence that `+'.  More
generally, some items are eligible only with some set of possible
lookaheads.

States 8 to 10 are similar:

     state 8
     
         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp '-' exp .   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)
     
         '*'	shift, and go to state 5
         '/'	shift, and go to state 6
     
         '/'	[reduce using rule 2 (exp)]
         $default	reduce using rule 2 (exp)
     
     
     
     state 9
     
         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp '*' exp .   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)
     
         '/'	shift, and go to state 6
     
         '/'	[reduce using rule 3 (exp)]
         $default	reduce using rule 3 (exp)
     
     
     
     state 10
     
         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)
         exp  ->  exp '/' exp .   (rule 4)
     
         '+'	shift, and go to state 3
         '-'	shift, and go to state 4
         '*'	shift, and go to state 5
         '/'	shift, and go to state 6
     
         '+'	[reduce using rule 4 (exp)]
         '-'	[reduce using rule 4 (exp)]
         '*'	[reduce using rule 4 (exp)]
         '/'	[reduce using rule 4 (exp)]
         $default	reduce using rule 4 (exp)

Observe that state 10 contains conflicts due to the lack of precedence
of `/' wrt `+', `-', and `*', but also because the associativity of `/'
is not specified.

Finally, the state 11 (plus 12) is named the "final state", or the
"accepting state":

     state 11
     
         $EOI	shift, and go to state 12
     
     
     
     state 12
     
         $default	accept

The end of input is shifted `$EOI shift,' and the parser exits
successfully (`go to state 12', that terminates).


File: wisent.info,  Node: Wisent Parsing,  Next: Wisent Semantic,  Prev: Wisent Grammar,  Up: Top

Wisent Parsing
**************

The Wisent's parser is what is called a "bottom-up" or "shift-reduce"
parser which repeatedly:

"shift"
     That is pushes the value of the last lexical token read (the
     look-ahead token) into a value stack, and reads a new one.

"reduce"
     That is replaces a nonterminal by its semantic value.  The values
     of the components which form the right hand side of a rule are
     popped from the value stack and reduced by the semantic action of
     this rule.  The result is pushed back on top of value stack.

The parser will stop on:

"accept"
     When all input has been successfully parsed.  The semantic value of
     the start nonterminal is on top of the value stack.

"error"
     When a syntax error (an unexpected token in input) has been
     detected.  At this point the parser issues an error message and
     either stops or calls a recovery routine to try to resume parsing.

The above elementary actions are driven by the LALR(1) automaton built
by `wisent-compile-grammar' from a context-free grammar.

The Wisent's parser is entered by calling the function:

 - Function: wisent-parse automaton lexer &optional error start
     Parse input using the automaton specified in AUTOMATON.

    AUTOMATON
          Is an LALR(1) automaton generated by `wisent-compile-grammar'
          (*note Wisent Grammar::).

    LEXER
          Is a function with no argument called by the parser to obtain
          the next terminal (token) in input (*note Writing a lexer::).

    ERROR
          Is an optional reporting function called when a parse error
          occurs.  It receives a message string to report.  It defaults
          to the function `wisent-message' (*note Report errors::).

    START
          Specify the start symbol (nonterminal) used by the parser as
          its goal.  It defaults to the start symbol defined in the
          grammar (*note Wisent Grammar::).

The following two normal hooks permit to do some useful processing
respectively before to start parsing, and after the parser terminated.

 - Variable: wisent-pre-parse-hook
     Normal hook run just before entering the LR parser engine.

 - Variable: wisent-post-parse-hook
     Normal hook run just after the LR parser engine terminated.

* Menu:

* Writing a lexer::
* Actions goodies::
* Report errors::
* Error recovery::
* Debugging actions::


File: wisent.info,  Node: Writing a lexer,  Next: Actions goodies,  Up: Wisent Parsing

What the parser must receive
============================

It is important to understand that the parser does not parse
characters, but lexical tokens, and does not know anything about
characters in text streams!

Reading input data to produce lexical tokens is performed by a lexer
(also called a scanner) in a lexical analysis step, before the syntax
analysis step performed by the parser.  The parser automatically calls
the lexer when it needs the next token to parse.

A Wisent's lexer is an Emacs Lisp function with no argument.  It must
return a valid lexical token of the form:

`(TOKEN-CLASS VALUE [START . END])'

TOKEN-CLASS
     Is a category of lexical token identifying a terminal as specified
     in the grammar (*note Wisent Grammar::).  It can be a symbol or a
     character literal.

VALUE
     Is the value of the lexical token.  It can be of any valid Emacs
     Lisp data type.

START
END
     Are the optionals beginning and end positions of VALUE in the
     input stream.

When there are no more tokens to read the lexer must return the token
`(list wisent-eoi-term)' to each request.

 - Variable: wisent-eoi-term
     Predefined constant, End-Of-Input terminal symbol.

`wisent-lex' is an example of a lexer that reads lexical tokens
produced by a semantic lexer, and translates them into lexical tokens
suitable to the Wisent parser.  See also *Note Wisent Lex::.

To call the lexer in a semantic action use the function `wisent-lexer'.
See also *Note Actions goodies::.


File: wisent.info,  Node: Actions goodies,  Next: Report errors,  Prev: Writing a lexer,  Up: Wisent Parsing

Variables and macros useful in grammar actions.
===============================================

 - Variable: wisent-input
     The last token read.  This variable only has meaning in the scope
     of `wisent-parse'.

 - Function: wisent-lexer
     Obtain the next terminal in input.

 - Function: wisent-region &rest positions
     Return the start/end positions of the region including POSITIONS.
     Each element of POSITIONS is a pair `(START-POS .  END-POS)' or
     `nil'.  The returned value is the pair
     `(MIN-START-POS . MAX-END-POS)' or `nil' if no POSITIONS are
     available.


File: wisent.info,  Node: Report errors,  Next: Error recovery,  Prev: Actions goodies,  Up: Wisent Parsing

The error reporting function
============================

When the parser encounters a syntax error it calls a user-defined
function.  It must be an Emacs Lisp function with one argument: a
string containing the message to report.

By default the parser uses this function to report error messages:

 - Function: wisent-message string &rest args
     Print a one-line message if `wisent-parse-verbose-flag' is set.
     Pass STRING and ARGS arguments to "message".

*Please Note:*
     `wisent-message' uses the following function to print lexical
     tokens:

      - Function: wisent-token-to-string token
          Return a printed representation of lexical token TOKEN.

     The general printed form of a lexical token is:

     `TOKEN(VALUE)@LOCATION'

To control the verbosity of the parser you can set to non-`nil' this
variable:

 - Option: wisent-parse-verbose-flag
     non-`nil' means to issue more messages while parsing.

Or interactively use the command:

 - Command: wisent-parse-toggle-verbose-flag
     Toggle whether to issue more messages while parsing.

When the error reporting function is entered the variable
`wisent-input' contains the unexpected token as returned by the lexer.

The error reporting function can be called from a semantic action too
using the special macro `wisent-error'.  When called from a semantic
action entered by error recovery (*note Error recovery::) the value of
the variable `wisent-recovering' is non-`nil'.


File: wisent.info,  Node: Error recovery,  Next: Debugging actions,  Prev: Report errors,  Up: Wisent Parsing

Error recovery
==============

The error recovery mechanism of the Wisent's parser conforms to the one
Bison uses.  See *Note Error Recovery: (bison)Error Recovery, in the
Bison manual for details.

To recover from a syntax error you must write rules to recognize the
special token `error'.  This is a terminal symbol that is automatically
defined and reserved for error handling.

When the parser encounters a syntax error, it pops the state stack
until it finds a state that allows shifting the `error' token.  After
it has been shifted, if the old look-ahead token is not acceptable to
be shifted next, the parser reads tokens and discards them until it
finds a token which is acceptable.

Strategies for error recovery depend on the choice of error rules in
the grammar.  A simple and useful strategy is simply to skip the rest
of the current statement if an error is detected:

     (stmnt (( error ?; )) ;; on error, skip until ';' is read
            )

It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed:

     (primary (( ?{ expr  ?} ))
              (( ?{ error ?} ))
              ...
              )

Note that error recovery rules may have actions, just as any other
rules can.  Here are some predefined hooks, variables, functions or
macros, useful in such actions:

 - Variable: wisent-nerrs
     The number of parse errors encountered so far.

 - Variable: wisent-recovering
     non-`nil' means that the parser is recovering.  This variable only
     has meaning in the scope of `wisent-parse'.

 - Function: wisent-error msg
     Call the user supplied error reporting function with message MSG
     (*note Report errors::).

     For an example of use, *Note wisent-skip-token::.

 - Function: wisent-errok
     Resume generating error messages immediately for subsequent syntax
     errors.

     The parser suppress error message for syntax errors that happens
     shortly after the first, until three consecutive input tokens have
     been successfully shifted.

     Calling `wisent-errok' in an action, make error messages resume
     immediately.  No error messages will be suppressed if you call it
     in an error rule's action.

     For an example of use, *Note wisent-skip-token::.

 - Function: wisent-clearin
     Discard the current lookahead token.  This will cause a new
     lexical token to be read.

     In an error rule's action the previous lookahead token is
     reanalyzed immediately.  `wisent-clearin' may be called to clear
     this token.

     For example, suppose that on a parse error, an error handling
     routine is called that advances the input stream to some point
     where parsing should once again commence.  The next symbol
     returned by the lexical scanner is probably correct.  The previous
     lookahead token ought to be discarded with `wisent-clearin'.

     For an example of use, *Note wisent-skip-token::.

 - Function: wisent-abort
     Abort parsing and save the lookahead token.

 - Function: wisent-set-region start end
     Change the region of text matched by the current nonterminal.
     START and END are respectively the beginning and end positions of
     the region occupied by the group of components associated to this
     nonterminal.  If START or END values are not a valid positions the
     region is set to `nil'.

     For an example of use, *Note wisent-skip-token::.

 - Variable: wisent-discarding-token-functions
     List of functions to be called when discarding a lexical token.
     These functions receive the lexical token discarded.  When the
     parser encounters unexpected tokens, it can discards them, based
     on what directed by error recovery rules.  Either when the parser
     reads tokens until one is found that can be shifted, or when an
     semantic action calls the function `wisent-skip-token' or
     `wisent-skip-block'.  For language specific hooks, make sure you
     define this as a local hook.

     For example, in semantic, this hook is set to the function
     `wisent-collect-unmatched-syntax' to collect unmatched lexical
     tokens (*note Useful functions::).

 - Function: wisent-skip-token
     Skip the lookahead token in order to resume parsing.  Return nil.
     Must be used in error recovery semantic actions.

     It typically looks like this:

          (wisent-message "%s: skip %s" $action
                          (wisent-token-to-string wisent-input))
          (run-hook-with-args
           'wisent-discarding-token-functions wisent-input)
          (wisent-clearin)
          (wisent-errok)))

 - Function: wisent-skip-block
     Safely skip a block in order to resume parsing.  Return nil.  Must
     be used in error recovery semantic actions.

     A block is data between an open-delimiter (syntax class `(') and a
     matching close-delimiter (syntax class `)'):

          (a parenthesized block)
          [a block between brackets]
          {a block between braces}

     The following example uses `wisent-skip-block' to safely skip a
     block delimited by `LBRACE' (`{') and `RBRACE' (`}') tokens, when
     a syntax error occurs in `other-components':

          (block ((LBRACE other-components RBRACE))
                 ((LBRACE RBRACE))
                 ((LBRACE error)
                  (wisent-skip-block))
                 )


File: wisent.info,  Node: Debugging actions,  Prev: Error recovery,  Up: Wisent Parsing

Debugging semantic actions
==========================

Each semantic action is represented by a symbol interned in an
"obarray" that is part of the LALR(1) automaton (*note Compiling a
grammar::).  `symbol-function' on a semantic action symbol return the
semantic action lambda expression.

A semantic action symbol name has the form `NONTERMINAL:INDEX', where
NONTERMINAL is the name of the nonterminal symbol the action belongs
to, and INDEX is an action sequence number within the scope of
NONTERMINAL.  For example, this nonterminal definition:

     input:
        line                     [`input:0']
      | input line
        (format "%s %s" $1 $2)   [`input:1']
      ;

Will produce two semantic actions, and associated symbols:

`input:0'
     A default action that returns `$1'.

`input:1'
     That returns `(format "%s %s" $1 $2)'.

Debugging uses the Lisp debugger to investigate what is happening
during execution of semantic actions.  Three commands are available to
debug semantic actions.  They receive two arguments:

   * The automaton that contains the semantic action.

   * The semantic action symbol.

 - Command: wisent-debug-on-entry automaton function
     Request AUTOMATON's FUNCTION to invoke debugger each time it is
     called.  FUNCTION must be a semantic action symbol that exists in
     AUTOMATON.

 - Command: wisent-cancel-debug-on-entry automaton function
     Undo effect of `wisent-debug-on-entry' on AUTOMATON's FUNCTION.
     FUNCTION must be a semantic action symbol that exists in AUTOMATON.

 - Command: wisent-debug-show-entry automaton function
     Show the source of AUTOMATON's semantic action FUNCTION.  FUNCTION
     must be a semantic action symbol that exists in AUTOMATON.


File: wisent.info,  Node: Wisent Semantic,  Next: GNU Free Documentation License,  Prev: Wisent Parsing,  Up: Top

How to use Wisent with Semantic
*******************************

This section presents how the Wisent's parser can be used to produce
"tags" for the semantic tool set.

semantic tags form a hierarchy of Emacs Lisp data structures that
describes a program in a way independent of programming languages.
Tags map program declarations, like functions, methods, variables, data
types, classes, includes, grammar rules, etc..

To use the Wisent parser with semantic you have to define your grammar
in "WY" form, a grammar format very close to the one used by Bison.

Please *note Semantic Grammar Framework Manual: (grammar-fw)top for
more information on semantic grammars.

* Menu:

* Grammar styles::
* Wisent Lex::


File: wisent.info,  Node: Grammar styles,  Next: Wisent Lex,  Up: Wisent Semantic

Grammar styles
==============

semantic parsing heavily depends on how you wrote the grammar.  There
are mainly two styles to write a Wisent's grammar intended to be used
with the semantic tool set: the "Iterative style" and the "Bison
style".  Each one has pros and cons, and in certain cases it can be
worth a mix of the two styles!

* Menu:

* Iterative style::
* Bison style::
* Mixed style::
* Start nonterminals::
* Useful functions::


File: wisent.info,  Node: Iterative style,  Next: Bison style,  Prev: Grammar styles,  Up: Grammar styles

Iterative style
---------------

The "iterative style" is the preferred style to use with semantic.  It
relies on an iterative parser back-end mechanism which parses start
nonterminals one at a time and automagically skip unexpected lexical
tokens in input.

Compared to rule-based iterative functions (*note Bison style::),
iterative parsers are better in that they can handle obscure errors
more cleanly.

Each start nonterminal must produces a "raw tag" by calling a
`TAG'-like grammar macro with appropriate parameters.  See also *Note
Start nonterminals::.

Then, each parsing iteration automatically translates a raw tag into
"expanded tags", updating the raw tag structure with internal
properties and buffer related data.

After parsing completed it results in a tree of expanded tags.

The following example is a snippet of the iterative style Java grammar
provided in the semantic distribution in the file `wisent-java-tags.wy'.

     ...
     ;; Alternate entry points
     ;;    - Needed by partial re-parse
     %start formal_parameter
     ...
     ;;    - Needed by EXPANDFULL clauses
     %start formal_parameters
     ...
     
     formal_parameter_list
       : PAREN_BLOCK
         (EXPANDFULL $1 formal_parameters)
       ;
     
     formal_parameters
       : LPAREN
         ()
       | RPAREN
         ()
       | formal_parameter COMMA
       | formal_parameter RPAREN
       ;
     
     formal_parameter
       : formal_parameter_modifier_opt type variable_declarator_id
         (VARIABLE-TAG $3 $2 nil :typemodifiers $1)
       ;

It shows the use of the `EXPANDFULL' grammar macro to parse a
`PAREN_BLOCK' which contains a `formal_parameter_list'.  `EXPANDFULL'
tells to recursively parse `formal_parameters' inside `PAREN_BLOCK'.
The parser iterates until it digested all available input data inside
the `PAREN_BLOCK', trying to match any of the `formal_parameters' rules:

   * `LPAREN'

   * `RPAREN'

   * `formal_parameter COMMA'

   * `formal_parameter RPAREN'

At each iteration it will return a `formal_parameter' raw tag, or `nil'
to skip unwanted (single `LPAREN' or `RPAREN' for example) or
unexpected input data.  Those raw tags will be automatically expanded
by the iterative back-end parser.


File: wisent.info,  Node: Bison style,  Next: Mixed style,  Prev: Iterative style,  Up: Grammar styles

Bison style
-----------

What we call the "Bison style" is the traditional style of Bison's
grammars.  Compared to iterative style, it is not straightforward to
use grammars written in Bison style in semantic.  Mainly because such
grammars are designed to parse the whole input data in one pass, and
don't use the iterative parser back-end mechanism (*note Iterative
style::).  With Bison style the parser is called once to parse the
grammar start nonterminal.

The following example is a snippet of the Bison style Java grammar
provided in the semantic distribution in the file `wisent-java.wy'.

     %start formal_parameter
     ...
     
     formal_parameter_list
       : formal_parameter_list COMMA formal_parameter
         (cons $3 $1)
       | formal_parameter
         (list $1)
       ;
     
     formal_parameter
       : formal_parameter_modifier_opt type variable_declarator_id
         (EXPANDTAG
          (VARIABLE-TAG $3 $2 :typemodifiers $1)
          )
       ;

The first consequence is that syntax errors are not automatically
handled by semantic.  Thus, it is necessary to explicitly handle them
at the grammar level, providing error recovery rules to skip unexpected
input data.

The second consequence is that the iterative parser can't do automatic
tag expansion, except for the start nonterminal value.  It is necessary
to explicitly expand tag from concerned semantic actions by calling the
grammar macro `EXPANDTAG' with a raw tag as parameter.  See also *Note
Start nonterminals::, for incremental re-parse considerations.

