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 Semantic Manual.
Copyright © 1988–1993, 1995, 1998–2004, 2007, 2012–2024 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 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License”.
(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU manual.”
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 Bison Manual.
Wisent can generate compilers compatible with the Semantic tool set. See the Semantic Manual.
It benefits from these Bison features:
Static Semantics in Compiler Error Recovery
June 1985, Report No. UCB/CSD 85/251.
Efficient Computation of LALR(1) Look-Ahead Sets
October 1982, ACM TOPLAS Vol 4 No 4, 615–49, https://doi.org/10.1145/69622.357187.
error.
Nevertheless there are some fundamental differences between Bison and Wisent.
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 (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’.
To be acceptable by Wisent a context-free grammar must respect a particular format. That is, must be represented as an Emacs Lisp list of the form:
(terminals assocs . non-terminals)
Is the list of terminal symbols used in the grammar.
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 (bison)Contextual Precedence, in the Bison manual.
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:
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 (see 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
) ;; ;
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 (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.
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 (see How to use Wisent with Semantic).
$action ¶The value of $action is the symbolic name of the current
semantic action (see Debugging semantic 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.
Here is an example to parse simple infix arithmetic expressions. See (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 (see How to use Wisent with 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}
;
%%
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 derive 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 (bison)Language and Grammar, in the Bison manual for more information.
Grammar translation (compilation) is achieved by the function:
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]
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 Wisent Parsing.
A state/nonterminal matrix telling the parser the next state to go to after reducing with each rule.
An alist which maps the allowed start symbols (nonterminals) to lexical tokens that will be first shifted into the parser stack.
An obarray of semantic action symbols. A semantic action is actually an Emacs Lisp function (lambda expression).
Normally, a grammar should produce an automaton where at each state the parser has only one action to do (see Wisent Parsing).
In certain cases, a grammar can produce an automaton where, at some states, there are more than one action possible. Such a grammar is ambiguous, and generates 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:
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 (bison)Shift/Reduce, in the Bison manual for more information.
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 (bison)Reduce/Reduce, in the Bison manual for more information.
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:
non-nil means to report verbose information on generated parser.
Or interactively use the command:
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 Understanding the automaton, for details on the verbose report.
To help debugging the grammar compiler itself, you can set this variable to print the content of some internal data structures:
non-nil means enable some debug stuff.
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 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).
The Wisent’s parser is what is called a bottom-up or shift-reduce parser which repeatedly:
That is pushes the value of the last lexical token read (the look-ahead token) into a value stack, and reads a new one.
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:
When all input has been successfully parsed. The semantic value of the start nonterminal is on top of the value stack.
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:
Parse input using the automaton specified in automaton.
Is an LALR(1) automaton generated by
wisent-compile-grammar (see Wisent Grammar).
Is a function with no argument called by the parser to obtain the next terminal (token) in input (see What the parser must receive).
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 (see The error reporting function).
Specify the start symbol (nonterminal) used by the parser as its goal. It defaults to the start symbol defined in the grammar (see Wisent Grammar).
The following two normal hooks permit doing some useful processing respectively before starting parsing, and after the parser terminated.
Normal hook run just before entering the LR parser engine.
Normal hook run just after the LR parser engine terminated.
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])
Is a category of lexical token identifying a terminal as specified in the grammar (see Wisent Grammar). It can be a symbol or a character literal.
Is the value of the lexical token. It can be of any valid Emacs Lisp data type.
Are the optional beginning and ending 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.
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 The Wisent Lex lexer.
To call the lexer in a semantic action use the function
wisent-lexer. See also Variables and macros useful in grammar actions..
The last token read.
This variable only has meaning in the scope of wisent-parse.
Obtain the next terminal in input.
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.
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:
Print a one-line message if wisent-parse-verbose-flag is set.
Pass string and args arguments to message.
wisent-message uses the following function to print lexical
tokens:
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:
non-nil means to issue more messages while parsing.
Or interactively use the command:
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 (see Error recovery) the
value of the variable wisent-recovering is non-nil.
The error recovery mechanism of the Wisent’s parser conforms to the one Bison uses. See (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:
(statement (( 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:
The number of parse errors encountered so far.
non-nil means that the parser is recovering.
This variable only has meaning in the scope of wisent-parse.
Call the user supplied error reporting function with message msg (see The error reporting function).
For an example of use, See wisent-skip-token.
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, See wisent-skip-token.
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, See wisent-skip-token.
Abort parsing and save the lookahead token.
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, See wisent-skip-token.
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 (see Useful functions).
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)))
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))
)
Each semantic action is represented by a symbol interned in an
obarray that is part of the LALR(1) automaton
(see 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:0A default action that returns $1.
input:1That 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:
Request automaton’s function to invoke debugger each time it is called. function must be a semantic action symbol that exists in automaton.
Undo effect of wisent-debug-on-entry on automaton’s function.
function must be a semantic action symbol that exists in automaton.
Show the source of automaton’s semantic action function. function must be a semantic action symbol that exists in automaton.
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 see Semantic Grammar Framework Manual, for more information on Semantic grammars.
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!
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 skips unexpected lexical tokens in input.
Compared to rule-based iterative functions (see 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
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 completes, 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 semantic/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:
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.
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 (see 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 semantic/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 tags from concerned semantic actions by
calling the grammar macro EXPANDTAG with a raw tag as
parameter. See also Start nonterminals, for incremental
re-parse considerations.
%start grammar
;; Reparse
%start prologue epilogue declaration nonterminal rule
...
%%
grammar:
prologue
| epilogue
| declaration
| nonterminal
| PERCENT_PERCENT
;
...
nonterminal:
SYMBOL COLON rules SEMI
(TAG $1 'nonterminal :children $3)
;
rules:
lifo_rules
(apply 'nconc (nreverse $1))
;
lifo_rules:
lifo_rules OR rule
(cons $3 $1)
| rule
(list $1)
;
rule:
rhs
(let* ((rhs $1)
name type comps prec action elt)
...
(EXPANDTAG
(TAG name 'rule :type type :value comps :prec prec :expr action)
))
;
This example shows how iterative and Bison styles can be combined in the same grammar to obtain a good compromise between grammar complexity and an efficient parsing strategy in an interactive environment.
‘nonterminal’ is parsed using iterative style via the main
‘grammar’ rule. The semantic action uses the TAG macro to
produce a raw tag, automagically expanded by Semantic.
But ‘rules’ part is parsed in Bison style! Why?
Rule delimiters are the colon (:), that follows the nonterminal
name, and a final semicolon (;). Unfortunately these
delimiters are not open-paren/close-paren type, and the
Emacs’ syntactic analyzer can’t easily isolate data between them to
produce a ‘RULES_PART’ parenthesis-block-like lexical token.
Consequently it is not possible to use EXPANDFULL to iterate in
‘RULES_PART’, like this:
nonterminal:
SYMBOL COLON rules SEMI
(TAG $1 'nonterminal :children $3)
;
rules:
RULES_PART ;; Map a parenthesis-block-like lexical token
(EXPANDFULL $1 'rules)
;
rules:
COLON
()
OR
()
SEMI
()
rhs
rhs
(let* ((rhs $1)
name type comps prec action elt)
...
(TAG name 'rule :type type :value comps :prec prec :expr action)
)
;
In such cases, when it is difficult for Emacs to obtain parenthesis-block-like lexical tokens, the best solution is to use the traditional Bison style with error recovery!
In some extreme cases, it can also be convenient to extend the lexer, to deliver new lexical tokens, to simplify the grammar.
When you write a grammar for Semantic, it is important to carefully indicate the start nonterminals. Each one defines an entry point in the grammar, and after parsing its semantic value is returned to the back-end iterative engine. Consequently:
The semantic value of a start nonterminal must be a produced by a TAG like grammar macro.
Start nonterminals are declared by %start statements. When
nothing is specified the first nonterminal that appears in the grammar
is the start nonterminal.
Generally, the following nonterminals must be declared as start symbols:
Of course!
EXPAND/EXPANDFULL
These grammar macros recursively parse a part of input data, based on rules of the given nonterminal.
For example, the following will parse ‘PAREN_BLOCK’ data using the ‘formal_parameters’ rules:
formal_parameter_list : PAREN_BLOCK (EXPANDFULL $1 formal_parameters) ;The semantic value of ‘formal_parameters’ becomes the value of the
EXPANDFULLexpression. It is a list of Semantic tags spliced in the tags tree.Because the automaton must know that ‘formal_parameters’ is a start symbol, you must declare it like this:
%start formal_parameters
The EXPANDFULL macro has a side effect it is important to know,
related to the incremental re-parse mechanism of Semantic: the
nonterminal symbol parameter passed to EXPANDFULL also becomes
the reparse-symbol property of the tag returned by the
EXPANDFULL expression.
When buffer’s data mapped by a tag is modified, Semantic
schedules an incremental re-parse of that data, using the tag’s
reparse-symbol property as start nonterminal.
The rules associated to such start symbols must be carefully reviewed to ensure that the incremental parser will work!
Things are a little bit different when the grammar is written in Bison style.
The reparse-symbol property is set to the nonterminal
symbol the rule that explicitly uses EXPANDTAG belongs to.
For example:
rule:
rhs
(let* ((rhs $1)
name type comps prec action elt)
...
(EXPANDTAG
(TAG name 'rule :type type :value comps :prec prec :expr action)
))
;
Set the reparse-symbol property of the expanded tag to
‘rule’. An important consequence is that:
Every nonterminal having any rule that calls EXPANDTAG
in a semantic action, should be declared as a start symbol!
Here is a description of some predefined functions it might be useful to know when writing new code to use Wisent in Semantic:
Add input lexical token to the cache of unmatched tokens, in
variable semantic-unmatched-syntax-cache.
See implementation of the function wisent-skip-token in
Error recovery, for an example of use.
The lexical analysis step of Semantic is performed by the general
function semantic-lex. For more information, see Semantic Language Development.
semantic-lex produces lexical tokens of the form:
(token-class start . end)
Is a symbol that identifies a lexical token class, like symbol,
string, number, or PAREN_BLOCK.
Are the start and end positions of mapped data in the input buffer.
The Wisent’s parser doesn’t depend on the nature of analyzed input stream (buffer, string, etc.), and requires that lexical tokens have a different form (see What the parser must receive):
(token-class value [start . end])
wisent-lex is the default Wisent’s lexer used in Semantic.
Return the next available lexical token in Wisent’s form.
The variable wisent-lex-istream contains the list of lexical
tokens produced by semantic-lex. Pop the next token available
and convert it to a form suitable for the Wisent’s parser.
Mapping of lexical tokens as produced by semantic-lex into
equivalent Wisent lexical tokens is straightforward:
(token-class start . end)
⇒ (token-class value start . end)
value is the input buffer-substring from start to
end.
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