bison_8.htm

Lex和Yacc的Manual

Not all rules and not all tokens have precedence.  If either the rule or
the look-ahead token has no precedence, then the default is to shift.

</P>


<H2><A NAME="SEC76" HREF="index.html#SEC76">Context-Dependent Precedence</A></H2>
<P>
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</P>
<P>
Often the precedence of an operator depends on the context.  This sounds
outlandish at first, but it is really very common.  For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary operator.

</P>
<P>
The Bison precedence declarations, <CODE>%left</CODE>, <CODE>%right</CODE> and
<CODE>%nonassoc</CODE>, can only be used once for a given token; so a token has
only one precedence declared in this way.  For context-dependent
precedence, you need to use an additional mechanism: the <CODE>%prec</CODE>
modifier for rules.
</P>
<P>
The <CODE>%prec</CODE> modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
It's not necessary for that symbol to appear otherwise in the rule.  The
modifier's syntax is:

</P>

<PRE>
%prec <VAR>terminal-symbol</VAR>
</PRE>

<P>
and it is written after the components of the rule.  Its effect is to
assign the rule the precedence of <VAR>terminal-symbol</VAR>, overriding
the precedence that would be deduced for it in the ordinary way.  The
altered rule precedence then affects how conflicts involving that rule
are resolved (see section <A HREF="bison_8.html#SEC71">Operator Precedence</A>).

</P>
<P>
Here is how <CODE>%prec</CODE> solves the problem of unary minus.  First, declare
a precedence for a fictitious terminal symbol named <CODE>UMINUS</CODE>.  There
are no tokens of this type, but the symbol serves to stand for its
precedence:

</P>

<PRE>
...
%left '+' '-'
%left '*'
%left UMINUS
</PRE>

<P>
Now the precedence of <CODE>UMINUS</CODE> can be used in specific rules:

</P>

<PRE>
exp:    ...
        | exp '-' exp
        ...
        | '-' exp %prec UMINUS
</PRE>



<H2><A NAME="SEC77" HREF="index.html#SEC77">Parser States</A></H2>
<P>
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</P>
<P>
The function <CODE>yyparse</CODE> is implemented using a finite-state machine.
The values pushed on the parser stack are not simply token type codes; they
represent the entire sequence of terminal and nonterminal symbols at or
near the top of the stack.  The current state collects all the information
about previous input which is relevant to deciding what to do next.

</P>
<P>
Each time a look-ahead token is read, the current parser state together
with the type of look-ahead token are looked up in a table.  This table
entry can say, "Shift the look-ahead token."  In this case, it also
specifies the new parser state, which is pushed onto the top of the
parser stack.  Or it can say, "Reduce using rule number <VAR>n</VAR>."
This means that a certain number of tokens or groupings are taken off
the top of the stack, and replaced by one grouping.  In other words,
that number of states are popped from the stack, and one new state is
pushed.

</P>
<P>
There is one other alternative: the table can say that the look-ahead token
is erroneous in the current state.  This causes error processing to begin
(see section <A HREF="bison_9.html#SEC81">Error Recovery</A>).

</P>


<H2><A NAME="SEC78" HREF="index.html#SEC78">Reduce/Reduce Conflicts</A></H2>
<P>
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<A NAME="IDX171"></A>

</P>
<P>
A reduce/reduce conflict 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.

</P>
<P>
For example, here is an erroneous attempt to define a sequence
of zero or more <CODE>word</CODE> groupings.

</P>

<PRE>
sequence: /* empty */
                { printf ("empty sequence\n"); }
        | maybeword
        | sequence word
                { printf ("added word %s\n", $2); }
        ;

maybeword: /* empty */
                { printf ("empty maybeword\n"); }
        | word
                { printf ("single word %s\n", $1); }
        ;
</PRE>

<P>
The error is an ambiguity: there is more than one way to parse a single
<CODE>word</CODE> into a <CODE>sequence</CODE>.  It could be reduced to a
<CODE>maybeword</CODE> and then into a <CODE>sequence</CODE> via the second rule.
Alternatively, nothing-at-all could be reduced into a <CODE>sequence</CODE>
via the first rule, and this could be combined with the <CODE>word</CODE>
using the third rule for <CODE>sequence</CODE>.

</P>
<P>
There is also more than one way to reduce nothing-at-all into a
<CODE>sequence</CODE>.  This can be done directly via the first rule,
or indirectly via <CODE>maybeword</CODE> and then the second rule.

</P>
<P>
You might think that this is a distinction without a difference, because it
does not change whether any particular input is valid or not.  But it does
affect which actions are run.  One parsing order runs the second rule's
action; the other runs the first rule's action and the third rule's action.
In this example, the output of the program changes.

</P>
<P>
Bison resolves a reduce/reduce conflict 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.  Here is the
proper way to define <CODE>sequence</CODE>:

</P>

<PRE>
sequence: /* empty */
                { printf ("empty sequence\n"); }
        | sequence word
                { printf ("added word %s\n", $2); }
        ;
</PRE>

<P>
Here is another common error that yields a reduce/reduce conflict:

</P>

<PRE>
sequence: /* empty */
        | sequence words
        | sequence redirects
        ;

words:    /* empty */
        | words word
        ;

redirects:/* empty */
        | redirects redirect
        ;
</PRE>

<P>
The intention here is to define a sequence which can contain either
<CODE>word</CODE> or <CODE>redirect</CODE> groupings.  The individual definitions of
<CODE>sequence</CODE>, <CODE>words</CODE> and <CODE>redirects</CODE> are error-free, but the
three together make a subtle ambiguity: even an empty input can be parsed
in infinitely many ways!

</P>
<P>
Consider: nothing-at-all could be a <CODE>words</CODE>.  Or it could be two
<CODE>words</CODE> in a row, or three, or any number.  It could equally well be a
<CODE>redirects</CODE>, or two, or any number.  Or it could be a <CODE>words</CODE>
followed by three <CODE>redirects</CODE> and another <CODE>words</CODE>.  And so on.

</P>
<P>
Here are two ways to correct these rules.  First, to make it a single level
of sequence:

</P>

<PRE>
sequence: /* empty */
        | sequence word
        | sequence redirect
        ;
</PRE>

<P>
Second, to prevent either a <CODE>words</CODE> or a <CODE>redirects</CODE>
from being empty:

</P>

<PRE>
sequence: /* empty */
        | sequence words
        | sequence redirects
        ;

words:    word
        | words word
        ;

redirects:redirect
        | redirects redirect
        ;
</PRE>



<H2><A NAME="SEC79" HREF="index.html#SEC79">Mysterious Reduce/Reduce Conflicts</A></H2>

<P>
Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:

</P>

<PRE>
%token ID

%%
def:    param_spec return_spec ','
        ;
param_spec:
             type
        |    name_list ':' type
        ;
return_spec:
             type
        |    name ':' type
        ;
type:        ID
        ;
name:        ID
        ;
name_list:
             name
        |    name ',' name_list
        ;
</PRE>

<P>
It would seem that this grammar can be parsed with only a single token
of look-ahead: when a <CODE>param_spec</CODE> is being read, an <CODE>ID</CODE> is 
a <CODE>name</CODE> if a comma or colon follows, or a <CODE>type</CODE> if another
<CODE>ID</CODE> follows.  In other words, this grammar is LR(1).

</P>
<P>
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<A NAME="IDX173"></A>
However, Bison, like most parser generators, cannot actually handle all
LR(1) grammars.  In this grammar, two contexts, that after an <CODE>ID</CODE>
at the beginning of a <CODE>param_spec</CODE> and likewise at the beginning of
a <CODE>return_spec</CODE>, are similar enough that Bison assumes they are the
same.  They appear similar because the same set of rules would be
active--the rule for reducing to a <CODE>name</CODE> and that for reducing to
a <CODE>type</CODE>.  Bison is unable to determine at that stage of processing
that the rules would require different look-ahead tokens in the two
contexts, so it makes a single parser state for them both.  Combining
the two contexts causes a conflict later.  In parser terminology, this
occurrence means that the grammar is not LALR(1).

</P>
<P>
In general, it is better to fix deficiencies than to document them.  But
this particular deficiency is intrinsically hard to fix; parser
generators that can handle LR(1) grammars are hard to write and tend to
produce parsers that are very large.  In practice, Bison is more useful
as it is now.

</P>
<P>
When the problem arises, you can often fix it by identifying the two
parser states that are being confused, and adding something to make them
look distinct.  In the above example, adding one rule to
<CODE>return_spec</CODE> as follows makes the problem go away:

</P>

<PRE>
%token BOGUS
...
%%
...
return_spec:
             type
        |    name ':' type
        /* This rule is never used.  */
        |    ID BOGUS
        ;
</PRE>

<P>
This corrects the problem because it introduces the possibility of an
additional active rule in the context after the <CODE>ID</CODE> at the beginning of
<CODE>return_spec</CODE>.  This rule is not active in the corresponding context
in a <CODE>param_spec</CODE>, so the two contexts receive distinct parser states.
As long as the token <CODE>BOGUS</CODE> is never generated by <CODE>yylex</CODE>,
the added rule cannot alter the way actual input is parsed.

</P>
<P>
In this particular example, there is another way to solve the problem:
rewrite the rule for <CODE>return_spec</CODE> to use <CODE>ID</CODE> directly
instead of via <CODE>name</CODE>.  This also causes the two confusing
contexts to have different sets of active rules, because the one for
<CODE>return_spec</CODE> activates the altered rule for <CODE>return_spec</CODE>
rather than the one for <CODE>name</CODE>.

</P>

<PRE>
param_spec:
             type
        |    name_list ':' type
        ;
return_spec:
             type
        |    ID ':' type
        ;
</PRE>



<H2><A NAME="SEC80" HREF="index.html#SEC80">Stack Overflow, and How to Avoid It</A></H2>
<P>
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<A NAME="IDX175"></A>
<A NAME="IDX176"></A>

</P>
<P>
The Bison parser stack can overflow if too many tokens are shifted and
not reduced.  When this happens, the parser function <CODE>yyparse</CODE>
returns a nonzero value, pausing only to call <CODE>yyerror</CODE> to report
the overflow.

</P>
<P>
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By defining the macro <CODE>YYMAXDEPTH</CODE>, you can control how deep the
parser stack can become before a stack overflow occurs.  Define the
macro with a value that is an integer.  This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.
It must be a constant expression whose value is known at compile time.

</P>
<P>
The stack space allowed is not necessarily allocated.  If you specify a
large value for <CODE>YYMAXDEPTH</CODE>, the parser actually allocates a small
stack at first, and then makes it bigger by stages as needed.  This
increasing allocation happens automatically and silently.  Therefore,
you do not need to make <CODE>YYMAXDEPTH</CODE> painfully small merely to save
space for ordinary inputs that do not need much stack.

</P>
<P>
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The default value of <CODE>YYMAXDEPTH</CODE>, if you do not define it, is
10000.

</P>
<P>
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You can control how much stack is allocated initially by defining the
macro <CODE>YYINITDEPTH</CODE>.  This value too must be a compile-time
constant integer.  The default is 200.

</P>
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