98024d1798
Add Caldera license. Approved by: David Taylor <davidt@caldera.com> Make roughly buildable under FreeBSD. The results are not perfect: the original Makefile referred to a refer file papers/Ind, which doesn't seem to have been kept, so the references to other publications are missing. In addition, the pagination is not correct, with the result that some .DS/.DE blocks leave large amounts of white space empty before them. Possibly this could be fixed by putting the (blank) footnotes at the end. PR: 35345 Requested by: Tony Finch <fanf@dotat.at>
184 lines
7.2 KiB
Plaintext
184 lines
7.2 KiB
Plaintext
.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions are
|
|
.\" met:
|
|
.\"
|
|
.\" Redistributions of source code and documentation must retain the above
|
|
.\" copyright notice, this list of conditions and the following
|
|
.\" disclaimer.
|
|
.\"
|
|
.\" Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\"
|
|
.\" All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\"
|
|
.\" This product includes software developed or owned by Caldera
|
|
.\" International, Inc. Neither the name of Caldera International, Inc.
|
|
.\" nor the names of other contributors may be used to endorse or promote
|
|
.\" products derived from this software without specific prior written
|
|
.\" permission.
|
|
.\"
|
|
.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
|
|
.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
|
|
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
|
|
.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
|
|
.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR
|
|
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
|
|
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
|
|
.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
|
|
.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
|
|
.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
|
|
.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)ss6 8.1 (Berkeley) 6/8/93
|
|
.\"
|
|
.\" $FreeBSD$
|
|
.SH
|
|
6: Precedence
|
|
.PP
|
|
There is one common situation
|
|
where the rules given above for resolving conflicts are not sufficient;
|
|
this is in the parsing of arithmetic expressions.
|
|
Most of the commonly used constructions for arithmetic expressions can be naturally
|
|
described by the notion of
|
|
.I precedence
|
|
levels for operators, together with information about left
|
|
or right associativity.
|
|
It turns out that ambiguous grammars with appropriate disambiguating rules
|
|
can be used to create parsers that are faster and easier to
|
|
write than parsers constructed from unambiguous grammars.
|
|
The basic notion is to write grammar rules
|
|
of the form
|
|
.DS
|
|
expr : expr OP expr
|
|
.DE
|
|
and
|
|
.DS
|
|
expr : UNARY expr
|
|
.DE
|
|
for all binary and unary operators desired.
|
|
This creates a very ambiguous grammar, with many parsing conflicts.
|
|
As disambiguating rules, the user specifies the precedence, or binding
|
|
strength, of all the operators, and the associativity
|
|
of the binary operators.
|
|
This information is sufficient to allow Yacc to resolve the parsing conflicts
|
|
in accordance with these rules, and construct a parser that realizes the desired
|
|
precedences and associativities.
|
|
.PP
|
|
The precedences and associativities are attached to tokens in the declarations section.
|
|
This is done by a series of lines beginning with a Yacc keyword: %left, %right,
|
|
or %nonassoc, followed by a list of tokens.
|
|
All of the tokens on the same line are assumed to have the same precedence level
|
|
and associativity; the lines are listed in
|
|
order of increasing precedence or binding strength.
|
|
Thus,
|
|
.DS
|
|
%left \'+\' \'\-\'
|
|
%left \'*\' \'/\'
|
|
.DE
|
|
describes the precedence and associativity of the four arithmetic operators.
|
|
Plus and minus are left associative, and have lower precedence than
|
|
star and slash, which are also left associative.
|
|
The keyword %right is used to describe right associative operators,
|
|
and the keyword %nonassoc is used to describe operators, like
|
|
the operator .LT. in Fortran, that may not associate with themselves; thus,
|
|
.DS
|
|
A .LT. B .LT. C
|
|
.DE
|
|
is illegal in Fortran, and such an operator would be described with the keyword
|
|
%nonassoc in Yacc.
|
|
As an example of the behavior of these declarations, the description
|
|
.DS
|
|
%right \'=\'
|
|
%left \'+\' \'\-\'
|
|
%left \'*\' \'/\'
|
|
|
|
%%
|
|
|
|
expr : expr \'=\' expr
|
|
| expr \'+\' expr
|
|
| expr \'\-\' expr
|
|
| expr \'*\' expr
|
|
| expr \'/\' expr
|
|
| NAME
|
|
;
|
|
.DE
|
|
might be used to structure the input
|
|
.DS
|
|
a = b = c*d \- e \- f*g
|
|
.DE
|
|
as follows:
|
|
.DS
|
|
a = ( b = ( ((c*d)\-e) \- (f*g) ) )
|
|
.DE
|
|
When this mechanism is used,
|
|
unary operators must, in general, be given a precedence.
|
|
Sometimes a unary operator and a binary operator
|
|
have the same symbolic representation, but different precedences.
|
|
An example is unary and binary \'\-\'; unary minus may be given the same
|
|
strength as multiplication, or even higher, while binary minus has a lower strength than
|
|
multiplication.
|
|
The keyword, %prec, changes the precedence level associated with a particular grammar rule.
|
|
%prec appears immediately after the body of the grammar rule, before the action or closing semicolon,
|
|
and is followed by a token name or literal.
|
|
It
|
|
causes the precedence of the grammar rule to become that of the following token name or literal.
|
|
For example, to make unary minus have the same precedence as multiplication the rules might resemble:
|
|
.DS
|
|
%left \'+\' \'\-\'
|
|
%left \'*\' \'/\'
|
|
|
|
%%
|
|
|
|
expr : expr \'+\' expr
|
|
| expr \'\-\' expr
|
|
| expr \'*\' expr
|
|
| expr \'/\' expr
|
|
| \'\-\' expr %prec \'*\'
|
|
| NAME
|
|
;
|
|
.DE
|
|
.PP
|
|
A token declared
|
|
by %left, %right, and %nonassoc need not be, but may be, declared by %token as well.
|
|
.PP
|
|
The precedences and associativities are used by Yacc to
|
|
resolve parsing conflicts; they give rise to disambiguating rules.
|
|
Formally, the rules work as follows:
|
|
.IP 1.
|
|
The precedences and associativities are recorded for those tokens and literals
|
|
that have them.
|
|
.IP 2.
|
|
A precedence and associativity is associated with each grammar rule; it is the precedence
|
|
and associativity of the last token or literal in the body of the rule.
|
|
If the %prec construction is used, it overrides this default.
|
|
Some grammar rules may have no precedence and associativity associated with them.
|
|
.IP 3.
|
|
When there is a reduce/reduce conflict, or there is a shift/reduce conflict
|
|
and either the input symbol or the grammar rule has no precedence and associativity,
|
|
then the two disambiguating rules given at the beginning of the section are used,
|
|
and the conflicts are reported.
|
|
.IP 4.
|
|
If there is a shift/reduce conflict, and both the grammar rule and the input character
|
|
have precedence and associativity associated with them, then the conflict is resolved
|
|
in favor of the action (shift or reduce) associated with the higher precedence.
|
|
If the precedences are the same, then the associativity is used; left
|
|
associative implies reduce, right associative implies shift, and nonassociating
|
|
implies error.
|
|
.PP
|
|
Conflicts resolved by precedence are not counted in the number of shift/reduce and reduce/reduce
|
|
conflicts reported by Yacc.
|
|
This means that mistakes in the specification of precedences may
|
|
disguise errors in the input grammar; it is a good idea to be sparing
|
|
with precedences, and use them in an essentially ``cookbook'' fashion,
|
|
until some experience has been gained.
|
|
The
|
|
.I y.output
|
|
file
|
|
is very useful in deciding whether the parser is actually doing
|
|
what was intended.
|