Intel XENIX 286 Programmers Guide (86) - Tenox.tc

XENIX Programming yacc: Compiler-Compiler
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 96nonassoc is used to describe operators, like the operator
.LT. in FORTRAN, that may not associate with themselves; thus
A .LT. B . LT . C
is illegal in FORTRAN, and such an operator would be described with the keyword
96nonassoc in yacc. As an example of the behavior of these declarations, the description
%right I I
=
%left I +
I l_l
%left 1 * 1 1 1 1
%%
expr : expr
I expr
I expr
I expr
I expr
j NAME
I I
=
I + I
I I
-
I * I
I
I I
expr
expr
expr
expr
expr
might be used to structure the input
as follows:
a = b = c*d - e-f*g
a = ( b = ( ((c*d)-e) - (f*g) ) )
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 96prec changes the
precedence level associated with a particular gram mar rule. The 96prec keyword
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:
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