X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/c85be41a072e4edd6b1669498c7fe6d11789796c..d78f0ac9d8c8830542faf9d00d0b6ef652dda45e:/doc/bison.texinfo diff --git a/doc/bison.texinfo b/doc/bison.texinfo index 09ca7ab1..ebb37ca6 100644 --- a/doc/bison.texinfo +++ b/doc/bison.texinfo @@ -264,7 +264,8 @@ The Bison Parser Algorithm Operator Precedence * Why Precedence:: An example showing why precedence is needed. -* Using Precedence:: How to specify precedence in Bison grammars. +* Using Precedence:: How to specify precedence and associativity. +* Precedence Only:: How to specify precedence only. * Precedence Examples:: How these features are used in the previous example. * How Precedence:: How they work. @@ -1858,8 +1859,8 @@ parentheses nested to arbitrary depth. Here is the Bison code for %token NUM %left '-' '+' %left '*' '/' -%left NEG /* negation--unary minus */ -%right '^' /* exponentiation */ +%precedence NEG /* negation--unary minus */ +%right '^' /* exponentiation */ %% /* The grammar follows. */ input: /* empty */ @@ -1892,15 +1893,16 @@ In the second section (Bison declarations), @code{%left} declares token types and says they are left-associative operators. The declarations @code{%left} and @code{%right} (right associativity) take the place of @code{%token} which is used to declare a token type name without -associativity. (These tokens are single-character literals, which +associativity/precedence. (These tokens are single-character literals, which ordinarily don't need to be declared. We declare them here to specify -the associativity.) +the associativity/precedence.) Operator precedence is determined by the line ordering of the declarations; the higher the line number of the declaration (lower on the page or screen), the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (@code{NEG}) is next, followed -by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator +by @samp{*} and @samp{/}, and so on. Unary minus is not associative, +only precedence matters (@code{%precedence}. @xref{Precedence, ,Operator Precedence}. The other important new feature is the @code{%prec} in the grammar @@ -2003,7 +2005,7 @@ the same as the declarations for the infix notation calculator. %left '-' '+' %left '*' '/' -%left NEG +%precedence NEG %right '^' %% /* The grammar follows. */ @@ -2251,8 +2253,8 @@ Here are the C and Bison declarations for the multi-function calculator. %right '=' %left '-' '+' %left '*' '/' -%left NEG /* negation--unary minus */ -%right '^' /* exponentiation */ +%precedence NEG /* negation--unary minus */ +%right '^' /* exponentiation */ @end group %% /* The grammar follows. */ @end smallexample @@ -4019,7 +4021,8 @@ Bison will convert this into a @code{#define} directive in the parser, so that the function @code{yylex} (if it is in this file) can use the name @var{name} to stand for this token type's code. -Alternatively, you can use @code{%left}, @code{%right}, or +Alternatively, you can use @code{%left}, @code{%right}, +@code{%precedence}, or @code{%nonassoc} instead of @code{%token}, if you wish to specify associativity and precedence. @xref{Precedence Decl, ,Operator Precedence}. @@ -4095,7 +4098,8 @@ of ``$end'': @cindex declaring operator precedence @cindex operator precedence, declaring -Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to +Use the @code{%left}, @code{%right}, @code{%nonassoc}, or +@code{%precedence} declaration to declare a token and specify its precedence and associativity, all at once. These are called @dfn{precedence declarations}. @xref{Precedence, ,Operator Precedence}, for general information on @@ -4131,6 +4135,10 @@ left-associativity (grouping @var{x} with @var{y} first) and means that @samp{@var{x} @var{op} @var{y} @var{op} @var{z}} is considered a syntax error. +@code{%precedence} gives only precedence to the @var{symbols}, and +defines no associativity at all. Use this to define precedence only, +and leave any potential conflict due to associativity enabled. + @item The precedence of an operator determines how it nests with other operators. All the tokens declared in a single precedence declaration have equal @@ -6221,7 +6229,8 @@ shift and when to reduce. @menu * Why Precedence:: An example showing why precedence is needed. -* Using Precedence:: How to specify precedence in Bison grammars. +* Using Precedence:: How to specify precedence and associativity. +* Precedence Only:: How to specify precedence only. * Precedence Examples:: How these features are used in the previous example. * How Precedence:: How they work. @end menu @@ -6276,8 +6285,9 @@ makes right-associativity. @node Using Precedence @subsection Specifying Operator Precedence @findex %left -@findex %right @findex %nonassoc +@findex %precedence +@findex %right Bison allows you to specify these choices with the operator precedence declarations @code{%left} and @code{%right}. Each such declaration @@ -6287,13 +6297,63 @@ those operators left-associative and the @code{%right} declaration makes them right-associative. A third alternative is @code{%nonassoc}, which declares that it is a syntax error to find the same operator twice ``in a row''. +The last alternative, @code{%precedence}, allows to define only +precedence and no associativity at all. As a result, any +associativity-related conflict that remains will be reported as an +compile-time error. The directive @code{%nonassoc} creates run-time +error: using the operator in a associative way is a syntax error. The +directive @code{%precedence} creates compile-time errors: an operator +@emph{can} be involved in an associativity-related conflict, contrary to +what expected the grammar author. The relative precedence of different operators is controlled by the -order in which they are declared. The first @code{%left} or -@code{%right} declaration in the file declares the operators whose +order in which they are declared. The first precedence/associativity +declaration in the file declares the operators whose precedence is lowest, the next such declaration declares the operators whose precedence is a little higher, and so on. +@node Precedence Only +@subsection Specifying Precedence Only +@findex %precedence + +Since @acronym{POSIX} Yacc defines only @code{%left}, @code{%right}, and +@code{%nonassoc}, which all defines precedence and associativity, little +attention is paid to the fact that precedence cannot be defined without +defining associativity. Yet, sometimes, when trying to solve a +conflict, precedence suffices. In such a case, using @code{%left}, +@code{%right}, or @code{%nonassoc} might hide future (associativity +related) conflicts that would remain hidden. + +The dangling @code{else} ambiguity (@pxref{Shift/Reduce, , Shift/Reduce +Conflicts}) can be solved explictly. This shift/reduce conflicts occurs +in the following situation, where the period denotes the current parsing +state: + +@example +if @var{e1} then if @var{e2} then @var{s1} . else @var{s2} +@end example + +The conflict involves the reduction of the rule @samp{IF expr THEN +stmt}, which precedence is by default that of its last token +(@code{THEN}), and the shifting of the token @code{ELSE}. The usual +disambiguation (attach the @code{else} to the closest @code{if}), +shifting must be preferred, i.e., the precedence of @code{ELSE} must be +higher than that of @code{THEN}. But neither is expected to be involved +in an associativity related conflict, which can be specified as follows. + +@example +%precedence THEN +%precedence ELSE +@end example + +The unary-minus is another typical example where associativity is +usually over-specified, see @ref{Infix Calc, , Infix Notation +Calculator: @code{calc}}. The @code{%left} directive is traditionaly +used to declare the precedence of @code{NEG}, which is more than needed +since it also defines its associativity. While this is harmless in the +traditional example, who knows how @code{NEG} might be used in future +evolutions of the grammar@dots{} + @node Precedence Examples @subsection Precedence Examples @@ -6355,8 +6415,8 @@ 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. -The Bison precedence declarations, @code{%left}, @code{%right} and -@code{%nonassoc}, can only be used once for a given token; so a token has +The Bison precedence declarations +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} modifier for rules. @@ -9865,7 +9925,7 @@ Specify the programming language for the generated parser. @end deffn @deffn {Directive} %left -Bison declaration to assign left associativity to token(s). +Bison declaration to assign precedence and left associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @end deffn @@ -9900,7 +9960,7 @@ parser file. @xref{Decl Summary}. @end deffn @deffn {Directive} %nonassoc -Bison declaration to assign nonassociativity to token(s). +Bison declaration to assign precedence and nonassociativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @end deffn @@ -9920,6 +9980,11 @@ Bison declaration to assign a precedence to a specific rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}. @end deffn +@deffn {Directive} %precedence +Bison declaration to assign precedence to token(s), but no associativity +@xref{Precedence Decl, ,Operator Precedence}. +@end deffn + @deffn {Directive} %pure-parser Deprecated version of @code{%define api.pure} (@pxref{Decl Summary, ,%define}), for which Bison is more careful to warn about unreasonable usage. @@ -9931,7 +9996,7 @@ Require a Version of Bison}. @end deffn @deffn {Directive} %right -Bison declaration to assign right associativity to token(s). +Bison declaration to assign precedence and right associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @end deffn