expression2.y

语法分析 语法分析

%{
{*
   计算简单的数学表达式的yacc程序
   编译：yacc Expression ExpressionY
 *}
unit ExpressionY2;

interface
uses
  SysUtils, LexLib, YaccLib, ExpressionL2;

{指定不同于整数的类型时，一定要使用%type <Real> expression 指定表达式的类型}
{同时要使用 %token <Real> Number来指定token的类型}
%}

%token NAME
%token <Real> NUMBER
%type <Real> expression 

%left '-' '+'
%left '*' '/'
%nonassoc UMINUS

%%

statement:	NAME '=' expression
	|	expression		{ writeln(format('= %f', [$1])); }
	;

expression:	expression '+' expression { $$ := $1 + $3; }
	|	expression '-' expression { $$ := $1 - $3; }
	|	expression '*' expression { $$ := $1 * $3; }
	|	expression '/' expression
				{	if($3 = 0) then
						yyerror('除零错误')
					else
						$$ := $1 / $3;
				}
	|	'-' expression %prec UMINUS	{ $$ := -$2; }
	|	'(' expression ')'	{ $$ := $2; }
	|	NUMBER			{ $$ := $1; }
	;
%%
end.