Calc.yp

#
# Calc.yp
# 
# Parse::Yapp input grammar example.
#
# This file is PUBLIC DOMAIN 
#
#
%right  '='
%left   '-' '+'
%left   '*' '/'
%left   NEG
%right  '^'

%%
input:  #empty
        |   input line  { $_[1][] = $_[2];
                    return $_[1]; }
;

line:       '\n'
        |   exp '\n'            { print "$_[1]\n" }
		|	error '\n' { $this->YYErrok() }
;

exp:        NUM
        |   VAR                 { $this->YYData->VARS->{$_[1]} = null }
        |   VAR '=' exp         { $this->YYData->VARS->{$_[1]} = $_[3] }
        |   exp '+' exp         { return $_[1] + $_[3] }
        |   exp '-' exp         { return $_[1] - $_[3] }
        |   exp '*' exp         { return $_[1] * $_[3] }
        |   exp '/' exp         {
                                    if ($_[3]) {
                                        return $_[1] / $_[3];
                                    }

                                    $this->YYData->ERRMSG = "Illegal division by zero.\n";
                                    $this->YYError();
                                }
        |   '-' exp %prec NEG   { return -$_[2] }
        |   exp '^' exp         { return $_[1] ** $_[3] }
        |   '(' exp ')'         { return $_[2] }
;

%%

public function _Error()
{
    if (!isset($this->YYData->ERRMSG)) {
        print "Syntax error.\n";
    } else {
        echo $this->YYData->ERRMSG, "\n";
        unset($this->YYData->ERRMSG);
    }
}

public function _Lexer(MyParser $parser)
{
    return ['', undef];
}

public function Run()
{
    return $this->YYParse(['yylex' => [$this, '_Lexer'], 'yyerror' => [$this, '_Error']]);
}