%right  '='
%left   '-' '+'
%left   '*' '/'
%left   NEG

%tree

%%
input:                
        |   input $line  { print $line->str."\n" }
;

line:     '\n'      
        | exp '\n'   
        | error '\n'  
;

exp:        NUM             
        |   VAR              
        |   VAR '=' exp         
        |   exp '+' exp         
        |   exp '-' exp        
        |   exp '*' exp       
        |   exp '/' exp      
        |   '-' exp %prec NEG 
        |   '(' exp ')'       
;

%%

sub _Lexer {
    my($parser)=shift;

    if ($parser->YYEndOfInput) {
      my $input = <STDIN>;
      return('', undef) unless $input;
      $parser->input($input);
    };

    for (${$parser->YYInput}) {
        m/\G[ \t]*/gc;
        m/\G([0-9]+(?:\.[0-9]+)?)/gc and return('NUM',$1);
        m/\G([A-Za-z][A-Za-z0-9_]*)/gc and return('VAR',$1);
        m/\G(.)/gcs and return($1,$1);
        return('', undef);
    }
}

__PACKAGE__->lexer(\&_Lexer);