%{
=head1 SYNOPSIS

This example shows how the dynamic conflict resolution
technique makes possible to change the behavior of the parser
at the programmer's command.

Compile it with:

    eyapp -C dynamicgrammar.eyp 

Run with:

    $ ./dynamicgrammar.pm -f input_for_dynamicgrammar.txt 

The file C<input_for_dynamicgrammar.txt> contains:

            2-1-1 # left: 0
            RIGHT
            2-1-1 # right: 2
            LEFT
            3-1-1 # left: 1
            RIGHT
            3-1-1 # right: 3

=head1 SEE ALSO

File C<examples/debuggingtut/dynamicgrammar0.eyp> is similar
to this grammar, but the shift-reduce conflict is left unsolved

=cut

my $reduce = 1;
%}

%whites /(\s*(?:#.*)?\s*)/
%token NUM = /(\d+)/

%conflict leftORright {
  # reduce if $reduce and next token is '-'
  if ($reduce && m{\G(?=-)}gc) 
       { $self->YYSetReduce('-', ':M' ); }
  else { $self->YYSetShift('-'); }
}

%expect 1

%%
p: c * {} ; 

c:
      $expr { print "$expr\n" }
    | RIGHT { $reduce = 0}
    | LEFT  { $reduce = 1}

;

expr: 
      '(' $expr ')'  { $expr } 
    | %name :M
      expr.left                         %PREC leftORright
                '-' expr.right          %PREC leftORright
         { $left - $right }

    | NUM
;

%%