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✏️ Editing: demo_calc.pl

use v5.10;
use warnings;

use Parse::RecDescent;

sub evalop
{
    my (@list) = @{$_[0]};
    my $val = shift(@list)->();
    while (@list)
    {
        my ($op, $arg2) = splice @list, 0, 2;
        $op->($val,$arg2->());
    }
    return $val;
}

my $parse = Parse::RecDescent->new(<<'EndGrammar');

<trace_parse>

main: expr /\s*\Z/ { $item[1]->() }
        | <error>

expr: /for(each)?/ lvar range expr
            { my ($vname,$expr) = @item[2,4];
              my ($from, $to) = @{$item[3]};
              sub { my $val;
                no strict "refs";
                for $$vname ($from->()..$to->())
                    { $val = $expr->() }
                return $val;
                  }
            }
        | lvar '=' addition
            { my ($vname, $expr) = @item[1,3];
              sub { no strict 'refs'; $$vname = $expr->() }
            }
        | addition

range: "(" expr ".." expr ")"
            { [ @item[2,4] ] }

addition: <leftop:multiplication add_op multiplication>
            { my $add = $item[1]; sub { ::evalop $add } }

add_op: '+' { sub { $_[0] += $_[1] } }
          | '-' { sub { $_[0] -= $_[1] } }

multiplication: <leftop:factor mult_op factor>
            { my $mult = $item[1]; sub { ::evalop $mult } }

mult_op: '*'    { sub { $_[0] *= $_[1] } }
           | '/'    { sub { $_[0] /= $_[1] } }

factor: number
          | rvar
          | '(' expr ')' { $item[2] }

number: /[-+]?\d+(\.\d+)?/  { sub { $item[1] } }

lvar:   /\$([a-z]\w*)/      { $1 }

rvar:   lvar            { sub { no strict 'refs'; ${$item[1]} } }

EndGrammar

print "> ";
while (<>) {    # FOR DEMO CHANGE TO: while (<DATA>)
  print $parse->main($_), "\n\n> ";
}

__DATA__
$x = 2
$y = 3
+1-1+1-1+1-1+1-1+1
7*7-6*8
121/(121/11)/121*11
1/(10-1/(1/(10-1)))
$x * $y
foreach $i (1..$y) $x = $x * 2 + $i
$x

MR SKB