module Rpn-0.0.1-cpan:ASAVIGE;

sub evaluate (Str $expr) returns Int {
    my @stack;
    for ($expr.split()) -> $tok {
        if $tok ~~ rx:Perl5/^-?\d+$/ {
            @stack.push($tok);
            next;
        }
        my $x = @stack.pop() err die "Stack underflow\n";
        my $y = @stack.pop() err die "Stack underflow\n";
        given $tok {
            when '+' { @stack.push($y + $x) }
            when '-' { @stack.push($y - $x) }
            when '*' { @stack.push($y * $x) }
            when '/' { @stack.push(int($y / $x)) }
            default  { die "Invalid token:\"$tok\"\n" }
        }
    }
    @stack.elems == 1 or die "Invalid stack:[@stack[]]\n";
    return @stack[0];
}