Re: Tales from writing a RPN evaluator in Perl 5, Perl 6 and Haskell

[Anonymous Monk]

Here's a Ruby version:

def evaluate expr
    stack = []
    expr.split(/\s+/).each do |token|
        stack << 
            case token
            when /^-?\d*$/
                token.to_i
            when /^[+\-*\/]$/
                right, left = stack.pop, stack.pop
                raise "Stack underflow" unless left and right
                left.send token, right
            else
                raise "Invalid token: #{token}"
            end
        
    end
    stack.length == 1 or raise "Invalid stack: #{stack}"
    stack[0]
end

[audreyt]

That's pretty nice... Here is a straight port from Ruby to Perl6:

sub evaluate ($expr) {
    my @stack;
    for $expr.split {
        @stack.push: do {
            when /^ -?\d* $/ { $_ }
            when <+ - * /> {
                my $right = @stack.pop;
                my $left  = @stack.pop;
                defined($left & $right) or fail "Stack underflow";
                $left.$_: $right;
            }
            fail "Invalid token: $_";
        }
    }
    @stack == 1 or fail "Invalid stack: @stack";
    @stack[0];
}