macro_rules! while_iter_ok {
    ($it:expr, |$x:pat| $f:expr) => {
        for x in $it {
            let $x = try!(x);
            $f;
        }
    };

    ($it:expr, |$x:ident : $t:ty| $f:expr) => {
        for x in $it {
            let $x: $t = try!(x);
            $f;
        }
    };
}