Shoelace formula for polygonal area

func area_by_shoelace (*p) {
    var x = p.map{_[0]}
    var y = p.map{_[1]}

    var s = (
        (x ~Z* y.rotate(+1)).sum -
        (x ~Z* y.rotate(-1)).sum
    )

    s.abs / 2
}

say area_by_shoelace([3,4], [5,11], [12,8], [9,5], [5,6])

Output:

30