=====
>>> main.whiley
import string from js::core

type Point is {int x, int y}

function get(bool f) -> (Point|string r)
ensures f ==> (r is string)
ensures !f ==> (r is Point):
    if f:
        return "Hello"
    else:
        return {x:0,y:0}

public export method test():
    Point|string xs = get(true)
    // Must be true
    assert xs is string
    // Let's try again
    Point|string ys = get(false)
    // Must be true
    assert ys is Point
    // Let's try other way
    Point|string zs = get(true)
    // Must not be true
    assert !(zs is Point)
    // Let's try again again
    Point|string ws = get(false)
    // Must not be true
    assert !(ws is string)
>>> js/core.whiley
package js

public type string is int[] where true
---