With pattern matching, the term on the left is a destructuring of the term on the right. Using a static instance like that isn't what you normally do in practice, but rather something like:

    myfoo = Foo(4, Bar(3, 8))
    with patterns:
        Foo(x, Bar(3, z)) << myfoo
        

So in this case, the assertion will only pass if the Foo contains a Bar in the second slot, and that Bar contains a 3 in its first slot, while also binding the 4 and 8 to their own names, x and z respectively.