F:带有懒惰评价的古怪

给出以下代码:

let bar =
    lazy(
        printfn "bar"        
        ())

let foo = 
    lazy(
        printfn "foo"
        bar)

在交互窗口中(每次重置一次),

当我呼唤 let res = foo.Force () 我得到:

foo
val res : Lazy<unit> = Value is not created.

当我打电话的时候 foo.Force() 我得到:

foo
bar
val it : Lazy<unit> =
  <ToString exception: Object reference not set to an instance of an object.>
    {IsValueCreated = false;
     Value = null;}

在这两种情况下,我都希望它只打印“foo”,但是第二种情况打印“foo\nbar”。这是怎么回事?