These concerns do not lessen what can be gleaned from this well-written, if occasionally overwritten, book, and the numerous print errors are small annoyances in a text printed relatively affordably.
In the case of an updatable closure, its info pointer may now be overwritten with a 'black hole' info pointer or, in a parallel system, a 'queue me' info pointer.
Therefore, it can be asynchronous and the information in it is nonconsumable and can be overwritten.
For files used for writing, things are worse since if parts of a file were overwritten this would violate the property of referential transparency.
The remaining ingredient is a way of making sure that the custom verification code of the abstraction will never allow the stack to be overwritten.
Note how each of these expressions has been overwritten by the value obtained by evaluating it.
When the value of the variable is required, the closure to which it points is evaluated, and the closure is overwritten with the resulting value.
A cell may only be overwritten with a value of its assigned type.
