How to declare a reference to a empty stack in OCaml?

Question

perhaps I am being stupid here, so the more general question I want to ask is that how to declare a reference to a empty value of certain type in OCaml. Usually I declare a reference to a custom defined empty value, for example if I have a type type point = Point of (int * int) I would declare a reference like this let a = ref (Point (0,0)). However this is not satisfying because I have to ''come up" with a empty value myself. Also, if you look at the reference ``Stack'' module in the standard library (http://caml.inria.fr/pub/docs/manual-ocaml/libref/Stack.html), there is no empty value in it. How do you deal with this?

Answer 1

If you really want it, you can do this:

let sr = ref (Stack.create ())
let () = Stack.push 42 !sr

OCaml's value restriction doesn't prevent it, it will initially have type '_a Stack.t ref, until you push into it.

But you hardly need it, Stack is already mutable.

Answer 2

好吧，堆栈有一个空值，因为Stack.create ()将创建一个空堆栈。关于您的一般问题，通常None被用作空值。当然，这会自动将您的价值提升到选项中。但这是有意的，因为如果您将值创建为空，然后您将通过引用更新它而没有类型保证，那么您将更新它。

实施例

let p0 = ref None in
...
p0 := Some (Point (0,0));
...