F＃：静态let和静态成员

Question

I'm a noob in F# and currently reading Expert in F# 3.0. Its the first compiled language I'm learning (I just know to program in R)

on chapter 6, p.117, we are introduced without much ceremony static let and static member. i don't really understand what it is

type Vector2D(dx : float, dy : float) =
     static let zero = Vector2D(0.0, 0.0)
     static let onex = Vector2D(1.0, 0.0)
     static let oney = Vector2D(0.0, 1.0)
     /// Get the zero vector
     static member Zero = zero
     /// Get a constant vector along the X axis of length one
     static member OneX = onex
     /// Get a constant vector along the Y axis of length one
     static member OneY = oney 

There is no example who to proceed further in the book.

I'm typing this in F# interactive. From there, how do I construct a variable x whose value is zero (or onex...)?

I'm trying the following. Nothing works

let x = Zero;;
let y = Vector2D(2.0,2.0);; /// ok
y.Zero;;
stdin(237,1): error FS0809: Property 'Zero' is static

in http://fsharpforfunandprofit.com/posts/classes/ there is this example,

type StaticExample() = 
    member this.InstanceValue = 1
    static member StaticValue = 2  // no "this"

// test
let instance = new StaticExample()
printf "%i" instance.InstanceValue
printf "%i" StaticExample.StaticValue

so i would have expected y.Zero;; to yield something above ?...

thanks. sorry the question is so basic. If someone can explain me what it is about...

Answer 1

So the basic difference is that static members don't belong to an instance of a class.

rather than y.Zero , you get Vector2D.Zero .

To a first approximation you can think of these examples of properties of the type.

Answer 2

static members belong to the type on which they are declared and are accessed by the fully qualified type and member name. In this instance

let zero = Vector2D.Zero
let oneX = Vector2D.OneX
let oneY = Vector2D.OneY

The Vector2D type does not currently have any instance members, but if it did, these would be accessible on an instance of Vector2D