为什么不允许F#记录具有AllowNullLiteralAttribute?

记录不能具有allownullliteraltribute属性是否有编译器实现的原因,或者这是一个选定的约束?

我确实有时会看到这种约束强制清理代码,但并不总是这样。

[<AllowNullLiteralAttribute>]
type IBTreeNode = {
    mutable left : IBTreeNode; mutable right : IBTreeNode; mutable value : int}
with
    member this.Insert x =
        if x < this.value then
            if this.left = null then
                this.left <- {left = null; right = null; value = x}
            else
                this.left.Insert x
        else
            if this.right = null then
                this.right <- {left = null; right = null; value = x}
            else
                this.right.Insert x

// Would be a lot of boilerplate if I wanted these public
[<AllowNullLiteralAttribute>]
type CBTreeNode(value) = 
    let mutable left = null
    let mutable right = null
    let mutable value = value
with
    member this.Insert x =
        if x < value then
            if left = null then
                left <- CBTreeNode(x)
            else
                left.Insert x
        else
            if right = null then
                right <- CBTreeNode(x)
            else
                right.Insert x

添加了一个不可变的版本对不赞成可变人群。在这种情况下大约快30%。

type OBTree =
    | Node of OBTree * OBTree * int
    | Null
with
    member this.Insert x =
        match this with
        | Node(left, right, value) when x <= value -> Node(left.Insert x, right, value)
        | Node(left, right, value) -> Node(left, right.Insert x, value)
        | Null -> Node(Null, Null, x)