recursion - 创建递归可区分联合值

Question

当我有这个代码时:

type HtmlNode = 
    | HtmlElement of name:string * attribute:HtmlAttribute list
    | HtmlText of content:string

and HtmlAttribute =  
    | HtmlAttribute of name:string * value:string * parent:HtmlNode

let createElement name attrs =
    let toAttributes element = [ for name, value in attrs -> HtmlAttribute(name, value, element)]
    let rec element = HtmlElement(name, attributes)
    and attributes = toAttributes element
    element

编译器给出以下错误:

Recursive values cannot appear directly as a construction of the type 'HtmlNode' within a recursive binding. This feature has been removed from the F# language. Consider using a record instead.

这是为什么？ let rec 应该支持递归值的创建，类似的东西也适用于记录。

Answer 1

我不知道为什么会这样改变，但一种解决方法是使用 seq而不是 list .

type HtmlNode = 
    | HtmlElement of name:string * attribute:HtmlAttribute seq
    | HtmlText of content:string

and HtmlAttribute =  
    | HtmlAttribute of name:string * value:string * parent:HtmlNode

let createElement name attrs =
    let rec element = HtmlElement(name, attributes)
    and attributes = seq { for name, value in attrs -> HtmlAttribute(name, value, element) }
    element