Creating recursive discriminated unions values

Question

When I have this code:

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

The compiler gives the following error:

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.

Why is that? The let rec is supposed to support the creation of recursive values, and something similar works with records.

Answer 1

I don't know why this was changed, but one workaround is to use seq instead of 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