Object expression, interface implementation, detailed syntax

Question

Object expression, interface implementation, detailed syntax

I often use verbose syntax to create single-line expressions that usually have multiple lines. I usually do this, so I can send the whole expression to F # interactive with Alt+'. For example, I can do this:

let longLength list = let rec helper result = function [] -> result | _ :: t -> helper (result + 1L) t in helper 0L list

In the sample code, suppose these namespaces were open:

open System
open System.Collections
open System.Collections.Generic

I want to translate this object expression into one line:

let emptyEnumerator = {
    new IEnumerator<int> with member t.Current = raise(InvalidOperationException())
    interface IEnumerator with
        member t.MoveNext() = false
        member t.Reset() = ()
        member t.Current = raise(InvalidOperationException())
    interface IDisposable with member t.Dispose() = ()
    }

I found that I can only do this using the detailed syntax. Typically, verbose syntax code also compiles with lightweight syntax, but it is not here. I also tried to create a named class with the same implementation and that almost identical code compiles with both easy and detailed syntax:

open System
open System.Collections
open System.Collections.Generic

#light "off"
type A() = class interface IEnumerator<int> with member t.Current = raise(InvalidOperationException()) end interface IEnumerator with member t.MoveNext() = false member t.Reset() = () member t.Current = raise(InvalidOperationException()) end interface IDisposable with member t.Dispose() = () end end
let b = { new IEnumerator<int> with member t.Current = raise(InvalidOperationException()) end interface IEnumerator with member t.MoveNext() = false member t.Reset() = () member t.Current = raise(InvalidOperationException()) end interface IDisposable with member t.Dispose() = () end end }

#light "on"
type C() = class interface IEnumerator<int> with member t.Current = raise(InvalidOperationException()) end interface IEnumerator with member t.MoveNext() = false member t.Reset() = () member t.Current = raise(InvalidOperationException()) end interface IDisposable with member t.Dispose() = () end end
let d = { new IEnumerator<int> with member t.Current = raise(InvalidOperationException()) end interface IEnumerator with member t.MoveNext() = false member t.Reset() = () member t.Current = raise(InvalidOperationException()) end interface IDisposable with member t.Dispose() = () end end }