lazylist.fs

namespace Collections

open System
open System.Collections

[<AbstractClass>]
type LazyList<'T>() =
    static let notimpl () = raise $ NotImplementedException()

    abstract member Item: int -> 'T
    abstract member Count: int

    interface IList with
        member this.Count = this.Count
        member this.Item
            with get index = box $ this.Item index
            and set index value = notimpl ()
        member this.IsReadOnly = true
        member this.IsFixedSize = false

        member this.GetEnumerator() =
            let index = ref -1
            { new IEnumerator with
                member e.Current = box $ this.Item !index
                member e.MoveNext () = incr index; !index < this.Count
                member e.Reset () = index := 0
            }

        member this.SyncRoot = notimpl ()
        member this.IsSynchronized = true

        member this.IndexOf(item) = -1
        member this.Contains(item) = false

        member this.Add(item) = notimpl ()
        member this.Clear() = notimpl ()
        member this.CopyTo(array, arrayIndex) = notimpl ()
        member this.Insert(index, item) = notimpl ()
        member this.Remove(item) = notimpl ()
        member this.RemoveAt(index) = notimpl ()