Custom IEnumerator in F# - f#

Question

More F# questions. I have the implementation of a binary reader below. I want it to work like an enumerable sequence. The code below gives me the following error and I have as usual no clue how to resolve it. I have a c# implementation where I had to implement two different overrides for .Current property. I guess I have to do the same here but not sure how. As always, thanks a million in advance for your help.
error FS0366: No implementation was given for Collections.IEnumerator.get_Current() : obj. Note that all interface members must be implemented and listed under an appropriate interface declaration, e.g. interface ... with member ....
namespace persisitence
open System.Collections.Generic
open System
open System.IO
type BinaryPersistenceIn<'T>(fn: string, serializer: ('T * BinaryReader) -> unit) as this =
let stream_ = File.Open(fn, FileMode.Open, FileAccess.Read)
let reader_ = new BinaryReader(stream_)
[<DefaultValue>] val mutable current_ : 'T
let eof() =
stream_.Position = stream_.Length
interface IEnumerator<'T> with
member this.MoveNext() =
let mutable ret = eof()
if stream_.CanRead && ret then
serializer(this.current_, reader_)
ret
member this.Current
with get() = this.current_
member this.Dispose() =
stream_.Close()
reader_.Close()
member this.Reset() =
stream_.Seek((int64) 0., SeekOrigin.Begin) |> ignore

Answer 1

As #Richard pointed out, you need to implement IEnumerator.Current.
Here's code in response to your question "how to do it". This should work:
A few notes: (thanks to #DaxFohl)
IEnumerator is in different namespace (see code).
MoveNext and Reset are really members of IEnumerator, not IEnumerator<'t>, so that's where they should be implemented.
Dispose, however, is on IEnumerator<'t> (surprise! :-)
-
type BinaryPersistenceIn<'T>(fn: string, serializer: ('T * BinaryReader) -> unit) as this =
...
interface IEnumerator<'T> with
...
member this.Current
with get() = this.current_
interface System.Collections.IEnumerator with
member this.Current
with get() = this.current_ :> obj
member this.MoveNext() = ...
member this.Reset() = ...
And in conclusion, I must add this: are you really sure you want to implement IEnumerator? This is a rather low-lever thing, easy to get wrong. Why not use a sequence computation expression instead?
let binaryPersistenceSeq (fn: string) (serializer: BinaryReader -> 'T) =
seq {
use stream_ = File.Open(fn, FileMode.Open, FileAccess.Read)
use reader_ = new BinaryReader(stream_)
let eof() = stream_.Position = stream_.Length
while not eof() do
if stream_.CanRead then
yield serializer reader_
}

Answer 2

IEnumerator<T> extends IEnumerator and IEnumerator has a Current property of type object.
You need to also implement IEnumerator.Current separately from IEnumerator<T>.Current.

Answer 3

This version of the code compiles... as to does it really work.. will find out.
type BinaryPersistenceIn<'T>(fn: string, serializer: ('T * BinaryReader) -> unit) =
let stream_ = File.Open(fn, FileMode.Open, FileAccess.Read)
let reader_ = new BinaryReader(stream_)
[<DefaultValue>] val mutable current_ : 'T
let eof() =
stream_.Position = stream_.Length
interface IEnumerator<'T> with
member this.MoveNext() =
let mutable ret = eof()
if stream_.CanRead && ret then
serializer(this.current_, reader_)
ret
member this.Current
with get() = this.current_
member this.Dispose() =
stream_.Close()
reader_.Close()
member this.Reset() =
stream_.Seek((int64) 0., SeekOrigin.Begin) |> ignore
member this.Current
with get() = this.current_ :> obj