Question

I have a factory class that creates objects with circular references. I’d like them to be immutable (in some sense of the word) too. So I use the following technique, using a closure of sorts:

[<AbstractClass>]
type Parent() =
  abstract Children : seq<Child>
and Child(parent) =
  member __.Parent = parent

module Factory =

  let makeParent() =
    let children = ResizeArray()
    let parent = 
      { new Parent() with
        member __.Children = Seq.readonly children }
    [Child(parent); Child(parent); Child(parent)] |> children.AddRange
    parent

I like this better than an internal AddChild method because there’s a stronger guarantee of immutability. Perhaps it’s neurotic, but I prefer closures for access control.

Are there any pitfalls to this design? Are there better, perhaps less cumbersome, ways to do this?

Answer 1

You can use F#’s support for recursive initialization even when creating an instance of abstract class:

let makeParent() =
  let rec children = seq [ Child(parent); Child(parent); Child(parent) ]
  and parent = 
    { new Parent() with
      member __.Children = children }
  parent

When compiling the code, F# uses lazy values, so the value children becomes a lazy value and the property Children accesses the value of this lazy computation. This is fine, because it can first create instance of Parent (referencing the lazy value) and then actually construct the sequence.

Doing the same thing with records wouldn’t work as nicely, because none of the computations would be delayed, but it works quite nicely here, because the sequence is not actually accessed when creating the Parent (if it was a record, this would be a field that would have to be evaluated).

The F# compiler cannot tell (in general) whether this is correct, so it emits a warning that can be disabled using #nowarn "40".

In general, I think that using let rec .. and .. to initialize recursive values is a good thing – it is a bit limited (one of the references must be delayed), but it forces you to keep the recursive references isolated and, I think, it keeps your code simpler.

EDIT To add an example when this may go wrong – if the constructor of Child tries to access the Children collection of its parent, then it forces evaluation of the lazy value before it can be created and you get a runtime error (which is what the warning says). Try adding this to the constructor of Child:

do printfn "%d" (Seq.length parent.Children)