Question

Is it possible to implement friend function and friend class (as in c++) in F#?

Update:
Since there is no friend function/class in f#, and friend is not even a reserved keyword for future expansion, I’m wondering is there any problems with the friend mechanism in F# that make the developers to decide not to implement it?
(such as in “protected” access modifier).

Suggestion 1: Brian, signature file –
I don’t think that this thing work properly. If you have a closure (e.g. lambda expression in A, which is a different object than the instance of A) that evaluate B.X , it’s won’t work

Suggestion 2: Massif (+Mitya0), InternalsVisibleTo –
It’s not clear to me, Are you writing this in the second class or it expose the class to the entire assembly?

Answer 1

Note that you can use signature files to mimic friend.

If you want A to be a friend of B, so A can access internals of B that others cannot see, you can do e.g.

// File1.fs
type B() =
    let x = 42  // private field
    member this.X = x // public getter

type A() =
    member this.PeekInto(b : B) =
        b.X

but also have

// File1.fsi
type B = 
    new : unit -> B
    // do not expose X in the signature

type A = 
    new : unit -> A
    PeekInto : B -> int

and now A‘s implementation can see B.X but the sequel of the program cannot see B.X.

Signature files are pretty awesome for creating arbitrary encapsulation boundaries.