type Cell<'T>(initial: 'T) =
    let mutable value = initial
    let mutable listeners : (unit -> unit) list = []  
    member _.Value = value
    member _.Set v =
        value <- v
        for l in listeners do l()
    
    member _.Subscribe(callback: unit -> unit) =
        listeners <- callback :: listeners

type RuleBuilder() =
    member _.Bind(cell: Cell<'T>, f: 'T -> unit) =
        f cell.Value
        cell.Subscribe(fun () -> f cell.Value)
    member _.Return(x) = x
    member _.Delay(f: unit -> unit) = f()
    member _.Zero() = () 

let rule = RuleBuilder()

// Define some reactive cells.
let a = Cell(3)
let b = Cell(4)
let sum = Cell(0)

// Create a reactive rule that computes the sum of a and b.
rule {
    let! va = a
    let! vb = b
    sum.Set (va + vb)
}

printfn "Initial sum: %d" sum.Value

// Changing 'a' will automatically update the computed sum.
a.Set 10
printfn "Updated sum: %d" sum.Value