Lazy Builder

lazy.fs

[<AutoOpen>]
module LazyBuilder =
  let inline force(x: Lazy<_>) = x.Force()
  type LazyCont<'a> = LazyCont of (unit -> Lazy<'a>)
  type LazyBuilder() =
    member __.Return x = lazy x
    member __.ReturnFrom (x: Lazy<_>) = x
    member __.Bind(Lazy x, f) = f x
    member __.Delay f = LazyCont f
    member __.Run (LazyCont f) = Lazy.Create (f >> force)
  let l = LazyBuilder()

let foo x = lazy x

let a = 
  l {
    printfn "1"
    let! b = foo 1
    printfn "%A" b
    return 1
}