コレクション初期化子もどき

Program.fs

open System.Collections.Generic

type CollectionBuilder< 'TCollection, 'TElem, 'TRet when 'TCollection : (member Add : 'TElem -> 'TRet)> () =
  [<DefaultValue>] val mutable xs : 'TCollection
  member inline this.Yield(x: 'TElem) =
    (^TCollection : (member Add : 'TElem -> 'TRet) (this.xs, x)) |> ignore
  member inline this.Delay(f) = f
  member inline this.Combine(_, rest) = rest ()
  member inline this.Run(f) = f (); this.xs
  member inline this.Zero() = ()

let inline collection (xs : 'TCollection when 'TCollection : (member Add : 'TElem -> 'TRet)) =
  CollectionBuilder(xs=xs)

type Collection2Builder< ^TCollection, 'TElem1, 'TElem2, 'TRet when 'TCollection : (member Add : 'TElem1 * 'TElem2 -> 'TRet)> () =
  [<DefaultValue>] val mutable xs : 'TCollection
  member inline this.Yield(x: 'TElem1 * 'TElem2) =
    let x1, x2 = x
    (^TCollection : (member Add : 'TElem1 * 'TElem2 -> 'TRet) (this.xs, x1, x2)) |> ignore
  member inline this.Delay(f) = f
  member inline this.Combine(_, rest) = rest ()
  member inline this.Run(f) = f (); this.xs
  member inline this.Zero() = ()

let inline collection2 (xs : 'TCollection when 'TCollection : (member Add : 'TElem1 * 'TElem2 -> 'TRet)) =
  Collection2Builder(xs=xs)

[<EntryPoint>]
let main argv =
  let d = collection2 (Dictionary()) {
    yield "hoge", 4
    yield "foo", 3
  }
  printfn "%A" d
  0