コンピュテーション式を使わない書き方

Json.fs

module Json

open System.Text

[<AbstractClass>]
type JNode() =
    abstract member Stringify: unit -> string

[<AbstractClass>]
type JValue() =
    inherit JNode()

    static member Null: JValue =
        { new JValue() with
            member _.Stringify() = "null" }

let encodeString (value: string) =
    let escaped = value.Replace("\"", "\\\"")
    "\"" + escaped + "\""

type JString(value: string) =
    inherit JValue()
    member _.Value: string = value
    override _.Stringify() = encodeString value

type JBool(value: bool) =
    inherit JValue()
    member _.Value: bool = value
    override _.Stringify() = value.ToString()

type JNumber(value: decimal) =
    inherit JValue()
    member _.Value: decimal = value
    override _.Stringify() = value.ToString()

type JArray(array: JNode array) =
    inherit JNode()

    override _.Stringify() =
        match array with
        | [||] -> "[]"
        | [| x |] -> $"[{x.Stringify()}]"
        | [| x; y |] -> $"[{x.Stringify()}, {y.Stringify()}]"
        | _ ->
            let folder (sb: StringBuilder) (jn: JNode) =
                if sb.Length = 0 then
                    sb.Append(jn.Stringify())
                else
                    sb.Append ", " |> ignore
                    sb.Append(jn.Stringify())

            let sb = Array.fold folder (StringBuilder()) array
            $"[{sb}]"

let encodeProperty (k: string, v: JNode) = $"{encodeString k}: {v.Stringify()}"

type JObject(props: (string * JNode) array) =
    inherit JNode()

    override _.Stringify() =
        match props with
        | [||] -> "{}"
        | [| k, v |] -> $"{{{encodeProperty (k, v)}}}"
        | _ ->
            let folder (sb: StringBuilder) (k, v) =
                if sb.Length = 0 then
                    sb.Append(encodeProperty (k, v))
                else
                    sb.Append ", " |> ignore
                    sb.Append(encodeProperty (k, v))

            let sb = Array.fold folder (StringBuilder()) props
            $"{{{sb}}}"