JsonClasses.fs

type Json =
    | Array of list<Json>
    | Double of double
    | False
    | Integer of int64
    | Null
    | Object of list<string*Json>
    | String of string
    | True

    static member inline ToJson x : Json =
        ((^a or ^b) :
            (static member ToJson : ^a * ^b -> Json) (Null, x))

    static member ToJson(_: Json, x: int) = Integer (int64 x)
    static member ToJson(_: Json, x: string) = String x
    static member inline ToJson(_: Json, x: list<_>) =
        Array (List.map Json.ToJson x)

    static member inline FromJson(x: Json) =
        ((^a or ^b) : (static member FromJson : ^a * ^b -> ^a)
            (Unchecked.defaultof<_>, x))

    static member FromJson(_: int, x: Json) =
        match x with
        | Integer x -> int x
        | _ -> invalidArg "x" "Conversion failed."

    static member FromJson(_: string, x: Json) =
        match x with
        | String x -> x
        | _ -> invalidArg "x" "Conversion failed."

    static member inline FromJson(_: list<_>, x: Json) =
        match x with
        | Array xs -> [for x in xs -> Json.FromJson x]
        | _ -> invalidArg "x" "Conversion failed."

type Person =
    {
        age: int
        name: string
    }

    static member ToJson(_: Json, x: Person) =
        Object [
            "age", Integer (int64 x.age);
            "name", String x.name
        ]

    static member FromJson(_:Person, j: Json) =
        match j with
        | Object ["age", Integer age; "name", String name] ->
            { age = int age; name = name }
        | _ ->
            failwith "Conversion failed."

let test =
    let p = {name="Vladimir Putin"; age=59}
    let r : list<Person> = Json.FromJson(Json.ToJson [p; p; p])
    r

See http://t0yv0.blogspot.com/2011/12/hacking-type-classes-in-f.html for context.