How To Model A Versioned Contract

Use this pattern for config files or messages that evolve over time and need an explicit version marker in the wire shape.

open CodecMapper.Schema

type SettingsV2 = {
    Version: int
    Mode: string
    Region: string option
}

let makeSettingsV2 version mode region =
    { Version = version; Mode = mode; Region = region }

let settingsV2Schema =
    define<SettingsV2>
    |> construct makeSettingsV2
    |> fieldWith "version" _.Version (
        int
        |> tryMap
            (fun value ->
                if value > 0 then Ok value
                else Error "version must be positive")
            id
    )
    |> field "mode" _.Mode
    |> field "region" _.Region
    |> build

Keep the version field explicit so contract evolution stays visible in one authored schema. If you also need omission policies or defaults, compose those helpers directly at the field boundary.

type SettingsV2 = { Version: int Mode: string Region: string option }

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

type 'T option = Option<'T>

val makeSettingsV2: version: int -> mode: string -> region: string option -> SettingsV2

val version: int

val mode: string

val region: string option

val settingsV2Schema: obj

union case Result.Ok: ResultValue: 'T -> Result<'T,'TError>

union case Result.Error: ErrorValue: 'TError -> Result<'T,'TError>

val id: x: 'T -> 'T