a.fs

//Experiment, based on http://withouttheloop.com/articles/2014-04-18-fsharp-csharp-statemachines/

open System
type Particulars = { buyer: string; seller: string }

type Draft = private Draft of Particulars
type Approved = private Approved of (Particulars * DateTime)
type Historical = private Historical of (Particulars * DateTime)

let createDraft = Draft
let approve timestamp (Draft p) = Approved (p, timestamp)
let retire timestamp (Approved (p,_)) = Historical (p, timestamp)

// usage
let contract = createDraft { buyer = "Acme"; seller = "Initech" }
let approved = approve DateTime.Now contract
let retired = retire DateTime.Now approved
retired |> printfn "%+A"

type Contract = Choice<Draft, Approved, Historical>

//let invalid = approve DateTime.Now approved