http://blog.bjartwolf.com/?p=4522

AgeDU.fs

open System

type PersonAge = PersonAge of int

type Person = { id: Guid; name: string; age: PersonAge}
 
let bjorn = { id=System.Guid.NewGuid(); name="Bjørn Einar"; age=PersonAge 34}
let changedName = {bjorn with name ="Bjørn the confused"}

let createAge age =
  // somecrazybusiness rules with age and using services to check if the
  // age is really correct and do all kinds of sideeffects
  match age with
  | i when i < 18 -> failwith "too young"
  | i when i > 120 -> failwith "no you are not!"
  | i -> PersonAge i

let changeAge (p:Person) (newAge:int) = 
  {p with age = (createAge newAge)}

let changedAgedBjorn = changeAge changedName 30