Generics in record "with" syntax

Question

If I've got a record with a generic field, is there any way to mimic the convenient with syntax when changing the generic?

i.e. if I have

type User<'photo> = // 'photo can be Bitmap or Url
  { first: string; last: string; address: string; phone: string; photo: 'photo }

I want to be able to write something like

let loadUser(user: User<Url>): User<Bitmap> =
  { user with
    photo = download user.photo }

But it looks like I've got to write this instead.

let loadUser(user: User<Url>): User<Bitmap> =
  { first = user.first
    last = user.last
    address = user.address
    phone = user.phone
    photo = download user.photo }

Is there a way to get the first syntax?

Answer 1

not directly but you can make your User into a functor (for the photo part):

let fmap (f : 'a -> 'b) (user: User<'a>): User<'b> =
  { first   = user.first
    last    = user.last
    address = user.address
    phone   = user.phone
    photo   = f user.photo }

once and write (for example):

let loadUser (user : User<Url>) : User<Bitmap> =
    fmap download user

also you could rename fmap into withPhoto if you prefer let loadUser = withPhoto download :D

now it might be a bit strange that the photo part is any kind of data/type so I would think about renamig this part just into value - but that's just me