Copying a union case but with a different value in F#

Question

In F#, I want to construct an instance (correct terminology?) of a discriminated union based on an existing instance. Example:

type union Currency =
    | Dollar of int
    | Euro of int

let lowPrice = Dollar 100 (* or, it could be *) let lowPrice = Euro 100
let highPrice = (* of the same union case as lowPrice but with value 200 *)

What code could I insert in place of the comment to create that effect?

Answer 1

You could do

let highPrice =
    let n = 200
    match lowPrice with
    | Dollar _ -> Dollar n
    | Euro _ -> Euro n

but units of measure are probably better.

EDIT

Alternatively, maybe you want

type MoneyType = Dollar | Euro
type Currency = Currency of MoneyType * int
let lowPrice = Currency(Dollar, 100)
let highPrice = 
    match lowPrice with
    | Currency(kind, _) -> Currency(kind, 200)