Converted https://gist.github.com/JefClaes/496388c579dc9ce86b56 to F#

gistfile1.fs

module Program
open Inventory
open System

[<EntryPoint>]
let main argv =
    let item = Inventory.Register (Inventory.InventoryItemName.Make "Domain Driven Design") 
    let itemAfterCheckingIn = Inventory.CheckIn (InventoryItemCount.Make 3) item
    let itemAfterCheckingOut = Inventory.CheckOut (InventoryItemCount.Make 1) itemAfterCheckingIn
    let deactivatedItem = Inventory.Deactivate itemAfterCheckingOut

    let x = Console.ReadLine()
    0 // return an integer exit code

module Inventory
    open System

    type InventoryItemName (value: string) =
        member this.Value = value
        static member Make(value: string) =
            match value with
            | "" -> invalidArg "value" "Name shouldn't be empty."
            | _ -> InventoryItemName value 

    type InventoryItemCount(value: int) =
        member this.Value = value
        static member (-) (countLeft: InventoryItemCount, countRight: InventoryItemCount) =
            InventoryItemCount.Make(countLeft.Value-countRight.Value)
        static member (+) (countLeft: InventoryItemCount, countRight: InventoryItemCount) =
            InventoryItemCount.Make(countLeft.Value+countRight.Value)
        static member Empty =
            InventoryItemCount.Make 0
        static member Make(value: int) =
            if value < 0 then
                invalidArg "value" "Count can't be negative."
            InventoryItemCount value

    type InventoryItem = { Id: Guid; Activated: bool; Name: InventoryItemName; Count: InventoryItemCount; }
              
    let Register name =
        { Id = Guid.NewGuid(); Activated = false; Name = name; Count = InventoryItemCount.Empty; }
    let CheckIn count item = 
        { item with Count = item.Count + count }
    let CheckOut count item =
        { item with Count = item.Count - count }
    let Deactivate item =
        { item with Activated = false }

Don't use tuples for functions arts !

let checkout count item = { ... }

And make inventoryItemCount a struct '

@thinkbeforecoding Updated!