Using Discriminated Union Labelled Fields

// Without labels
type Customer =
    | Registered of string * string option * bool
    | Guest of string

// With labels
type Customer =
    | Registered of Name:string * Email:string option * IsEligible:bool
    | Guest of Name:string

(*
Feature: Applying a discount

Scenario: Eligible Registered Customers get 10% discount 
when they spend £100 or more

Given the following Registered Customers
|Customer Id|Email          |Is Eligible|
|John       |[email protected]  |true       |
|Mary       |[email protected]  |true       |
|Richard    |               |false      |
|Alison     |[email protected]|false      |

When <Customer Id> spends <Spend>
Then their order total will be <Total>

Examples:
|Customer Id| Spend | Total |
|Mary       |  99.00|  99.00|
|John       | 100.00|  90.00|
|Richard    | 100.00| 100.00|
|Sarah      | 100.00| 100.00|
*)

type Customer =
    | Registered of Name:string * Email:string option * IsEligible:bool
    | Guest of Name:string

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (name, email, isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

In this case, I'm only interested in the IsEligible flag, so will wildcards work? Yes they do:

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (_, _, isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

// Compiler Error
let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (Name = name, Email = email, IsEligible = isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (Name = name; Email = email; IsEligible = isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (Name = _; Email = _; IsEligible = isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

// Compiler Error
let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (_; _; IsEligible = isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (IsEligible = isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (IsEligible = true) when spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

let tryGetEligibleEmail customer =
    match customer with
    | Registered (IsEligible = true; Email = email) -> Some email 
    | _ -> None

// let john = Registered ( "John", Some "[email protected]", true )

let john = Registered ( Name = "John", Email = Some "[email protected]", IsEligible = true )
let mary = Registered ( Name = "Mary", Email = Some "[email protected]", IsEligible = true )
let richard = Registered ( Name = "Richard", Email = None, IsEligible = false )
let alison = Registered ( Name = "Alison", Email = Some "[email protected]", IsEligible = false )
let sarah = Guest ( Name = "Sarah" )

let assertJohn = calculateOrderTotal john 100.0M = 90.0M
let assertMary = calculateOrderTotal mary 99.0M = 99.0M
let assertRichard = calculateOrderTotal richard 100.0M = 100.0M
let assertSarah = calculateOrderTotal sarah 100.0M = 100.0M

let assertMaryEmail = tryGetEligibleEmail mary = Some "[email protected]"
let assertRichardEmail = tryGetEligibleEmail richard = None
let assertAlisonEmail = tryGetEligibleEmail alison = None
let assertSarahEmail = tryGetEligibleEmail sarah = None

type Customer =
    | Registered of string * Email:string option * IsEligible:bool
    | Guest of Name:string

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (name, email, isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (_, _, isEligible) when isEligible && spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

// Compiler Error
let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (name; Email = email; IsEligible = true) when spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount

let calculateOrderTotal customer spend =
    let discount = 
        match customer with
        | Registered (Email = email; IsEligible = true) when spend >= 100M -> spend * 0.1M 
        | _ -> 0M
    spend - discount