How can I ignore the value of a discriminated union case in FsUnit's assert?

Question

How can I ignore the value of a discriminated union case in FsUnit's assert?

Take for example:

type TransactionAttempt = { 
    Deposited:float
    Requires:float 
}

type RequestResult =
    | Denied of TransactionAttempt
    | Granted of Product

In my test, I want to do this:

let display = balance |> select Pepsi
display |> should equal Denied

I wouldn't mind doing this:

display |> should equal (Denied _)

However, I'm forced to do this:

display |> should equal (Denied {Deposited=0.25; Requires=1.00})

Note how explicit I have to be with the expression above.

Hence, I just want to know if it was denied. I don't care about the details.

Here's the actual test:

[<Test>]
let ``Vending machine reflects more money required for selection``() =
   // Setup
   let balance = Quarter |> insert []

   // Test
   let display = balance |> select Pepsi

   // Verify
   display |> should equal (Denied {Deposited=0.25; Requires=1.00})

Here's the function:

let select product balance =
    let attempt = { Deposited=balance
                    Requires=product |> getPrice }

    let paidInFull = attempt.Deposited >= attempt.Requires

    if not paidInFull then 
        Denied attempt
    else Granted product

Here's the entire domain:

module Machine

type Deposit =
    | Nickel
    | Dime
    | Quarter
    | OneDollarBill
    | FiveDollarBill

type TransactionAttempt = { 
    Deposited:float
    Requires:float 
}

type State =
    | OutOfService
    | PaymentReceived of Deposit
    | WaitingForSelection
    | NotPaidInFull of TransactionAttempt

type Product =
    | Pepsi
    | Coke
    | Sprite
    | MountainDew

type RequestResult =
    | Denied of TransactionAttempt
    | Granted of Product

(* Functions *)
open System

let display = function
    | OutOfService            -> "Out of Service"
    | WaitingForSelection     -> "Make selection"
    | NotPaidInFull attempt   -> sprintf "%s Required" ((attempt.Requires - attempt.Deposited).ToString("C2"))
    | PaymentReceived deposit -> match deposit with
                                 | Nickel         -> "5¢"
                                 | Dime           -> "10¢"
                                 | Quarter        -> "25¢"
                                 | OneDollarBill  -> "$1.00"
                                 | FiveDollarBill -> "$5.00"

let getBalance coins =
    coins |> List.fold (fun acc d -> match d with
                                     | Nickel         -> acc + 0.05
                                     | Dime           -> acc + 0.10
                                     | Quarter        -> acc + 0.25
                                     | OneDollarBill  -> acc + 1.00
                                     | FiveDollarBill -> acc + 5.00) 0.00
let insert balance coin =
    coin::balance |> getBalance

let getPrice = function
    | Pepsi       -> 1.00
    | Coke        -> 1.00
    | Sprite      -> 1.00
    | MountainDew -> 1.00

let select product balance =
    let attempt = { Deposited=balance
                    Requires=product |> getPrice }

    let paidInFull = attempt.Deposited >= attempt.Requires

    if not paidInFull then 
        Denied attempt
    else Granted product

Answer 1

The simplest thing I can come up with is to write a predicate for the check you want:

let isDenied du =
    match du with
    | Denied _ -> true
    | _ -> false

Or since let f x = match x with ... is equivalent to let f = function ..., that could be:

let isDenied = function Denied _ -> true | _ -> false

Then your test looks like:

display |> isDenied |> should be True

Note that True, with a capital T, is a constraint. If you're comparing to the Boolean value, then it would be

display |> isDenied |> should equal true

If you find you're having to write LOTS of these custom predicates, there's probably a more general solution involving writing a custom constraint for NUnit or XUnit or whatever testing framework you're using on the back end. But as a relative newbie to F#, you should probably go with the simple solution first, and generalize it later.

P.S. If you do decide to write a custom constraint, look at https://github.com/fsprojects/FsUnit/blob/master/src/FsUnit.NUnit/FsUnit.fs (if you're using NUnit) or https://github.com/fsprojects/FsUnit/blob/master/src/FsUnit.Xunit/FsUnit.fs (if you're using XUnit) for inspiration.