Using FsCheck to Enumerate Invalid Instances

enumInvalidInstances.fs

//  In this example, we're using FsCheck in an unorthodox way.
//  Typically you tell FsCheck to look for something that violates
//  your rule.  Here, we're reappropriating FsCheck's fuzz testing
//  capabilities to enumerate everything that has been deemed 
//  "valid" to see if it's actually valid to help us figure out the rules.

type Money = decimal
type CouponType = FreeMail | FreeSide

type PaymentOption =
    | Cash of Amount:Money
    | Coupon of CouponType
    | Credit of Amount:Money * CardNumber:string * Expiration:string
    | GiftCard of Amount:Money * Balance:Money * CardNumber:Guid

//  We'll first assume everything is valid
let isValidPayment(paymentType:PaymentOption) = true

//  We pass this function to tell FsCheck everything is valid
//  so it keeps throwing data at us.  We only print the ones that
//  we've deemed valid.
let showValidPayment(paymentType) = 
    if isValidPayment(paymentType) then
        printfn "%A" paymentType
    true

FsCheck.Check.Quick(showValidPayment)