F# 小于模式匹配中的运算符

Question

由于某种原因，此模式匹配中的小于运算符将不起作用。这是我唯一的错误，它让我发疯。我可能遗漏了一些非常明显的东西，但感谢所有帮助。

let CheckAccount account = 
match account with
| {Balance < 10.00} -> Console.WriteLine("Balance is Low")
| {Balance >= 10.00 and <= 100.00} -> Console.WriteLine("Balance is OK")
| {Balance > 100.00} -> Console.WriteLine("Balance is High")

这是类型：

type Account = {AccountNumber:string 
            mutable Balance:float} 
            member this.Withdraw(amnt:float) = 
                if amnt > this.Balance then
                    Console.WriteLine("Unable to withdraw. The Amount you wish to withdraw is greater than your current balance.")
                else
                    this.Balance <- this.Balance - amnt
                    Console.WriteLine("You have withdrawn £" + amnt.ToString() + ". Your balance is now: £" + this.Balance.ToString())
            member this.Deposit(amnt:float) =
                this.Balance <- this.Balance + amnt
                Console.WriteLine("£" + amnt.ToString() + " Deposited. Your new Balance is: £" + this.Balance.ToString())
            member this.Print = 
                Console.WriteLine("Account Number: " + this.AccountNumber)
                Console.WriteLine("Balance: £" + this.Balance.ToString())

Answer 1

您可以使用模式匹配来提取余额值，将其绑定到一个新名称，然后使用 when 子句比较这些值：

let CheckAccount account = 
  match account with
  | {Balance = b} when b < 10.00 -> Console.WriteLine("Balance is Low")
  | {Balance = b} when b >= 10.00 && b <= 100.00 -> Console.WriteLine("Balance is OK")
  | {Balance = b} when b > 100.00 -> Console.WriteLine("Balance is High")

我要说的是，在这种情况下，您实际上并没有从使用模式匹配中得到多少。如果您使用 if 编写相同的代码，那么它可能看起来会更好。

您可以使用更高级的方法并定义可让您比较值的活动模式：

let (|LessThan|_|) k value = if value < k then Some() else None
let (|MoreThan|_|) k value = if value > k then Some() else None

那么你可以改用那些：

let CheckAccount account = 
  match account with
  | {Balance = LessThan 10.0} -> Console.WriteLine("Balance is Low")
  | {Balance = LessThan 100.0 & MoreThan 10.0 } -> Console.WriteLine("Balance is OK")

这实际上相当有趣 - 因为您可以使用 & 构造来组合多个活动模式，如 LessThan 100.0 & MoreThan 10.0.

F# 小于模式匹配中的运算符

F# less than operator in pattern matching

f#

pattern-matching