F 中的数据驱动编程

由于 F# 中的类型推理和部分应用，数据驱动编程简洁易读。

让我们想象一下，我们正在出售汽车保险。在我们尝试将其出售给客户之前，我们会通过检查客户的性别和年龄来确定客户是否是我们公司的有效潜在客户。

一个简单的客户模型：

type Sex =
  | Male
  | Female

type Customer =
  {
    Name    : string
    Born    : System.DateTime
    Sex     : Sex
  }

接下来，我们要定义一个排除列表（表），以便如果客户匹配排除列表中的任何行，则客户无法购买我们的汽车保险。

//  If any row in this list matches the Customer, the customer isn't eligible for the car insurance.
let exclusionList =
  let __          _   = true
  let olderThan   x y = x < y
  let youngerThan x y = x > y
  [|
//  Description                         Age               Sex
    "Not allowed for senior citizens" , olderThan   65  , __
    "Not allowed for children"        , youngerThan 16  , __
    "Not allowed for young males"     , youngerThan 25  , (=) Male
  |]

由于类型推断和部分应用，排除列表灵活且易于理解。

最后，我们定义了一个使用排除列表（表）将客户分成两个桶的功能：潜在客户和拒绝客户。

// Splits customers into two buckets: potential customers and denied customers.
// The denied customer bucket also includes the reason for denying them the car insurance
let splitCustomers (today : System.DateTime) (cs : Customer []) : Customer []*(string*Customer) [] =
  let potential = ResizeArray<_> 16 // ResizeArray is an alias for System.Collections.Generic.List
  let denied    = ResizeArray<_> 16

  for c in cs do
    let age = today.Year - c.Born.Year
    let sex = c.Sex
    match exclusionList |> Array.tryFind (fun (_, testAge, testSex) -> testAge age && testSex sex) with
    | Some (description, _, _)  -> denied.Add (description, c)
    | None                      -> potential.Add c

  potential.ToArray (), denied.ToArray ()

总结一下，让我们定义一些客户，看看他们是否是我们汽车保险的潜在客户：

let customers =
  let c n s y m d: Customer = { Name = n; Born = System.DateTime (y, m, d); Sex = s }
  [|
//    Name                      Sex     Born
    c "Clint Eastwood Jr."      Male    1930 05 31
    c "Bill Gates"              Male    1955 10 28
    c "Melina Gates"            Female  1964 08 15
    c "Justin Drew Bieber"      Male    1994 03 01
    c "Sophie Turner"           Female  1996 02 21
    c "Isaac Hempstead Wright"  Male    1999 04 09
  |]

[<EntryPoint>]
let main argv =
  let potential, denied = splitCustomers (System.DateTime (2014, 06, 01)) customers
  printfn "Potential Customers (%d)\n%A" potential.Length potential
  printfn "Denied Customers (%d)\n%A"    denied.Length    denied
  0

这打印：

Potential Customers (3)
[|{Name = "Bill Gates";
   Born = 1955-10-28 00:00:00;
   Sex = Male;}; {Name = "Melina Gates";
                  Born = 1964-08-15 00:00:00;
                  Sex = Female;}; {Name = "Sophie Turner";
                                   Born = 1996-02-21 00:00:00;
                                   Sex = Female;}|]
Denied Customers (3)
[|("Not allowed for senior citizens", {Name = "Clint Eastwood Jr.";
                                       Born = 1930-05-31 00:00:00;
                                       Sex = Male;});
  ("Not allowed for young males", {Name = "Justin Drew Bieber";
                                   Born = 1994-03-01 00:00:00;
                                   Sex = Male;});
  ("Not allowed for children", {Name = "Isaac Hempstead Wright";
                                Born = 1999-04-09 00:00:00;
                                Sex = Male;})|]