Design pattern implementation in F#

Data-driven programming in F#

Thanks to type-inference and partial application in F# data-driven programming is succinct and readable.

Let’s imagine we are selling car insurance. Before we try selling it to a customer, we try to determine if the customer is a valid potential customer for our company by checking the customer’s sex and age.

A simple customer model:

type Sex =
  | Male
  | Female

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

Next we want to define an exclusion list (table) so that if a customer matches any row in the exclusion list the customer is cannot buy our car insurance.

//  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
  |]

Because of type-inference and partial application, the exclusion list is flexible yet easy to understand.

Finally, we define a function that uses the exclusion list (a table) to split the customers into two buckets: potential and denied customers.

// 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 ()

To wrap up, let’s define some customers and see if they are any potential customers for our car insurance amongst them:

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

This prints:

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;})|]