module Functional

open System.IO
open System.Linq

// Write something on a file.
let appendFile (filename: string) (text: string) = 
    use sw = new StreamWriter(filename, true)
    sw.WriteLine(text)

// Recursive function.
let rec fac n = 
    if n <= 1 then 1
    else n * fac (n - 1)

// Mutual recursive functions.
let rec isEven n =
    if n = 0 then true
    elif n = 1 then false
    else isOdd (n - 1)
and isOdd n =
    if n = 0 then false
    elif n = 1 then true
    else isEven (n - 1)

// Algebraic data type.
type Suit =
    | Diamond
    | Spade
    | Heart
    | Club

type PlayingCard =
    | Ace of Suit
    | King of Suit
    | Queen of Suit
    | Jack of Suit
    | NumCard of int * Suit
    member this.Value = // Property.
        match this with
            | Ace (_) -> 14
            | King (_) -> 13
            | Queen (_) -> 12
            | Jack (_) -> 11
            | NumCard (n, _) when n >= 2 && n <= 10 -> n
            | NumCard (_) -> failwith "Card has invalid value!"

let deckOfCards = 
    [
        for suit in [ Spade; Club; Heart; Diamond ] do
            yield Ace(suit)
            yield King(suit)
            yield Queen(suit)
            yield Jack(suit)
            for n in [2 .. 10] do
                yield NumCard(n, suit)
    ]

// Records with custom equality.
[<CustomEquality; NoComparison>]
type Person = 
    { First: string; Last: string; Age: int }
    override this.Equals(other) =
        match other with
        | :? Person as p -> p.First = this.First && p.Last = this.Last
        | _ -> false
    override this.GetHashCode() =
        this.First.GetHashCode() ^^^ this.Last.GetHashCode()

let steve = { First = "Steve"; Last = "Holt"; Age = 17 }
let stevesTwin = { steve with First = "Paul" }

// Queries. Should be used with SQL or other external data sources.
let youngPersonNames persons =
   query {
      for p in persons do
      where (p.Age < 15)
      select p.First
   }

// Infinite lazy sequence.
let allPositiveInts =
    seq {
        for i in 1 .. System.Int32.MaxValue do
            yield i
    }
let rec fibs x y =
   seq {
      yield x
      yield! fibs y (x + y)
   }
let testYieldBang =
    seq {
        yield 10
        yield! [1; 2; 3] // Yields a collection.
    }
// Generator.
let testUnfold =
    Seq.unfold (fun a -> if a > 10 then None else Some(10 * a, a + 1)) 1

// Functions composition.
let SizeOfFolder = 
    let getFiles folder = 
        Directory.GetFiles(folder, "*.*", SearchOption.AllDirectories)
    getFiles
    >> Array.map (fun file -> (new FileInfo(file)).Length)
    >> Array.sum

// Matches literals.
[<Literal>]
let Bill = "Bill Gates" // Must begin with a capital!
let greet name =
   match name  with
   | Bill -> "Hello rich Bill!"
   | name -> sprintf "Hello poor %s" name

// Matches tuples.
let testXor x y =
   match x, y with
   | a, b when a <> b -> true
   | _ -> false