使用 type 關鍵字在 F 中建立型別

F# 使用 type 關鍵字建立不同型別的型別。

1. 輸入別名
2. 歧視的聯合型別
3. 記錄型別
4. 介面型別
5. 類型別
6. 結構型別

儘可能使用等效 C# 程式碼的示例：

// Equivalent C#:
//  using IntAliasType = System.Int32;
type IntAliasType = int // As in C# this doesn't create a new type, merely an alias

type DiscriminatedUnionType =
  | FirstCase
  | SecondCase  of int*string

  member x.SomeProperty = // We can add members to DU:s
    match x with
    | FirstCase         -> 0
    | SecondCase (i, _) -> i

type RecordType =
  {
    Id    : int
    Name  : string
  }
  static member New id name : RecordType = // We can add members to records
    { Id = id; Name = name } // { ... } syntax used to create records

// Equivalent C#:
//  interface InterfaceType
//  {
//    int     Id    { get; }
//    string  Name  { get; }
//    int Increment (int i);
//  }  
type InterfaceType =
  interface // In order to create an interface type, can also use [<Interface>] attribute
    abstract member Id        : int
    abstract member Name      : string
    abstract member Increment : int -> int
  end

// Equivalent C#:
//  class ClassType : InterfaceType
//  {
//    static int increment (int i)
//    {
//      return i + 1;
//    }
//  
//    public ClassType (int id, string name)
//    {
//      Id    = id    ;
//      Name  = name  ;
//    }
//  
//    public int    Id    { get; private set; }
//    public string Name  { get; private set; }
//    public int   Increment (int i)
//    {
//      return increment (i);
//    }
//  }
type ClassType (id : int, name : string) = // a class type requires a primary constructor
  let increment i = i + 1 // Private helper functions

  interface InterfaceType with // Implements InterfaceType
    member x.Id           = id
    member x.Name         = name
    member x.Increment i  = increment i

// Equivalent C#:
//  class SubClassType : ClassType
//  {
//    public SubClassType (int id, string name) : base(id, name)
//    {
//    }
//  }
type SubClassType (id : int, name : string) =
  inherit ClassType (id, name) // Inherits ClassType

// Equivalent C#:
//  struct StructType
//  {
//    public StructType (int id)
//    {
//      Id = id;
//    }
//  
//    public int Id { get; private set; }
//  }
type StructType (id : int) =
  struct  // In order create a struct type, can also use [<Struct>] attribute
    member x.Id = id
  end