Other posts in the se­ries:

• Part I - Background

  • Part II - Tuples

    • Part III - Records
    • Part IV - Type Unions{.}
    • Part V - The Match op­er­a­tor{.} Now that we know what Tuples are, we can start talk­ing about Record, as they use a de­riv­a­tive of Tuples un­der the cover. But first, what is a record?

    Well, in C# par­lance a Record is a sort of im­mutable value ob­ject. I talked at length about these fel­lows in this se­ries of blog posts. In func­tional par­lance, a Record is a Tuple that lets you ac­cess its items by name.

    You code Records like this:

    public class Order: Record<string, int> {
    public Order(string item, int qty): base(item,qty) {}
    public string Item { get { return state.Item1;}}
    public int Quantity { get { return state.Item2; } }
    

  } pub­lic class Customer: Record<string, IEnumerable<Order>> { pub­lic Customer(string name, IEnumerable<Order> or­ders) : base(name, or­ders) { } pub­lic string Name { get { re­turn state.Item1; } } pub­lic IEnumerable<Order> Orders { get { re­turn state.Item2; } } }

  You need to do three things:
  
    1. Inherit from a generic Record class specifying the types of the properties as parameters
    2. Add a constructor that calls back to the Record constructor
    3. Add getters to retrieve the values of the properties (no setters as it is immutable)
  
  This may seem like plenty of work, but in return you get structural equality. Coding that by hand every time you use a record would be a royal pain. You lose control of the base class, but that is often not a problem as in functional-land you more often use type unions than inheritance.
  
  How is it implemented?
  
  First of all it is an abstract class with as many generic parameters as properties that you need in your Record. Let's use two as an example.
  
  <pre class="code"><span style="color:rgb(0,0,255);">public</span> <span style="color:rgb(0,0,255);">abstract</span> <span style="color:rgb(0,0,255);">class</span> <span style="color:rgb(43,145,175);">Record</span>&lt;T1, T2&gt; {
  

    This abstract class has a field of type STuple:
    
    <pre class="code"><span style="color:rgb(0,0,255);">protected</span> <span style="color:rgb(0,0,255);">readonly</span> <span style="color:rgb(43,145,175);">STuple</span>&lt;T1, T2&gt; state;

    What is a STuple? Well it is exactly the same as the Tuple described in Part II, but coded as a struct instead of a class. The reason to use a struct is to not allocate an additional object on the stack. This allows this solution to be as ‘performant' as simply having coded the fields on the class itself. Or at least I think so
    
    The Record class also has a constructor that simply initialize the STuple:
    
    <pre class="code"><span style="color:rgb(0,0,255);">public</span> Record(T1 t1, T2 t2) { state = <span style="color:rgb(43,145,175);">F</span>.STuple(t1, t2); }</pre>
    
    <pre class="code"><font face="Verdana">where</font></pre>
    
    <pre class="code"><span style="color:rgb(0,0,255);">internal</span> <span style="color:rgb(0,0,255);">static</span> <span style="color:rgb(43,145,175);">STuple</span>&lt;T1, T2&gt; STuple&lt;T1, T2&gt;(T1 t1, T2 t2) {
        <span style="color:rgb(0,0,255);">return</span> <span style="color:rgb(0,0,255);">new</span> <span style="color:rgb(43,145,175);">STuple</span>&lt;T1, T2&gt;(t1, t2);
    }</pre>
    
    The Equals method is very much the same as the Tuple's one, just delegating to the same DSEqual function that checks equality for Tuples.
    
    <pre class="code"><span style="color:rgb(0,0,255);">public</span> <span style="color:rgb(0,0,255);">override</span> <span style="color:rgb(0,0,255);">bool</span> Equals(<span style="color:rgb(0,0,255);">object</span> right) {
        <span style="color:rgb(43,145,175);">Utils</span>.CheckNull(right);
        <span style="color:rgb(0,0,255);">if</span> (<span style="color:rgb(0,0,255);">object</span>.ReferenceEquals(<span style="color:rgb(0,0,255);">this</span>, right))
            <span style="color:rgb(0,0,255);">return</span> <span style="color:rgb(0,0,255);">true</span>;
        <span style="color:rgb(0,0,255);">if</span> (<span style="color:rgb(0,0,255);">this</span>.GetType() != right.GetType())
            <span style="color:rgb(0,0,255);">return</span> <span style="color:rgb(0,0,255);">false</span>;
        <span style="color:rgb(0,0,255);">var</span> rightT = right <span style="color:rgb(0,0,255);">as</span> <span style="color:rgb(43,145,175);">Record</span>&lt;T1, T2&gt;;
        <span style="color:rgb(0,0,255);">return</span> <span style="color:rgb(43,145,175);">F</span>.DSEquals(<span style="color:rgb(0,0,255);">this</span>.state, rightT.state);
    }</pre>
    
    That's it. Not too difficult as most of the implementation is based on the Tuple's code. Next post will hopefully be more interesting. It is about Type Unions.