A C# library to write functional code - Part III - Records

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.

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13 Comments

Comments

Microsoft news and tips &#38;r

2008-04-21T14:21:13Z

PingBack from http://​mi­crosoft­news.askpc­doc.com/?​p=3525

gOODiDEA.NET

2008-04-21T21:04:17Z

.NET RestLess - A Simple REST Framework ASP.NET AJAX Overview And Technical Tips A C# li­brary to write

gOODiDEA

2008-04-21T21:04:54Z

.NETRestLess-ASimpleRESTFrameworkASP.NETAJAXOverviewAndTechnicalTipsAC#libraryto…

Charlie Calvert's Community Bl

2008-04-23T16:58:14Z

Welcome to the forty-third is­sue of Community Convergence. The last few weeks have been con­sumed by the

Eamon Nerbonne

2008-04-24T05:11:28Z

By the way, there’s an­other rea­son your hash-code al­go­rithm is un­wise: Assuming the com­po­nent hash-codes are uni­formly dis­trib­uted over the in­te­gers from 0 to 2^32-1 (thus in­ter­pret­ing the signed in­te­gers as un­signed in­te­gers here, for sim­plic­ity), your will not be:  a bi­nary or never low­ers the num­ber of “on” bits; each bit of your re­sult hash code has only a 25% chance to be off - which low­ers the en­tropy of the over­all hash code to be around 26 bits, as op­posed to 32 bits.
Finally, tu­ples of tu­ples will hash par­tic­u­larly badly; and so will tu­ples with higher than 2-arity.

lucabol

2008-04-24T13:13:10Z

Eamon: you are right. It is bad. As I said be­fore, I was lazy.

ktmt

2008-04-25T11:13:00Z

Wow, a lit­tle more and we’ll fi­nally have the easy-to-use record con­struct of Pascal in the 1980s.  Just de­clare a Record type and you’re set.  I have (seriously) of­ten wished for this in C# in­stead of cum­ber­some, sep­a­rate struct or class de­f’s.

amiya

2008-05-08T13:55:11Z

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TimNew

2008-05-20T12:06:19Z

This’s a re­ally ex­cel­lent in­tro­duc­tion. I think I be­gin to know what the func­tional code is af­ter read­ing your posts and code. But I have some ques­tions about this:
For the GetHashCode(), why don’t you try “xor” other than “or”? I think “xor” will get a bet­ter job than the “or”~
For some­thing about all the code, and I found TypeUnion<T> is re­ally use­less! Since I’ll never have the change to use a mem­ber union!

pligg.com

2008-05-23T18:55:17Z

Previous posts: Part I - Background Part II - Tuples 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 im­mutabl

adamjcooper.com/blog

2008-06-03T15:38:39Z

The Quest for Quick-and-Easy Class-Based Immutable Value Objects in C# - Part 1: Introduction

adamjcooper.com/blog

2008-06-03T17:00:02Z

The Quest for Quick-and-Easy Immutable Value Objects in C#

Luca Bolognese's WebLog

2008-07-15T05:46:24Z

Other posts in the se­ries: Part I - Background Part II - Tuples Part III - Records Part IV - Type Unions