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<h1 id="procedures-in-records">Procedures in records</h1>
<p>By R. S. Doiel, 2020-07-07</p>
<p>This is the tenth post in the <a
href="../../04/11/Mostly-Oberon.html">Mostly Oberon</a> series. Mostly
Oberon documents my exploration of the Oberon Language, Oberon System
and the various rabbit holes I will inevitably fall into.</p>
<p>In my last post I looked at how Oberon-07 supports the passing of
procedures as parameters in a procedure. In this one I am looking at how
we can include procedures as a part of an Oberon RECORD.</p>
<p>Let’s modify our module name <a href="Noises.Mod">Noises.Mod</a> to
explore this. Copy “Noises.Mod” to “Creatures.Mod”. Replace the “MODULE
Noises;” line with “MODULE Creatures;” and the final “END Noises.”
statement with “END Creatures.”.</p>
<pre><code>
    MODULE Creatures;
    
    (* rest of code here *)

    END Creatures.
</code></pre>
<p>The key to supporting records with procedures as record attributes is
once again Oberon’s type system. The type <code>Noise</code> we created
in the previous post can also be used to declare a record attribute
similar to how we use this new type to pass the procedure. In this
exploration will create a linked list of “Creature” types which include
a “MakeNoise” attribute.</p>
<p>First let’s define our “Creature” as a type as well as a
<code>CreatureList</code>. Add the following under our <code>TYPE</code>
definition in <a href="Creatures.Mod">Creatures.Mod</a>.</p>
<pre><code>
    Creature = POINTER TO CreatureDesc;
    CreatureDesc = RECORD
                     name : ARRAY 32 OF CHAR;
                     noises : Noises;
                   END;
</code></pre>
<p>Let’s create a new <code>MakeCreature</code> procedure that will
create a populate a single <code>Creature</code> type record.</p>
<pre><code>
    PROCEDURE MakeCreature(name : ARRAY OF CHAR; noise : Noise; VAR creature : Creature);
    BEGIN
      NEW(creature);
      creature.name := name;
      creature.noise := noise;
    END MakeCreature;
</code></pre>
<p>Now lets modify <code>MakeNoise</code> to accept the
<code>Creature</code> type RECORD rather than a name and a noise
procedure parameter.</p>
<pre><code>
    PROCEDURE MakeNoise(creature : Creator);
    BEGIN
      creature.noise(creature.name);
    END MakeNoise;
</code></pre>
<p>How does this all work? The two “Noise” procedures “BarkBark” and
“ChirpChirp” remain as in our original “Noises” module. But our new
<code>MakeNoise</code> procedure looks takes a <code>Creature</code>
record rather than accepting a name and procedure as parameters. This
makes the code a little more concise as well as lets you evolve the
creature record type using an object oriented approach.</p>
<p>Our revised module should look like this.</p>
<pre><code>
    MODULE Noises;
      IMPORT Out;
    
    TYPE 
      Noise = PROCEDURE(who : ARRAY OF CHAR);

      Creature = RECORD
                   name : ARRAY 32 OF CHAR;
                   noises : Noises;
                 END;
    
    VAR
      dog, bird : Creature;

    PROCEDURE BarkBark(who : ARRAY OF CHAR);
    BEGIN
      Out.String(who);
      Out.String(&quot;: Bark, bark&quot;);Out.Ln();
    END BarkBark;
    
    PROCEDURE ChirpChirp(who : ARRAY OF CHAR);
    BEGIN
      Out.String(who);
      Out.String(&quot;: Chirp, chirp&quot;);Out.Ln();
    END ChirpChirp;
    
    PROCEDURE MakeNoise(creature : Creature);
    BEGIN
      (* Call noise with the animal name *)
      creature.noise(creature.name);
    END MakeNoise;

    PROCEDURE MakeCreature(name : ARRAY OF CHAR; noise : Noise; VAR creature : Creature);
    BEGIN
      NEW(creature);
      creature.name := name;
      creature.noise := noise;
    END MakeCreaturel
    
    BEGIN
      MakeCreature(&quot;Fido&quot;, BarkBark, dog);
      MakeCreature(&quot;Tweety&quot;, ChirpChirp, bird);
      MakeNoise(dog);
      MakeNoise(bird);
    END Noises.
</code></pre>
<p>Where to go from here? Think about evolving <a
href="Creatures.Mod">Creatures</a> so that you can create a dynamic set
of creatures that mix and match their behaviors. Another idea would be
to add a “MutateCreature” procedure that would let you change the noise
procedure to something new.</p>
<h3 id="next-and-previous">Next and Previous</h3>
<ul>
<li>Next <a href="../../08/15/Portable-Oberon-07.html">Portable
Oberon-07</a></li>
<li>Previous <a
href="../../06/20/Procedures-as-parameters.html">Procedures as
parameters</a></li>
</ul>
  </article>
</section>

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