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<a href="./Home">MLton 20130715</a>
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<h1>GenerativeDatatype</h1>
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<div class="paragraph"><p>In <a href="StandardML">Standard ML</a>, datatype declarations are said to be
<em>generative</em>, because each time a datatype declaration is evaluated,
it yields a new type.  Thus, any attempt to mix the types will lead to
a type error at compile-time.  The following program, which does not
type check, demonstrates this.</p></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span></span><span class="kr">functor</span> <span class="nn">F</span> <span class="p">()</span> <span class="p">=</span>
   <span class="kr">struct</span>
      <span class="kr">datatype</span> <span class="kt">t</span> <span class="p">=</span> <span class="nc">T</span>
   <span class="kr">end</span>
<span class="kr">structure</span> <span class="nn">S1</span> <span class="p">=</span> <span class="n">F</span> <span class="p">()</span>
<span class="kr">structure</span> <span class="nn">S2</span> <span class="p">=</span> <span class="n">F</span> <span class="p">()</span>
<span class="kr">val</span> <span class="p">_:</span> <span class="nn">S1</span><span class="p">.</span><span class="n">t</span> <span class="p">-&gt;</span> <span class="nn">S2</span><span class="p">.</span><span class="n">t</span> <span class="p">=</span> <span class="kr">fn</span> <span class="n">x</span> <span class="p">=&gt;</span> <span class="n">x</span>
</pre></div></div></div>
<div class="paragraph"><p>Generativity also means that two different datatype declarations
define different types, even if they define identical constructors.
The following program does not type check due to this.</p></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span></span><span class="kr">datatype</span> <span class="kt">t</span> <span class="p">=</span> <span class="nc">A</span> <span class="p">|</span> <span class="nc">B</span>
<span class="kr">val</span> <span class="nv">a1</span> <span class="p">=</span> <span class="n">A</span>
<span class="kr">datatype</span> <span class="kt">t</span> <span class="p">=</span> <span class="nc">A</span> <span class="p">|</span> <span class="nc">B</span>
<span class="kr">val</span> <span class="nv">a2</span> <span class="p">=</span> <span class="n">A</span>
<span class="kr">val</span> <span class="p">_</span> <span class="p">=</span> <span class="kr">if</span> <span class="n">true</span> <span class="kr">then</span> <span class="n">a1</span> <span class="kr">else</span> <span class="n">a2</span>
</pre></div></div></div>
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<h2 id="_also_see">Also see</h2>
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<li>
<p>
<a href="GenerativeException">GenerativeException</a>
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