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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../inform7n.html">Inform7</a></li><li><a href="index.html">runtime</a></li><li><a href="index.html#2">Chapter 2: Emission</a></li><li><b>Equality Schemas</b></li></ul></div>
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<p class="purpose">To define how to compile a comparison of two values.</p>
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. </b>For most word-value kinds, it's easy to compare two values to see if they are
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equal: all we need is the <span class="extract"><span class="extract-syntax">==</span></span> operator. But for pointer-value kinds, that
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would simply tell us whether they point to the same block of data on the
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heap, whereas we need in fact to compare the blocks they point to. So the
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kind system makes it possible for each individual kind to decide how values
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should be compared, returning an I6 schema prototype to compare <span class="extract"><span class="extract-syntax">*1</span></span> and <span class="extract"><span class="extract-syntax">*2</span></span>.
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</p>
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<p class="commentary">What happens at run-time when we test to see if value V equals value W,
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or change storage object S so that it now contains value T, depends on the
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kind of values we are discussing. If there were only word-based values in
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Inform (as was the case until September 2007), there would be little to
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do here, as the comparison would simply compile to <span class="extract"><span class="extract-syntax">V == W</span></span>, while the
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storage would be a matter of either <span class="extract"><span class="extract-syntax">S = W;</span></span> or some more exotic case
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along the lines of <span class="extract"><span class="extract-syntax">StorageRoutineWrite(S, W);</span></span>.
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</p>
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<p class="commentary">But once pointers to blocks are allowed, this becomes more interesting.
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Now the comparison needs to be a deep one, that is, we want to test whether
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two texts (say) have the same textual content — not whether we are
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holding two pointers to the same blocks in memory, which is what a simple
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comparison would achieve. Such a test is called "deep comparison", and
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similarly, we must assign by transferring the contents of the blocks of
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data, not merely the pointer to them, which is a "deep copy".
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="identifier-syntax">text_stream</span><span class="plain-syntax"> *</span><span class="function-syntax">EqualitySchemas::interpret_equality</span><span class="plain-syntax">(</span><span class="identifier-syntax">kind</span><span class="plain-syntax"> *</span><span class="identifier-syntax">left</span><span class="plain-syntax">, </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> *</span><span class="identifier-syntax">right</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">LOGIF</span><span class="plain-syntax">(</span><span class="identifier-syntax">KIND_CHECKING</span><span class="plain-syntax">, </span><span class="string-syntax">"Interpreting equality test of kinds %u, %u\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">left</span><span class="plain-syntax">, </span><span class="identifier-syntax">right</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">left</span><span class="plain-syntax">, </span><span class="identifier-syntax">K_truth_state</span><span class="plain-syntax">)) || (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">right</span><span class="plain-syntax">, </span><span class="identifier-syntax">K_truth_state</span><span class="plain-syntax">)))</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">I</span><span class="string-syntax">"(*1 && true) == (*2 && true)"</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">kind_constructor</span><span class="plain-syntax"> *</span><span class="identifier-syntax">L</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">, *</span><span class="identifier-syntax">R</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">left</span><span class="plain-syntax">) && (</span><span class="identifier-syntax">right</span><span class="plain-syntax">)) { </span><span class="identifier-syntax">L</span><span class="plain-syntax"> = </span><span class="identifier-syntax">left</span><span class="plain-syntax">-></span><span class="identifier-syntax">construct</span><span class="plain-syntax">; </span><span class="identifier-syntax">R</span><span class="plain-syntax"> = </span><span class="identifier-syntax">right</span><span class="plain-syntax">-></span><span class="identifier-syntax">construct</span><span class="plain-syntax">; }</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">kind_constructor_comparison_schema</span><span class="plain-syntax"> *</span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">for</span><span class="plain-syntax"> (</span><span class="identifier-syntax">dtcs</span><span class="plain-syntax"> = </span><span class="identifier-syntax">L</span><span class="plain-syntax">-></span><span class="identifier-syntax">first_comparison_schema</span><span class="plain-syntax">; </span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">; </span><span class="identifier-syntax">dtcs</span><span class="plain-syntax"> = </span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">-></span><span class="identifier-syntax">next_comparison_schema</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Str::len</span><span class="plain-syntax">(</span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">-></span><span class="identifier-syntax">comparator_unparsed</span><span class="plain-syntax">) > </span><span class="constant-syntax">0</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">-></span><span class="identifier-syntax">comparator</span><span class="plain-syntax"> = </span><span class="identifier-syntax">KindConstructors::parse</span><span class="plain-syntax">(</span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">-></span><span class="identifier-syntax">comparator_unparsed</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">Str::clear</span><span class="plain-syntax">(</span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">-></span><span class="identifier-syntax">comparator_unparsed</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">R</span><span class="plain-syntax"> == </span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">-></span><span class="identifier-syntax">comparator</span><span class="plain-syntax">) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">dtcs</span><span class="plain-syntax">-></span><span class="identifier-syntax">comparison_schema</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">KindConstructors::uses_block_values</span><span class="plain-syntax">(</span><span class="identifier-syntax">L</span><span class="plain-syntax">)) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">KindConstructors::allow_word_as_pointer</span><span class="plain-syntax">(</span><span class="identifier-syntax">L</span><span class="plain-syntax">, </span><span class="identifier-syntax">R</span><span class="plain-syntax">)) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">local_block_value</span><span class="plain-syntax"> *</span><span class="identifier-syntax">pall</span><span class="plain-syntax"> =</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">Frames::allocate_local_block_value</span><span class="plain-syntax">(</span><span class="identifier-syntax">Kinds::base_construction</span><span class="plain-syntax">(</span><span class="identifier-syntax">L</span><span class="plain-syntax">));</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">text_stream</span><span class="plain-syntax"> *</span><span class="identifier-syntax">promotion</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Str::new</span><span class="plain-syntax">();</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">WRITE_TO</span><span class="plain-syntax">(</span><span class="identifier-syntax">promotion</span><span class="plain-syntax">, </span><span class="string-syntax">"*=-BlkValueCompare(*1, BlkValueCast(%S, *#2, *2))==0"</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">pall</span><span class="plain-syntax">-></span><span class="identifier-syntax">to_refer</span><span class="plain-syntax">-></span><span class="identifier-syntax">prototype</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">promotion</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">text_stream</span><span class="plain-syntax"> *</span><span class="identifier-syntax">cr</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Kinds::Behaviour::get_comparison_routine</span><span class="plain-syntax">(</span><span class="identifier-syntax">left</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Str::len</span><span class="plain-syntax">(</span><span class="identifier-syntax">cr</span><span class="plain-syntax">) == </span><span class="constant-syntax">0</span><span class="plain-syntax">) ||</span>
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<span class="plain-syntax"> (</span><span class="identifier-syntax">Str::eq_wide_string</span><span class="plain-syntax">(</span><span class="identifier-syntax">cr</span><span class="plain-syntax">, </span><span class="identifier-syntax">L</span><span class="string-syntax">"signed"</span><span class="plain-syntax">)) ||</span>
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<span class="plain-syntax"> (</span><span class="identifier-syntax">Str::eq_wide_string</span><span class="plain-syntax">(</span><span class="identifier-syntax">cr</span><span class="plain-syntax">, </span><span class="identifier-syntax">L</span><span class="string-syntax">"UnsignedCompare"</span><span class="plain-syntax">))) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">I</span><span class="string-syntax">"*=-*1 == *2"</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">I</span><span class="string-syntax">"*=- *_1(*1, *2) == 0"</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">}</span>
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</pre>
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