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Started in on compiling propositions
2021-04-09 12:09:44 +03:00
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../compiler.html">Inform7</a></li><li><a href="index.html">imperative</a></li><li><a href="index.html#3">Chapter 3: Propositions</a></li><li><b>Compile Propositions</b></li></ul></div>
<p class="purpose">To compile a proposition within the body of the current function.</p>
<ul class="toc"><li><a href="3-cp.html#SP1">&#167;1. Compiling a test of a proposition</a></li><li><a href="3-cp.html#SP4">&#167;4. Making a proposition true</a></li><li><a href="3-cp.html#SP5">&#167;5. Checking the validity of a description</a></li></ul><hr class="tocbar">
<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>&#167;1. Compiling a test of a proposition. </b>Given a proposition \(\phi\), and a value \(v\), we compile a valid condition
to decide whether or not \(\phi(v)\) is true. This is essentially how Inform
compiles something like "if all the doors are closed" &mdash; where the proposition
has no free variable, so \(v\) is never substituted &mdash; or the test for something
like "taking an open container", where "an open container" is \(\phi\) and \(v\)
is the item being taken.
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CompilePropositions::to_test_as_condition</span><button class="popup" onclick="togglePopup('usagePopup1')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup1">Usage of <span class="code-font"><span class="function-syntax">CompilePropositions::to_test_as_condition</span></span>:<br/><a href="3-cp.html#SP3">&#167;3</a><br/>Deciding to Defer - <a href="3-dtd.html#SP18">&#167;18</a>, <a href="3-dtd.html#SP21">&#167;21</a><br/>Compile Blocks and Lines - <a href="4-cbal.html#SP4_3_5_1_1">&#167;4.3.5.1.1</a><br/>Compile Invocations - <a href="4-ci.html#SP1_3_1_2_1_1_1">&#167;1.3.1.2.1.1.1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">v</span><span class="plain-syntax">, </span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">prop</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">LOGIF</span><span class="plain-syntax">(</span><span class="identifier-syntax">PREDICATE_CALCULUS_WORKINGS</span><span class="plain-syntax">, </span><span class="string-syntax">"Compiling as test: $D\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">prop</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Propositions::copy</span><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">prop</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="comment-syntax"> the empty proposition is always true</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::val</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">K_truth_state</span><span class="plain-syntax">, </span><span class="identifier-syntax">LITERAL_IVAL</span><span class="plain-syntax">, </span><span class="constant-syntax">1</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> } </span><span class="reserved-syntax">else</span><span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><a href="3-dtd.html#SP6" class="function-link"><span class="function-syntax">Deferrals::defer_test_of_proposition</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">v</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">) == </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">v</span><span class="plain-syntax">) </span><span class="identifier-syntax">Binding::substitute_var_0_in</span><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="identifier-syntax">v</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TRAVERSE_VARIABLE</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">first_atom</span><span class="plain-syntax"> = </span><span class="identifier-syntax">prop</span><span class="plain-syntax">, *</span><span class="identifier-syntax">last_atom</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TRAVERSE_PROPOSITION</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">) </span><span class="identifier-syntax">last_atom</span><span class="plain-syntax"> = </span><span class="identifier-syntax">atom</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><a href="3-cp.html#SP2" class="function-link"><span class="function-syntax">CompilePropositions::to_test_segment</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="identifier-syntax">first_atom</span><span class="plain-syntax">, </span><span class="identifier-syntax">last_atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>&#167;2. </b>The following is used recursively to handle conjunctions and negations: it
tests the portion of a proposition between two atoms (including the two ends).
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CompilePropositions::to_test_segment</span><button class="popup" onclick="togglePopup('usagePopup2')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup2">Usage of <span class="code-font"><span class="function-syntax">CompilePropositions::to_test_segment</span></span>:<br/><a href="3-cp.html#SP1">&#167;1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">from_atom</span><span class="plain-syntax">,</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">to_atom</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">active</span><span class="plain-syntax"> = </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">, </span><span class="identifier-syntax">bl</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">penultimate_atom</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TRAVERSE_VARIABLE</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TRAVERSE_PROPOSITION</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">atom</span><span class="plain-syntax"> == </span><span class="identifier-syntax">from_atom</span><span class="plain-syntax">) </span><span class="identifier-syntax">active</span><span class="plain-syntax"> = </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">active</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">bl</span><span class="plain-syntax"> == </span><span class="constant-syntax">0</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">atom</span><span class="plain-syntax"> != </span><span class="identifier-syntax">from_atom</span><span class="plain-syntax">) &amp;&amp;</span>
<span class="plain-syntax"> (</span><span class="identifier-syntax">Propositions::implied_conjunction_between</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom_prev</span><span class="plain-syntax">, </span><span class="identifier-syntax">atom</span><span class="plain-syntax">))) {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::inv_primitive</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">AND_BIP</span><span class="plain-syntax">); </span><span class="identifier-syntax">Produce::down</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">());</span>
<span class="plain-syntax"> </span><a href="3-cp.html#SP2" class="function-link"><span class="function-syntax">CompilePropositions::to_test_segment</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="identifier-syntax">from_atom</span><span class="plain-syntax">, </span><span class="identifier-syntax">atom_prev</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><a href="3-cp.html#SP2" class="function-link"><span class="function-syntax">CompilePropositions::to_test_segment</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="identifier-syntax">atom</span><span class="plain-syntax">, </span><span class="identifier-syntax">to_atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::up</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">());</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">atom</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">element</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NEGATION_CLOSE_ATOM</span><span class="plain-syntax">) </span><span class="identifier-syntax">bl</span><span class="plain-syntax">--;</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">atom</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">element</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NEGATION_OPEN_ATOM</span><span class="plain-syntax">) </span><span class="identifier-syntax">bl</span><span class="plain-syntax">++;</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">atom</span><span class="plain-syntax"> == </span><span class="identifier-syntax">to_atom</span><span class="plain-syntax">) { </span><span class="identifier-syntax">active</span><span class="plain-syntax"> = </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">; </span><span class="identifier-syntax">penultimate_atom</span><span class="plain-syntax"> = </span><span class="identifier-syntax">atom_prev</span><span class="plain-syntax">; }</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">from_atom</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">element</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NEGATION_OPEN_ATOM</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">to_atom</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">element</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NEGATION_CLOSE_ATOM</span><span class="plain-syntax">)) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">from_atom</span><span class="plain-syntax"> == </span><span class="identifier-syntax">penultimate_atom</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="comment-syntax"> the negation of empty proposition is always false</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::val</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">K_truth_state</span><span class="plain-syntax">, </span><span class="identifier-syntax">LITERAL_IVAL</span><span class="plain-syntax">, </span><span class="constant-syntax">0</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> } </span><span class="reserved-syntax">else</span><span class="plain-syntax"> {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::inv_primitive</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">NOT_BIP</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::down</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">());</span>
<span class="plain-syntax"> </span><a href="3-cp.html#SP2" class="function-link"><span class="function-syntax">CompilePropositions::to_test_segment</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="identifier-syntax">from_atom</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">next</span><span class="plain-syntax">, </span><span class="identifier-syntax">penultimate_atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::up</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">());</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">active</span><span class="plain-syntax"> = </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TRAVERSE_PROPOSITION</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">atom</span><span class="plain-syntax"> == </span><span class="identifier-syntax">from_atom</span><span class="plain-syntax">) </span><span class="identifier-syntax">active</span><span class="plain-syntax"> = </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">active</span><span class="plain-syntax">) </span><a href="3-ca.html#SP1" class="function-link"><span class="function-syntax">CompileAtoms::code_to_perform</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">TEST_ATOM_TASK</span><span class="plain-syntax">, </span><span class="identifier-syntax">atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">atom</span><span class="plain-syntax"> == </span><span class="identifier-syntax">to_atom</span><span class="plain-syntax">) </span><span class="identifier-syntax">active</span><span class="plain-syntax"> = </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>&#167;3. </b>The following wrapper tests if a variable \(v\) matches a description \(\phi\), which is a
case we need often enough to be worth its own function.
</p>
<p class="commentary">If the variable has kind \(K\) then we conjoin to form \({\it kind}_K(v)\land\phi(v)\).
This will cause typechecking to fail if, say, the description "an open door" is
tested on a variable of kind "number". But we issue no problem message in that
case: we simply compile the condition as falsity. In this way, the test correctly
fails and without performing any type-unsafe operations on the value of the variable.
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CompilePropositions::to_test_if_variable_matches</span><button class="popup" onclick="togglePopup('usagePopup3')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup3">Usage of <span class="code-font"><span class="function-syntax">CompilePropositions::to_test_if_variable_matches</span></span>:<br/>Compile Invocations - <a href="4-ci.html#SP1_3_1_2_1_1_1">&#167;1.3.1.2.1.1.1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">v</span><span class="plain-syntax">, </span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">desc</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">desc</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">) </span><span class="identifier-syntax">internal_error</span><span class="plain-syntax">(</span><span class="string-syntax">"VMD against null description"</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">v</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">) </span><span class="identifier-syntax">internal_error</span><span class="plain-syntax">(</span><span class="string-syntax">"VMD on null variable"</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Lvalues::get_storage_form</span><span class="plain-syntax">(</span><span class="identifier-syntax">v</span><span class="plain-syntax">) != </span><span class="identifier-syntax">NONLOCAL_VARIABLE_NT</span><span class="plain-syntax">) &amp;&amp;</span>
<span class="plain-syntax"> (</span><span class="identifier-syntax">Lvalues::get_storage_form</span><span class="plain-syntax">(</span><span class="identifier-syntax">v</span><span class="plain-syntax">) != </span><span class="identifier-syntax">LOCAL_VARIABLE_NT</span><span class="plain-syntax">))</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">internal_error</span><span class="plain-syntax">(</span><span class="string-syntax">"VMD on non-variable"</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">LOG_INDENT</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">prop</span><span class="plain-syntax"> = </span><span class="identifier-syntax">SentencePropositions::from_spec</span><span class="plain-syntax">(</span><span class="identifier-syntax">desc</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> *</span><span class="identifier-syntax">K</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Specifications::to_kind</span><span class="plain-syntax">(</span><span class="identifier-syntax">v</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">prop</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Propositions::concatenate</span><span class="plain-syntax">(</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">KindPredicates::new_atom</span><span class="plain-syntax">(</span><span class="identifier-syntax">K</span><span class="plain-syntax">, </span><span class="identifier-syntax">Terms::new_variable</span><span class="plain-syntax">(0)), </span><span class="identifier-syntax">prop</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">LOGIF</span><span class="plain-syntax">(</span><span class="identifier-syntax">DESCRIPTION_COMPILATION</span><span class="plain-syntax">, </span><span class="string-syntax">"[VMD: $P (%u) matches $D]\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">v</span><span class="plain-syntax">, </span><span class="identifier-syntax">K</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">TypecheckPropositions::type_check</span><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">,</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TypecheckPropositions::tc_no_problem_reporting</span><span class="plain-syntax">()) == </span><span class="identifier-syntax">NEVER_MATCH</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Produce::val</span><span class="plain-syntax">(</span><span class="identifier-syntax">Emit::tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">K_truth_state</span><span class="plain-syntax">, </span><span class="identifier-syntax">LITERAL_IVAL</span><span class="plain-syntax">, </span><span class="constant-syntax">0</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> } </span><span class="reserved-syntax">else</span><span class="plain-syntax"> {</span>
<span class="plain-syntax"> </span><a href="3-cp.html#SP1" class="function-link"><span class="function-syntax">CompilePropositions::to_test_as_condition</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">v</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">LOG_OUTDENT</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>&#167;4. Making a proposition true. </b>Given a proposition \(\phi\) with no free variables, compile code to make the
state of affairs it describes true. This is essentially how Inform compiles
something like "now all the doors are closed".
</p>
<p class="commentary">This is simpler, but a question arises of how to force \(\lnot(\phi\land\psi)\) to
be true. It would be sufficient to falsify either one of \(\phi\) or \(\psi\)
alone, but for the sake of symmetry we falsify both. (We took the same
decision when asserting propositions about the initial state of the model.)
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CompilePropositions::to_make_true</span><button class="popup" onclick="togglePopup('usagePopup4')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup4">Usage of <span class="code-font"><span class="function-syntax">CompilePropositions::to_make_true</span></span>:<br/>Deciding to Defer - <a href="3-dtd.html#SP19">&#167;19</a><br/>Compile Blocks and Lines - <a href="4-cbal.html#SP4_3_2">&#167;4.3.2</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">prop</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">LOGIF</span><span class="plain-syntax">(</span><span class="identifier-syntax">PREDICATE_CALCULUS_WORKINGS</span><span class="plain-syntax">, </span><span class="string-syntax">"Compiling as 'now': $D\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><a href="3-dtd.html#SP8" class="function-link"><span class="function-syntax">Deferrals::defer_now_proposition</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">) == </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">parity</span><span class="plain-syntax"> = </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TRAVERSE_VARIABLE</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TRAVERSE_PROPOSITION</span><span class="plain-syntax">(</span><span class="identifier-syntax">atom</span><span class="plain-syntax">, </span><span class="identifier-syntax">prop</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">switch</span><span class="plain-syntax"> (</span><span class="identifier-syntax">atom</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">element</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NEGATION_OPEN_ATOM:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NEGATION_CLOSE_ATOM:</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">parity</span><span class="plain-syntax"> = (</span><span class="identifier-syntax">parity</span><span class="plain-syntax">)?</span><span class="identifier-syntax">FALSE:TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">break</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">default:</span>
<span class="plain-syntax"> </span><a href="3-ca.html#SP1" class="function-link"><span class="function-syntax">CompileAtoms::code_to_perform</span></a><span class="plain-syntax">(</span>
<span class="plain-syntax"> (</span><span class="identifier-syntax">parity</span><span class="plain-syntax">)?</span><span class="identifier-syntax">NOW_ATOM_TRUE_TASK:NOW_ATOM_FALSE_TASK</span><span class="plain-syntax">, </span><span class="identifier-syntax">atom</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">break</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP5" class="paragraph-anchor"></a><b>&#167;5. Checking the validity of a description. </b>The following utility routine checks that a proposition contains exactly one
free variable, producing problem messages if not &mdash; all of that is really just
defensive programming; if Inform is correctly written, none of these conditions
can ever occur &mdash; but also typechecking the proposition, which does do something
and can indeed throw problems.
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CompilePropositions::verify_descriptive</span><button class="popup" onclick="togglePopup('usagePopup5')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup5">Usage of <span class="code-font"><span class="function-syntax">CompilePropositions::verify_descriptive</span></span>:<br/>Deciding to Defer - <a href="3-dtd.html#SP12">&#167;12</a>, <a href="3-dtd.html#SP14">&#167;14</a>, <a href="3-dtd.html#SP15">&#167;15</a>, <a href="3-dtd.html#SP17">&#167;17</a>, <a href="3-dtd.html#SP20">&#167;20</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">pcalc_prop</span><span class="plain-syntax"> *</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="reserved-syntax">char</span><span class="plain-syntax"> *</span><span class="identifier-syntax">billing</span><span class="plain-syntax">,</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">constructor</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">constructor</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">) </span><span class="identifier-syntax">internal_error</span><span class="plain-syntax">(</span><span class="string-syntax">"description with null constructor"</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="comment-syntax"> best guess at the text to quote in any problem message</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">wording</span><span class="plain-syntax"> </span><span class="identifier-syntax">EW</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Node::get_text</span><span class="plain-syntax">(</span><span class="identifier-syntax">constructor</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Wordings::empty</span><span class="plain-syntax">(</span><span class="identifier-syntax">EW</span><span class="plain-syntax">)) &amp;&amp; (</span><span class="identifier-syntax">constructor</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">down</span><span class="plain-syntax">))</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">EW</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Node::get_text</span><span class="plain-syntax">(</span><span class="identifier-syntax">constructor</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">down</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Binding::is_well_formed</span><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">, </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">) == </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">)</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">internal_error</span><span class="plain-syntax">(</span><span class="string-syntax">"malformed proposition in description verification"</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">N</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Binding::number_free</span><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">N</span><span class="plain-syntax"> == </span><span class="constant-syntax">1</span><span class="plain-syntax">)</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TypecheckPropositions::type_check</span><span class="plain-syntax">(</span><span class="identifier-syntax">prop</span><span class="plain-syntax">,</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">TypecheckPropositions::tc_problem_reporting</span><span class="plain-syntax">(</span><span class="identifier-syntax">EW</span><span class="plain-syntax">,</span>
<span class="plain-syntax"> </span><span class="string-syntax">"involve a range of objects matching a description"</span><span class="plain-syntax">));</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">N</span><span class="plain-syntax"> &gt; </span><span class="constant-syntax">1</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::quote_source</span><span class="plain-syntax">(1, </span><span class="identifier-syntax">current_sentence</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::quote_text</span><span class="plain-syntax">(2, </span><span class="identifier-syntax">billing</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::quote_wording</span><span class="plain-syntax">(3, </span><span class="identifier-syntax">EW</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">StandardProblems::handmade_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">BelievedImpossible</span><span class="plain-syntax">));</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::issue_problem_segment</span><span class="plain-syntax">(</span>
<span class="plain-syntax"> </span><span class="string-syntax">"In %1, you are asking for %2, but this should range over a simpler "</span>
<span class="plain-syntax"> </span><span class="string-syntax">"description than '%3', please - it should not include any determiners such "</span>
<span class="plain-syntax"> </span><span class="string-syntax">"as 'at least three', 'all' or 'most'. (The range is always taken to be all "</span>
<span class="plain-syntax"> </span><span class="string-syntax">"of the things matching the description.)"</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::issue_problem_end</span><span class="plain-syntax">();</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">N</span><span class="plain-syntax"> &lt; </span><span class="constant-syntax">1</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::quote_source</span><span class="plain-syntax">(1, </span><span class="identifier-syntax">current_sentence</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::quote_text</span><span class="plain-syntax">(2, </span><span class="identifier-syntax">billing</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::quote_wording</span><span class="plain-syntax">(3, </span><span class="identifier-syntax">EW</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">StandardProblems::handmade_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">BelievedImpossible</span><span class="plain-syntax">));</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::issue_problem_segment</span><span class="plain-syntax">(</span>
<span class="plain-syntax"> </span><span class="string-syntax">"In %1, you are asking for %2, but '%3' looks as if it ranges over only a "</span>
<span class="plain-syntax"> </span><span class="string-syntax">"single specific object, not a whole collection of objects."</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">Problems::issue_problem_end</span><span class="plain-syntax">();</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax">}</span>
</pre>
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