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<title>DM4 §28: How nouns are parsed</title>
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<a id="p216" name="p216"></a>
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<h2>§28 How nouns are parsed</h2>
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<blockquote>The Naming of Cats is a difficult matter,<br>
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It isn't just one of your holiday games;<br>
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You may think at first I'm as mad as a hatter<br>
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When I tell you, a cat must have THREE DIFFERENT NAMES.<br>
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— T. S. Eliot (1888–1965), <i>The Naming of Cats</i></blockquote>
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<p class="normal"><span class="atleft"><img src="dm4-216_1.jpg" alt=""></span>
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Suppose we have a tomato defined with</p>
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<p class="lynxonly"></p>
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<pre class="code">name 'fried' 'green' 'tomato',</pre>
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<p class="normal">but which is going to redden later and need to be
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referred to as “red tomato”. The <code>name</code> property
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holds an array of dictionary words, so that</p>
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<p class="lynxonly"></p>
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tomato.&name-->0 == 'fried'
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tomato.&name-->1 == 'green'
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tomato.&name-->2 == 'tomato'
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<p class="normal">(Recall that <code>X.#Y</code> tells you the number of
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<code>-></code> entries in such a property array, in this case six,
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so that <code>X.#Y/2</code> tells you the number of <code>--></code>
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entries, in this case three.) You are quite free to alter this array
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<p class="lynxonly"></p>
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<pre class="code">tomato.&name-->1 = 'red';</pre>
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<p class="normal">The down side of this technique is that it's clumsy,
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when all's said and done, and not so very flexible, because you can't
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change the length of the <code>tomato.&name</code> array during
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play. Of course you <em>could</em> define the tomato</p>
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<p class="lynxonly"></p>
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with name 'fried' 'green' 'tomato' 'blank.' 'blank.' 'blank.'
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'blank.' 'blank.' 'blank.' 'blank.' 'blank.' 'blank.'
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'blank.' 'blank.' 'blank.' 'blank.' 'blank.' 'blank.',
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<p class="normal">or something similar, giving yourself another (say)
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fifteen “slots” to put new names into, but this is
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inelegant even by Inform standards. Instead, an object like the tomato
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can be given a <code>parse_name</code> routine, allowing complete flexibility
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for the designer to specify just what names it does and doesn't match.
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It is time to begin looking into the parser and how it works.</p>
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<a id="p217" name="p217"></a>
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<p class="dotbreak">� � � � �</p>
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<p class="normal">The Inform parser has two cardinal principles:
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firstly, it is designed to be as “open-access” as possible,
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because a parser cannot ever be general enough for every game without
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being highly modifiable. This means that there are many levels on
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which you can augment or override what it does. Secondly, it tries
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to be generous in what it accepts from the player, understanding the
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broadest possible range of commands and making no effort to be strict
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in rejecting ungrammatical requests. For instance, given a shallow
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pool nearby, “examine shallow” has an adjective without
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a noun: but it's clear what the player means. In general, all sensible
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commands should be accepted but it is not important whether or not
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nonsensical ones are rejected.</p>
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<p class="indent">The first thing the parser does is to read in text
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from the keyboard and break it up into a stream of words: so the text
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“wizened man, eat the grey bread” becomes</p>
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<p class="syntax"><tt>wizened</tt> / <tt>man</tt> / <tt>,</tt> /
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<tt>eat</tt> / <tt>the</tt> / <tt>grey</tt> / <tt>bread</tt></p>
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<p class="normal">and these words are numbered from 1. At all times
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the parser keeps a “word number” marker to keep its place
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along this line, and this is held in the variable <code>wn</code>.
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The routine <code>NextWord()</code> returns the word at the current
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position of the marker, and moves it forward, i.e., adds 1 to <code>wn</code>.
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For instance, the parser may find itself at word 6 and trying to
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match “grey bread” as the name of an object. Calling
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<code>NextWord()</code> returns the value <code>'grey'</code> and
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calling it again gives <code>'bread'</code>.</p>
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<p class="indent">Note that if the player had mistyped “grye bread”,
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“grye” being a word which isn't mentioned anywhere in the
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program or created by the library, then <code>NextWord()</code> returns
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0 for ‘not in the dictionary’. Inform creates the dictionary
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of a story file by taking all the <code>name</code> words of objects, all
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the verbs and prepositions from grammar lines, and all the words used
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in constants like <code>'frog'</code> written in the source code,
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and then sorting these into alphabetical order.</p>
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<p class="aside"><span class="warning">▲</span>
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However, the story file's dictionary only has 9-character resolution.
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(And only 6 if Inform has been told to compile an early-model story
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file: see <a href="s45.html">§45</a>.) Thus the values of
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<code>'polyunsaturate'</code> and <code>'polyunsaturated'</code>
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are equal. Also, upper case and lower case letters are considered the
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same. Although dictionary words are permitted to contain numerals
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or typewriter symbols like <code>-</code>, <code>:</code> or
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<code>/</code>, these cost as much as two ordinary letters, so
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<code>'catch-22'</code> looks the same as <code>'catch-2'</code> or
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<code>'catch-207'</code>.</p>
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<p class="aside"><span class="warning">▲▲</span>
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A dictionary word can even contain spaces, full stops or commas, but
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if so it is ‘untypeable’. For instance, <code>'in,out'</code>
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is an untypeable word because if the player were to type something
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like “go in,out”, the text would be broken up into four
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words, <code>go</code> /
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<a id="p218" name="p218"></a>
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<code>in</code> / <code>,</code> / <code>out</code>.
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Thus <code>'in,out'</code> may be in the story file's dictionary but
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it will never match against any word of what the player typed. Surprisingly,
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this can be useful, as it was at the end of <a href="s18.html">§18</a>.</p>
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<p class="dotbreak">� � � � �</p>
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<p class="normal">Since the story file's dictionary isn't always perfect,
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there is sometimes no alternative but to actually look at the player's
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text one character at a time: for instance, to check that a 12-digit
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phone number has been typed correctly and in full.</p>
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<p class="indent">The routine <code>WordAddress(wordnum)</code> returns
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a byte array of the characters in the word, and <code>WordLength(wordnum)</code>
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tells you how many characters there are in it. Given the above example
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text of “wizened man, eat the grey bread”:</p>
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<p class="lynxonly"></p>
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WordAddress(4)->0 == 'e'
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WordAddress(4)->1 == 'a'
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WordAddress(4)->2 == 't'
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<p class="normal">because word number 4 is “eat”. (Recall
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that the comma is considered as a word in its own right.)</p>
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<p class="aside"><span class="warning">▲</span>
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The parser provides a basic routine for comparing a word against the
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texts <code>'0'</code>, <code>'1'</code>, <code>'2'</code>, …,
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<code>'9999'</code>, <code>'10000'</code> or, in other words, against
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small numbers. This is the library routine <code>TryNumber(wordnum)</code>,
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which tries to parse the word at <code>wordnum</code> as a number and
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returns that number, if it finds a match. Besides numbers written
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out in digits, it also recognises the texts <code>'one'</code>,
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<code>'two'</code>, <code>'three'</code>, …, <code>'twenty'</code>.
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If it fails to recognise the text as a number, it returns −1,000;
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if it finds a number greater than 10,000, it rounds down and returns
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<p class="dotbreak">� � � � �</p>
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<p class="normal">To return to the naming of objects, the parser normally
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recognises any arrangement of some or all of the <code>name</code> words of an
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object as a noun which refers to it: and the more words, the better the
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match is considered to be. Thus “fried green tomato” is
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a better match than “fried tomato” or “green tomato”
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but all three are considered to match. On the other hand, so is
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“fried green”, and “green green tomato green fried
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green” is considered a very good match indeed. The method is quick
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and good at understanding a wide variety of sensible texts, though
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poor at throwing out foolish ones. (An example of the parser's strategy
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of being generous rather than strict.) To be more precise, here is what
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happens when the parser wants to match some text against an object:</p>
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<a id="p219" name="p219"></a>
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<ol style="list-style-type:decimal">
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<li>If the object provides a <code>parse_name</code> routine, ask
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this routine to determine how good a match there is.</li>
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<li>If there was no <code>parse_name</code> routine, or if there was
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but it returned −1, ask the entry point routine <code>ParseNoun</code>,
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if the game has one, to make the decision.</li>
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<li>If there was no <code>ParseNoun</code> entry point, or if there
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was but it returned −1, look at the <code>name</code> of the object
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and match the longest possible sequence of words given in the <code>name</code>.</li>
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<p class="normal">So: a <code>parse_name</code> routine, if provided,
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is expected to try to match as many words as possible starting from
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the current position of <code>wn</code> and reading them in one at
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a time using the <code>NextWord()</code> routine. Thus it must not stop
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just because the first word makes sense, but must keep reading and find
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out how many words in a row make sense. It should return:
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<p class="lynxonly"></p>
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<div class="inset"><table>
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<tr><td>0</td><td> if the text didn't make any sense at all,</td></tr>
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<tr><td><i>k</i></td><td> if <i>k</i> words in a row of the text seem to refer to the
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<tr><td>−1</td><td> to tell the parser it doesn't want to decide
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<p class="normal">The word marker <code>wn</code> can be left anywhere
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afterwards. For example, here is the fried tomato with which this section
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<p class="lynxonly"></p>
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parse_name [ n colour;
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if (self.ripe) colour = 'red'; else colour = 'green';
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while (NextWord() == 'tomato' or 'fried' or colour) n++;
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<p class="normal">The effect of this is that if <code>tomato.ripe</code> is
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true then the tomato responds to the names “tomato”, “fried”
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and “red”, and otherwise to “tomato”, “fried”
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and “green”.</p>
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<p class="indent">As a second example of how <code>parse_name</code> can
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be useful, suppose you define:</p>
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<p class="lynxonly"></p>
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Object -> "fly in amber"
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with name 'fly' 'in' 'amber';
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<p class="normal">If the player then types “put fly in amber in
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hole”, the parser will be thrown, because it will think “fly
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in amber in” is all just naming the object and then it won't
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know what the word “hole” is doing at the end. However:</p>
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<p class="lynxonly"></p>
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Object -> "fly in amber"
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if (NextWord() ~= 'fly' or 'amber') return 0;
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if (NextWord() == 'in' && NextWord() == 'amber')
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<a id="p220" name="p220"></a>
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<p class="normal">Now the word “in” is only recognised as
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part of the fly's name if it is followed by the word “amber”,
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and the ambiguity goes away. (“amber in amber” is also
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recognised, but then it's not worth the bother of excluding.)</p>
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<p class="aside"><span class="warning">▲</span>
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<code>parse_name</code> is also used to spot plurals: see
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<a href="s29.html">§29</a>.</p>
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<a id="ex71" name="ex71"></a>
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<p class="aside"><span class="warning"><b>•</b>
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<b><a href="sa6.html#ans71">EXERCISE 71</a></b></span><br>
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Rewrite the tomato's <code>parse_name</code> to insist that the adjectives
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must come before the noun, which must be present.</p>
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<a id="ex72" name="ex72"></a>
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<p class="aside"><span class="warning"><b>•</b>
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<b><a href="sa6.html#ans72">EXERCISE 72</a></b></span><br>
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Create a musician called Princess who, when kissed, is transformed
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into “<tt>/?%?/</tt> (the artiste formerly known as Princess)”.</p>
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<a id="ex73" name="ex73"></a>
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<p class="aside"><span class="warning"><b>•</b>
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<b><a href="sa6.html#ans73">EXERCISE 73</a></b></span><br>
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Construct a drinks machine capable of serving cola, coffee or tea,
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using only one object for the buttons and one for the possible drinks.</p>
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<a id="ex74" name="ex74"></a>
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<p class="aside"><span class="warning"><b>•</b>
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<b><a href="sa6.html#ans74">EXERCISE 74</a></b></span><br>
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Write a <code>parse_name</code> routine which looks through <code>name</code>
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in just the way that the parser would have done anyway if there hadn't
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been a <code>parse_name</code> in the first place.</p>
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<a id="ex75" name="ex75"></a>
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<p class="aside"><span class="warning"><b>•</b>▲
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<b><a href="sa6.html#ans75">EXERCISE 75</a></b></span><br>
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Some adventure game parsers split object names into ‘adjectives’
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and ‘nouns’, so that only the pattern ‹<span class="token">0 or more adjectives</span>›
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‹<span class="token">1 or more nouns</span>› is recognised. Implement this.</p>
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<a id="ex76" name="ex76"></a>
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<p class="aside"><span class="warning"><b>•</b>
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<b><a href="sa6.html#ans76">EXERCISE 76</a></b></span><br>
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During debugging it sometimes helps to be able to refer to objects by
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their internal numbers, so that “put object 31 on object 5”
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would work. Implement this.</p>
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<a id="ex77" name="ex77"></a>
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<p class="aside"><span class="warning"><b>•</b>▲
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<b><a href="sa6.html#ans77">EXERCISE 77</a></b></span><br>
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How could the word “<tt>#</tt>” be made a wild-card,
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meaning “match any single object”?</p>
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<a id="ex78" name="ex78"></a>
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<p class="aside"><span class="warning"><b>•</b>▲▲
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<b><a href="sa6.html#ans78">EXERCISE 78</a></b></span><br>
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And how could “<tt>*</tt>” be a wild-card for “match
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any collection of objects”? (Note: you need to have read
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<a href="s29.html">§29</a> to answer this.)</p>
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<p class="aside"><span class="warning"><b>•</b>
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<b>REFERENCES</b></span><br>
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Straightforward <code>parse_name</code> examples are the chess pieces
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object and the kittens class of ‘Alice Through the Looking-Glass’.
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Lengthier ones are found in ‘Balances’, especially
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in the white cubes class.
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<span class="warning"><b>•</b></span>Miron Schmidt's library
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extension <tt>"calyx_adjectives.h"</tt>, based on earlier
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work by Andrew Clover, provides for objects to have “adnames”
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as well as “names”: “adnames” are usually
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adjectives, and are regarded as being less good
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<a id="p221" name="p221"></a>
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matches for an object
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than “names”. In this system “get string”
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would take either a string bag or a ball of string, but if both were
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present would take the ball of string, because “string”
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is in that case a noun rather than an adjective.</p>
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