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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>1 Simple Goals and Queries</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">▼</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">Racklog:<span class="mywbr"> </span> Prolog-<wbr></wbr>Style Logic Programming</a></td></tr></table></div><div class="tocviewsublistonly" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Simple Goals and Queries</a></td></tr><tr><td align="right">2 </td><td><a href="predicates.html" class="tocviewlink" data-pltdoc="x">Predicates</a></td></tr><tr><td align="right">3 </td><td><a href="racket-w-logic.html" class="tocviewlink" data-pltdoc="x">Using Conventional Racket Expressions in Racklog</a></td></tr><tr><td align="right">4 </td><td><a href="backtracking.html" class="tocviewlink" data-pltdoc="x">Backtracking</a></td></tr><tr><td align="right">5 </td><td><a href="unification.html" class="tocviewlink" data-pltdoc="x">Unification</a></td></tr><tr><td align="right">6 </td><td><a href="and-or.html" class="tocviewlink" data-pltdoc="x">Conjuctions and Disjunctions</a></td></tr><tr><td align="right">7 </td><td><a href="lv-manip.html" class="tocviewlink" data-pltdoc="x">Manipulating Racklog Variables</a></td></tr><tr><td align="right">8 </td><td><a href="cut.html" class="tocviewlink" data-pltdoc="x">The Cut (<span class="RktSym"><span class="RktStxLink">!</span></span>)</a></td></tr><tr><td align="right">9 </td><td><a href="set-of.html" class="tocviewlink" data-pltdoc="x">Set Predicates</a></td></tr><tr><td align="right">10 </td><td><a href="Racklog_Module_Language.html" class="tocviewlink" data-pltdoc="x">Racklog Module Language</a></td></tr><tr><td align="right">11 </td><td><a href="glossary.html" class="tocviewlink" data-pltdoc="x">Glossary of Racklog Primitives</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr></table></div></div></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="index.html" title="backward to "Racklog: Prolog-Style Logic Programming"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Racklog: Prolog-Style Logic Programming"" data-pltdoc="x">up</a> <a href="predicates.html" title="forward to "2 Predicates"" data-pltdoc="x">next →</a></span> </div><h3 x-source-module="(lib "racklog/racklog.scrbl")" x-part-tag=""simple"">1<tt> </tt><a name="(part._simple)"></a>Simple Goals and Queries</h3><p>Racklog objects are the same as Racket objects. However, there
are two subsets of these objects that are of special
interest to Racklog: <span style="font-style: italic">goals</span> and <span style="font-style: italic">predicates</span>. We
will first look at some simple goals.
<a href="predicates.html" data-pltdoc="x">Predicates</a> will introduce predicates and ways
of making complex goals using predicates.</p><p>A goal is an object whose truth or falsity we can check. A
goal that turns out to be true is said to succeed.
A goal that turns out to be false is said to
fail.</p><p><div class="SIntrapara">Two simple goals that are provided in Racklog are:
</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktSym"><a href="glossary.html#%28def._%28%28lib._racklog%2Fmain..rkt%29._~25true%29%29" class="RktValLink" data-pltdoc="x">%true</a></span></td></tr><tr><td><span class="RktSym"><a href="glossary.html#%28def._%28%28lib._racklog%2Fmain..rkt%29._~25fail%29%29" class="RktValLink" data-pltdoc="x">%fail</a></span></td></tr></table></blockquote></div></p><p>The goal <span class="RktSym"><a href="glossary.html#%28def._%28%28lib._racklog%2Fmain..rkt%29._~25true%29%29" class="RktValLink" data-pltdoc="x">%true</a></span> succeeds. The goal <span class="RktSym"><a href="glossary.html#%28def._%28%28lib._racklog%2Fmain..rkt%29._~25fail%29%29" class="RktValLink" data-pltdoc="x">%fail</a></span>
always fails.</p><p>(The names of all Racklog primitive objects
start with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">%</span><span class="hspace"></span></span>. This is to avoid clashes with the names
of conventional Racket objects of related meaning.
User-created objects in Racklog are not required to
follow this convention.)</p><p>A Racklog user can <span style="font-style: italic">query</span> a goal by wrapping it in a
<span class="RktSym"><a href="glossary.html#%28form._%28%28lib._racklog%2Fmain..rkt%29._~25which%29%29" class="RktStxLink" data-pltdoc="x">%which</a></span>-form.</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="glossary.html#%28form._%28%28lib._racklog%2Fmain..rkt%29._~25which%29%29" class="RktStxLink" data-pltdoc="x">%which</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="glossary.html#%28def._%28%28lib._racklog%2Fmain..rkt%29._~25true%29%29" class="RktValLink" data-pltdoc="x">%true</a></span><span class="RktPn">)</span></p></blockquote><p>evaluates to <span class="RktRes">()</span>, indicating success, whereas:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="glossary.html#%28form._%28%28lib._racklog%2Fmain..rkt%29._~25which%29%29" class="RktStxLink" data-pltdoc="x">%which</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="glossary.html#%28def._%28%28lib._racklog%2Fmain..rkt%29._~25fail%29%29" class="RktValLink" data-pltdoc="x">%fail</a></span><span class="RktPn">)</span></p></blockquote><p>evaluates to <span class="RktVal">#f</span>, indicating failure.</p><p>Note 1: The second subexpression of the <span class="RktSym"><a href="glossary.html#%28form._%28%28lib._racklog%2Fmain..rkt%29._~25which%29%29" class="RktStxLink" data-pltdoc="x">%which</a></span>-form
is the empty list <span class="RktRes">()</span>. Later (<a href="predicates.html#%28part._solving-goals%29" data-pltdoc="x">Solving Goals</a>),
we will see <span class="RktSym"><a href="glossary.html#%28form._%28%28lib._racklog%2Fmain..rkt%29._~25which%29%29" class="RktStxLink" data-pltdoc="x">%which</a></span>es
with other lists as the second subform.</p><p>Henceforth, we will use the notation:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktSym">E</span></td></tr><tr><td><p><span class="RktRes">'F</span></p></td></tr></table></blockquote><p>to say that <span class="RktSym">E</span> <span style="font-style: italic">evaluates to</span> <span class="RktSym">F</span>. Thus,</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="glossary.html#%28form._%28%28lib._racklog%2Fmain..rkt%29._~25which%29%29" class="RktStxLink" data-pltdoc="x">%which</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="glossary.html#%28def._%28%28lib._racklog%2Fmain..rkt%29._~25true%29%29" class="RktValLink" data-pltdoc="x">%true</a></span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'()</span></p></td></tr></table></blockquote><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="index.html" title="backward to "Racklog: Prolog-Style Logic Programming"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Racklog: Prolog-Style Logic Programming"" data-pltdoc="x">up</a> <a href="predicates.html" title="forward to "2 Predicates"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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