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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>4&nbsp;Debug</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"/><script type="text/javascript" src="scribble-common.js"></script><script type="text/javascript" src="manual-racket.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="scribble-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,&quot;tocview_0&quot;);">&#9660;</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">Sugar:<span class="mywbr"> &nbsp;</span> readability &amp; convenience library</a></td></tr></table></div><div class="tocviewsublistonly" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1&nbsp;</td><td><a href="Installation___updates.html" class="tocviewlink" data-pltdoc="x">Installation &amp; updates</a></td></tr><tr><td align="right">2&nbsp;</td><td><a href="Coercion.html" class="tocviewlink" data-pltdoc="x">Coercion</a></td></tr><tr><td align="right">3&nbsp;</td><td><a href="Container.html" class="tocviewlink" data-pltdoc="x">Container</a></td></tr><tr><td align="right">4&nbsp;</td><td><a href="Debug.html" class="tocviewselflink" data-pltdoc="x">Debug</a></td></tr><tr><td align="right">5&nbsp;</td><td><a href="File.html" class="tocviewlink" data-pltdoc="x">File</a></td></tr><tr><td align="right">6&nbsp;</td><td><a href="Len.html" class="tocviewlink" data-pltdoc="x">Len</a></td></tr><tr><td align="right">7&nbsp;</td><td><a href="License___source_code.html" class="tocviewlink" data-pltdoc="x">License &amp; source code</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">report</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="versionbox"><span class="version">6.0.1.13</span></div><div class="navsettop"><span class="navleft"><div class="nosearchform"></div>&nbsp;&nbsp;</span><span class="navright">&nbsp;&nbsp;<a href="Container.html" title="backward to &quot;3 Container&quot;" data-pltdoc="x">&larr; prev</a>&nbsp;&nbsp;<a href="index.html" title="up to &quot;Sugar: readability &amp; convenience library&quot;" data-pltdoc="x">up</a>&nbsp;&nbsp;<a href="File.html" title="forward to &quot;5 File&quot;" data-pltdoc="x">next &rarr;</a></span>&nbsp;</div><h3>4<tt>&nbsp;</tt><a name="(part._.Debug)"></a><a name="(mod-path._sugar/debug)"></a>Debug</h3><p><table cellspacing="0" cellpadding="0" class="defmodule"><tr><td align="left"><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="stt"> </span><a href="Debug.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">sugar/debug</span></a><span class="RktPn">)</span></td><td align="right"><span class="RpackageSpec"><span class="Smaller">&nbsp;package:</span> <span class="stt">sugar</span></span></td></tr></table></p><p>Debugging utilities.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._sugar/debug..rkt)._report))"></a><span title="Provided from: sugar/debug | Package: sugar"><span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">report</a></span></span><span class="hspace">&nbsp;</span><span class="RktVar">expr</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Print the name and value of <span class="RktVar">expr</span> to <span class="RktSym"><a href="http://docs.racket-lang.org/reference/port-ops.html#%28def._%28%28quote._~23~25kernel%29._current-error-port%29%29" class="RktValLink" data-pltdoc="x">current-error-port</a></span>, but also return the evaluated result of <span class="RktVar">expr</span> as usual. This lets you see the value of an expression or variable at runtime without disrupting any of the surrounding code.</div></p><p>For instance, suppose you wanted to see how <span class="RktSym">first-condition?</span> was being evaluted in this expression:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28quote._~23~25kernel%29._if%29%29" class="RktStxLink" data-pltdoc="x">if</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">first-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">second-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">one-thing</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">other-thing</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>You can wrap it in <span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxLink" data-pltdoc="x">report</a></span> and find out:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28quote._~23~25kernel%29._if%29%29" class="RktStxLink" data-pltdoc="x">if</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxLink" data-pltdoc="x">report</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">first-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">second-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">one-thing</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">other-thing</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>This code will run the same way as before. But when it reaches <span class="RktSym">first-condition?</span>, you willl see in <span class="RktSym"><a href="http://docs.racket-lang.org/reference/port-ops.html#%28def._%28%28quote._~23~25kernel%29._current-error-port%29%29" class="RktValLink" data-pltdoc="x">current-error-port</a></span>:</p><p><span class="RktErr">(first-condition? x) = #t</span></p><p>You can also add standalone calls to <span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxLink" data-pltdoc="x">report</a></span> as a debugging aid at points where the return value will be irrelevant, for instance:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxLink" data-pltdoc="x">report</a></span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28quote._~23~25kernel%29._if%29%29" class="RktStxLink" data-pltdoc="x">if</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxLink" data-pltdoc="x">report</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">first-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">second-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">one-thing</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">other-thing</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p><span class="RktErr">x = 42
<br/>(first-condition? x) = #t</span></p><p>But don&rsquo;t do this, because the result of the <span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28quote._~23~25kernel%29._if%29%29" class="RktStxLink" data-pltdoc="x">if</a></span> expression will be skipped in favor of the last expression, which will be the value of <span class="RktVar">x</span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28quote._~23~25kernel%29._if%29%29" class="RktStxLink" data-pltdoc="x">if</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxLink" data-pltdoc="x">report</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">first-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">second-condition?</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">one-thing</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym">other-thing</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="Debug.html#%28form._%28%28lib._sugar%2Fdebug..rkt%29._report%29%29" class="RktStxLink" data-pltdoc="x">report</a></span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr></table></blockquote><div class="navsetbottom"><span class="navleft"><div class="nosearchform"></div>&nbsp;&nbsp;</span><span class="navright">&nbsp;&nbsp;<a href="Container.html" title="backward to &quot;3 Container&quot;" data-pltdoc="x">&larr; prev</a>&nbsp;&nbsp;<a href="index.html" title="up to &quot;Sugar: readability &amp; convenience library&quot;" data-pltdoc="x">up</a>&nbsp;&nbsp;<a href="File.html" title="forward to &quot;5 File&quot;" data-pltdoc="x">next &rarr;</a></span>&nbsp;</div></div></div><div id="contextindicator">&nbsp;</div></body></html>
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