108 lines
5.1 KiB
HTML
108 lines
5.1 KiB
HTML
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<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head>
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<title>.</title>
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<style type="text/css">
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* { color: #000; background: #fff; max-width: 700px; }
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tt, pre { background: #dedede; color: #111; font-family: monospace;
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white-space: pre; display: block; width: 100%; }
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.indentedcode { margin-left: 2em; margin-right: 2em; }
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.codeblock {
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background: #dedede; color: #111; font-family: monospace;
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box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19);
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padding: 7px;
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display: block;
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}
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.indentedlist { margin-left: 2em; color: #000; }
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span { white-space: pre; }
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.text { color: #000; white-space: pre; background: #dedede; }
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.colon { color: #000; background: #dedede; }
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.note { color: #000; background: #dedede; }
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.str { color: #000; text-decoration: underline; background: #dedede; }
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.num { color: #000; background: #dedede; font-weight: bold; font-style: italic; }
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.fnum { color: #000; font-weight: bold; background: #dedede; }
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.ptr { color: #000; font-weight: bold; background: #dedede; }
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.fetch { color: #000; font-style: italic; background: #dedede; }
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.store { color: #000; font-style: italic; background: #dedede; }
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.char { color: #000; background: #dedede; }
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.inst { color: #000; background: #dedede; }
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.defer { color: #000; background: #dedede; }
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.imm { color: #000; font-weight: bold; background: #dedede; }
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.prim { color: #000; font-weight: bolder; background: #dedede; }
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.tt { white-space: pre; font-family: monospace; background: #dedede; }
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.h1, .h2, .h3, .h4 { white-space: normal; }
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.h1 { font-size: 125%; }
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.h2 { font-size: 120%; }
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.h3 { font-size: 115%; }
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.h4 { font-size: 110%; }
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.hr { display: block; height: 2px; background: #000000; }
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</style>
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</head><body>
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<p><span class="h1">Loops</span>
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<br/><br/>
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RETRO provides several words for creating loops.
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<br/><br/>
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<br/><br/>
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An unconditional loop begins with <span class="tt">repeat</span> and ends with <span class="tt">again</span>.
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<br/><br/>
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<tt class='indentedcode'>:test repeat #1 n:put sp again ;</tt>
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<tt class='indentedcode'>test</tt>
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<br/><br/>
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Unconditional loops must be inside a definition or quote. To exit
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one of these, use <span class="tt">0;</span>, <span class="tt">-if;</span> or <span class="tt">if;</span>.
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<br/><br/>
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<tt class='indentedcode'>:test #100 repeat 0; dup n:put sp n:dec again ;</tt>
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<tt class='indentedcode'>test</tt>
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<br/><br/>
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<tt class='indentedcode'>:test #100 repeat dup #50 eq? [ 'done! s:put nl ] if; n:dec again ;</tt>
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<tt class='indentedcode'>test</tt>
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<br/><br/>
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You can also achieve this via recursion:
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<br/><br/>
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<tt class='indentedcode'>:test 0; dup n:put sp n:dec test ;</tt>
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<tt class='indentedcode'>#100 test</tt>
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<br/><br/>
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Be careful with recursion as the virtual machine will have a limited
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amount of space for the address stack and recursing too many times
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can cause a stack overflow.
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<br/><br/>
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<br/><br/>
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There are two conditional looping combinators: <span class="tt">while</span> and <span class="tt">until</span>.
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Both take a quote and execute it, checking a returned flag to decide
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when to stop running.
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<br/><br/>
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<tt class='indentedcode'>#0 [ dup n:put sp n:inc dup #10 eq? ] until</tt>
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<tt class='indentedcode'>#10 [ dup n:put sp n:dec dup n:-zero? ] while</tt>
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<br/><br/>
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<br/><br/>
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There are two combinators for counted loops. These are <span class="tt">times</span> and
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<span class="tt">indexed-times</span>.
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<br/><br/>
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<tt class='indentedcode'>#0 #10 [ dup n:put sp n:inc ] times nl</tt>
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<tt class='indentedcode'>#10 [ I n:put sp ] indexed-times</tt>
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<br/><br/>
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The <span class="tt">indexed-times</span> provides an index via the <span class="tt">I</span>, <span class="tt">J</span>, and
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<span class="tt">K</span> words. <span class="tt">I</span> will be the index of the current loop, with <span class="tt">J</span> and
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<span class="tt">K</span> being the indexes of the next two older loops.
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<br/><br/>
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The loop indexes can be accessed outside the loop body:
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<br/><br/>
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<tt class='indentedcode'>:display I n:square n:put sp ;</tt>
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<tt class='indentedcode'>:squares [ display ] indexed-times nl ;</tt>
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<tt class='indentedcode'>#100 squares</tt>
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<br/><br/>
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<span class="h2">Tradeoffs</span>
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<br/><br/>
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The unconditional loop form is more efficient as it's just a
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simple jump operation. The <span class="tt">times</span> counted loops are a little
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slower, but can be cleaner and more readable in many cases. The
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<span class="tt">indexed-times</span> form is significantly slower than the other
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two forms.
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</p>
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</body></html>
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