94 lines
4.9 KiB
HTML
94 lines
4.9 KiB
HTML
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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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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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</style>
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</head><body>
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<p><br/><br/>
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Word classes are one of the two elements at the heart of
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RETRO's interpreter.
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<br/><br/>
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There are different types of words in a Forth system. At a
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minimum there are data words, regular words, and immediate
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words. There are numerous approaches to dealing with this.
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<br/><br/>
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In RETRO I define special words which receive a pointer and
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decide how to deal with it. These are grouped into a <span class="tt">class:</span>
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namespace.
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<br/><br/>
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<br/><br/>
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When a word is found in the dictionary, RETRO will push a
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pointer to the definition (the <span class="tt">d:xt</span> field) to the stack
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and then call the word specified by the <span class="tt">d:class</span> field.
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<br/><br/>
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The word called is responsible for processing the pointer
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passed to it.
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<br/><br/>
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As a simple case, let's look at <span class="tt">immediate</span> words. These are
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words which will always be called when encountered. A common
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strategy is to have an immediacy bit which the interpreter
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will look at, but RETRO uses a class for this. The class is
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defined:
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='colon'>:class:immediate</span> <span class='note'>(a-)</span> <span class='prim'>call</span> <span class='imm'>;</span> </span><br/>
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<span class="tt">```</span></span><br/><br/>
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Or a normal word. These should be called at interpret time
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or compiled into definitions. The handler for this can look
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like:
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='colon'>:class:word</span> <span class='note'>(a-)</span> compiling? <span class='imm'>[</span> compile:call <span class='imm'>]</span> <span class='imm'>[</span> <span class='prim'>call</span> <span class='imm'>]</span> choose <span class='imm'>;</span> </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<br/><br/>
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The ability to add new classes is useful. If I wanted to add
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a category of word that preserves an input value, I could do
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it with a class:
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='colon'>:class:duplicating</span> <span class='note'>(a-)</span> </span><br/>
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<span class="tt"> compiling? <span class='imm'>[</span> <span class='ptr'>&dup</span> compile:call <span class='imm'>]</span> <span class='imm'>[</span> <span class='ptr'>&dup</span> dip <span class='imm'>]</span> choose </span><br/>
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<span class="tt"> class:word <span class='imm'>;</span> </span><br/>
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<span class="tt"> </span><br/>
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<span class="tt"><span class='colon'>:duplicating</span> <span class='ptr'>&class:duplicating</span> reclass <span class='imm'>;</span> </span><br/>
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<span class="tt"> </span><br/>
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<span class="tt"><span class='colon'>:.</span> n:put nl <span class='imm'>;</span> duplicating </span><br/>
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<span class="tt"><span class='num'>#100</span> <span class='fnum'>.</span> <span class='fnum'>.</span> <span class='fnum'>.</span> </span><br/>
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<span class="tt">```</span></span></p>
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</body></html>
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