93 lines
4.9 KiB
HTML
93 lines
4.9 KiB
HTML
<?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="h2">Prefixes as a Language Element</span>
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<br/><br/>
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A big change in RETRO 12 was the elimination of the traditional
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parser from the language. This was a sacrifice due to the lack
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of an I/O model. RETRO has no way to know <strong>how</strong> input is given
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to the <span class="tt">interpret</span> word, or whether anything else will ever be
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passed into it.
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<br/><br/>
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And so <span class="tt">interpret</span> operates only on the current token. The core
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language does not track what came before or attempt to guess at
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what might come in the future.
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<br/><br/>
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This leads into the prefixes. RETRO 11 had a complicated system
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for prefixes, with different types of prefixes for words that
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parsed ahead (e.g., strings) and words that operated on the
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current token (e.g., <span class="tt">@</span>). RETRO 12 eliminates all of these in
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favor of just having a single prefix model.
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<br/><br/>
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The first thing <span class="tt">interpret</span> does is look to see if the first
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character in a token matches a <span class="tt">prefix:</span> word. If it does, it
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passes the rest of the token as a string pointer to the prefix
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specific handler to deal with. If there is no valid prefix
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found, it tries to find it in the dictionary. Assuming that it
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finds the words, it passes the <span class="tt">d:xt</span> field to the handler that
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<span class="tt">d:class</span> points to. Otherwise it calls <span class="tt">err:notfound</span>.
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<br/><br/>
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This has an important implication: <strong>words can not reliably
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have names that start with a prefix character.</strong>
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<br/><br/>
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It also simplifies things. Anything that would normally parse
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becomes a prefix handler. So creating a new word? Use the <span class="tt">:</span>
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prefix. Strings? Use <span class="tt">'</span>. Pointers? Try <span class="tt">&</span>. And so on. E.g.,
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<br/><br/>
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<tt class='indentedcode'>In ANS | In RETRO</tt>
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<tt class='indentedcode'>: foo ... ; | :foo ... ;</tt>
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<tt class='indentedcode'>' foo | &foo</tt>
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<tt class='indentedcode'>: bar ... ['] foo ; | :bar ... &foo ;</tt>
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<tt class='indentedcode'>s" hello world!" | 'hello_world!</tt>
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<br/><br/>
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If you are familiar with ColorForth, prefixes are a similar
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idea to colors, but can be defined by the user as normal words.
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<br/><br/>
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After doing this for quite a while I rather like it. I can see
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why Chuck Moore eventually went towards ColorForth as using
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color (or prefixes in my case) does simplify the implementation
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in many ways.
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</p>
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</body></html>
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