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