retroforth/doc/book/tech-notes/prefixes

## Sigils as a Language Element

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 *how* input is given
to the `interpret` word, or whether anything else will ever be
passed into it.

And so `interpret` 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.

This leads into the sigils. RETRO 11 had a complicated system
for sigils, with different types of sigilss for words that
parsed ahead (e.g., strings) and words that operated on the
current token (e.g., `@`). RETRO 12 eliminates all of these in
favor of just having a single sigil model.

The first thing `interpret` does is look to see if the first
character in a token matches a `sigil:` word. If it does, it
passes the rest of the token as a string pointer to the sigil
specific handler to deal with. If there is no valid sigil
found, it tries to find it in the dictionary. Assuming that it
finds the words, it passes the `d:xt` field to the handler that
`d:class` points to. Otherwise it calls `err:notfound`.

This has an important implication: *words can not reliably
have names that start with a sigil character.*

It also simplifies things. Anything that would normally parse
becomes a sigil handler. So creating a new word? Use the `:`
sigil. Strings? Use `'`. Pointers? Try `&`. And so on. E.g.,

    In ANS                  | In RETRO
    : foo ... ;             | :foo ... ;
    ' foo                   | &foo
    : bar ... ['] foo ;     | :bar ... &foo ;
    s" hello world!"        | 'hello_world!

If you are familiar with ColorForth, sigils are a similar
idea to colors, but can be defined by the user as normal words.

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 sigils in my case) does simplify the implementation
in many ways.