86 lines
2.8 KiB
Text
86 lines
2.8 KiB
Text
# Syntax
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RETRO has more syntax than a traditional Forth due to ideas
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borrowed from ColorForth and some design decisions. This has
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some useful traits, and helps to make the language more
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consistent.
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## Tokens
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Input is divided into a series of whitespace delimited tokens.
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Each of these is then processed individually. There are no
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parsing words in RETRO.
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Tokens may have a single character *prefix*, which RETRO will
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use to decide how to process the token.
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## Prefixes
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Prefixes are single characters added to the start of a token
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to guide the compiler. The use of these is a major way in
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which RETRO differs from traditional Forth.
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When a token is passed to `interpret`, RETRO first takes the
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intitial character and looks to see if there is a word that
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matches this. If so, it will pass the rest of the token to
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that word to handle.
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In a traditional Forth, the interpret process is something
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like:
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get token
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is token in the dictionary?
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yes:
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is it immediate?
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yes: call the word.
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no: are we interpreting?
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yes: call the word
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no: compile a call to the word
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no:
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is it a number?
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yes: are we interpreting?
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yes: push the number to the stack
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no: compile the number as a literal
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no: report an error ("not found")
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In RETRO, the interpret process is basically:
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get token
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does the first character match a `prefix:` word?
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yes: pass the token to the prefix handler
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no: is token a word in the dictionary?
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yes: push the XT to the stack and call the
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class handler
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no: report an error ("not found")
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All of the actual logic for how to deal with tokens is moved
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to the individual prefix handlers, and the logic for handling
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words is moved to word class handlers.
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This means that prefixes are used for a lot of things. Numbers?
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Handled by a `#` prefix. Strings? Use the `'` prefix. Comments?
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Use `(`. Making a new word? Use the `:` prefix.
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The major prefixes are:
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| Prefix | Used For |
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| ------ | ----------------------------- |
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| @ | Fetch from variable |
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| ! | Store into variable |
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| & | Pointer to named item |
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| # | Numbers |
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| $ | ASCII characters |
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| ' | Strings |
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| ( | Comments |
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| : | Define a word |
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The individual prefixes will be covered in more detail in the
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later chapters on working with different data types.
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## Word Classes
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Word classes are words which take a pointer and do something
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with it. These are covered in detail in their own chapter,
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but essentially they decide *how* to execute or compile specific
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types of words.
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