69 lines
2.2 KiB
Markdown
69 lines
2.2 KiB
Markdown
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Stacks
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Words
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Prefixes
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Quotes
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Combinators
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Interpreter Loop:
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- repeat
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- get token
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- does token start with prefix?
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- yes: send token to prefix handler
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- no: is token a name in the dictionary?
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- yes: push the xt of the word to stack, call class handler
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- no: call `err:notfound`
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Tokens are strings, separated by whitespace. (At a minimum, spaces; most
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interfaces will also treat tab, cr, and lf as whitespace as well).
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Prefixes tell RETRO how to handle tokens. Default prefixes are:
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# Convert token to a number
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$ Return first character in token
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' Return token as a string, replacing _ with spaces
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& Return a pointer to a named item
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: Start a new definition
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` Convert token into a number and inline it into the definition
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@ Fetch a value in a named variable
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! Store a value into a named variable
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( Token is a comment
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Tokens without a valid prefix are expected to be words in the dictionary.
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RETRO will search for the token; if found it pushes a pointer (the *xt*)
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to the stack and calls the class handler for the word. If not found, an
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error handler is called.
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Prefix handlers are named words that handle specific prefixes. They get
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named like:
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prefix:#
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And are called when the character after the `prefix:` identifier is the
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first character of a token. Being handled as words allows for adding or
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replacing them as desired.
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Class handlers determine how RETRO handles words. A dictionary entry
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tracks a few elements:
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- a link to the previous entry (link)
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- a pointer to the word's start (xt)
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- a pointer to the word's class handler (class)
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- the name of the word (name)
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A class handler is a word that does something with an xt based on the
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system state. For instance, normal words might have a handler like:
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:class:word (a-)
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compiling? [ compile:call ] [ call ] choose ;
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And *immediate* words would have a handler like:
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:class:immediate (a-) call ;
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As with prefix handlers, new classes can be added and used at any time.
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RETRO does not expose a parser as part of the core language, so there
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are no parsing words. Use of prefixes replaces this.
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