notes on the interpreter design

FossilOrigin-Name: b3de33b2ab32fa194274c33701589ece2f92160bda46123472af6f44f43c5aeb

doc/Interpreter.md

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