3.4. dynamic symbols

Symbol definitions in alv normally follow ‘lexical scoping’ rules. That means that symbols are looked up by following the scopes outwards according to the syntactical nesting of expressions in the source code.

In the following snippet, for example, the symbol hello is resolved inside print-hello by checking first the innermost scope (the function body), and then the surrounding scope (the whole file), where the value "original message" is found:

(def hello "original message")
(defn print-hello () (print hello))

(do
  (def hello "overwritten message")
  (print-hello))

original message

On the other hand, there are also dynamic symbols. Dynamic symbols are symbols whose name starts and ends with an asterisk, like *clock* and *sym*. Where functions are called, dynamic symbols are not looked up in the scope that contains the function definition, but rather the scope containing the function call site:

(def *hello* "original message")
(defn print-hello () (print *hello*))

(do
  (def *hello* "overwritten message")
  (print-hello))

overwritten message

This allows symbols to be dynamically overwritten.