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(librep.info)Scope and Extent

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Scope and Extent
----------------

   In the `librep' dialect of Lisp by default variables have "lexical
scope". This means that bindings are associated with textual regions of
programs, and may be accessed by any forms within this associated
textual region. Moreover, the bindings are persistent, even when the
flow of control is currently outside the associated region.

   Consider the following example:

     (let
         ((counter 0))
       (defun count ()
         (setq counter (1+ counter))
         counter))

the value of the `counter' variable persists, and is incremented each
time the `count' function is called. The `counter' variable is
accessible from nowhere but the forms written inside the `let'
statement declaring it.

     (count)
       => 1
     (count)
       => 2

   An alternative method of scoping variables is also available. Any
variables declared using the `defvar' special form are said to be
"special" variables, they have "indefinite scope" and "dynamic extent",
often simplified to "dynamic scope". What this means is that references
to these variables may occur anywhere in a program (i.e. bindings
established in one function are visible within functions called from
the original function) and that references may occur at any point in
time between the binding being created and it being unbound.

   Dynamic scoping is easy to abuse, making programs hard to understand
and debug. A quick example of the use of dynamic scope,

     (defvar *foo-var* nil)
     
     (defun foo (x)
       (let
           ;; a dynamically-scoped binding
           ((*foo-var* (* x 20)))
         (bar x)
         ...
     
     (defun bar (y)
       ;; Since this function is called from
       ;; the function `foo' it can refer
       ;; to `*foo-var*'
       (setq y (+ y *foo-var*))
       ...

As shown in the previous example, a common convention is to mark
special variables by enclosing their names within asterisks.