Fight globalization! (18 Feb 2006)

A few days ago, I talked everybody to sleep about special variables and dynamic bindings in Lisp. I somehow managed to avoid this topic for years, but then I finally had to understand it to fix subtle issues in our code when dealing with what I thought were simple global variables.

In Lisp, you usually declare a global variable using defvar and defparameter - but this way, the variable not only becomes global, but also special. They are probably called special because of the special effects that they display - see my blog entry for an idea of the confusion this caused to a simple-minded C++ programmer (me).

Most of the time, I would use defvar to emulate the effect of a "file-global" static variable in C++, and fortunately, this can be implemented in a much cleaner fashion using a let statement at the right spot. Example:

  // C++, file foobar.C
  static int globalFoo = 42;
  
  int foobar(void)
  {
    return globalFoo * globalFoo;
  }
  int foobar2(int newVal)
  {
    globalFoo = newVal;
  }

  ;; Lisp
  (let ((globalFoo 42))
    (defun foobar1()
      (* globalFoo globalFoo))

    (defun foobar2(newVal)
      (setf globalFoo newVal))
  )

The let statement establishes a binding for globalFoo which is only accessible within foobar1 and foobar2. This is even better than a static global variable in C++ at file level, because this way precisely the functions which actually have a business with globalFoo are able to use it; the functions foobar1 and foobar2 now share a variable. We don't have to declare a global variable anymore and thereby achieve better encapsulation and at the same time avoid special variables with their amusing special effects. Life is good!

This introduces another interesting concept in Lisp: Closures, i.e. functions with references to variables in their lexical context. More on this hopefully soon.


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