Claus Brod: Fight globalization! (18 Feb 2006)

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---+++ [[BlogOnSoftware20060218][Fight globalization!]] (18 Feb 2006)

<summary>
[[BlogOnSoftware200602#11][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.
</summary>

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 [[BlogOnSoftware200602#11][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:

<pre>
  // 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))
  )
</pre>

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 [[BlogOnSoftware20060223][soon]].

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