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(emacs-lisp-intro.info)Building Robots

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Building Robots: Extending the Metaphor
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   It is sometimes helpful to think of a running program as a robot that
does a job.  In doing its job, a recursive function calls on a second
robot to help it.  The second robot is identical to the first in every
way, except that the second robot helps the first and has been passed
different arguments than the first.

   In a recursive function, the second robot may call a third; and the
third may call a fourth, and so on.  Each of these is a different
entity; but all are clones.

   Since each robot has slightly different instructions--the arguments
will differ from one robot to the next--the last robot should know when
to stop.

   Let's expand on the metaphor in which a computer program is a robot.

   A function definition provides the blueprints for a robot.  When you
install a function definition, that is, when you evaluate a `defun'
special form, you install the necessary equipment to build robots.  It
is as if you were in a factory, setting up an assembly line.  Robots
with the same name are built according to the same blueprints.  So they
have, as it were, the same `model number', but a different `serial
number'.

   We often say that a recursive function `calls itself'.  What we mean
is that the instructions in a recursive function cause the Lisp
interpreter to run a different function that has the same name and does
the same job as the first, but with different arguments.

   It is important that the arguments differ from one instance to the
next; otherwise, the process will never stop.