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Programming in Emacs Lisp

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3.6.3 Uninitialized Variables in a let Statement

If you do not bind the variables in a let statement to specific initial values, they will automatically be bound to an initial value of nil, as in the following expression:

(let ((birch 3)
      (oak 'some))
   "Here are %d variables with %s, %s, and %s value."
   birch pine fir oak))

Here, the varlist is ((birch 3) pine fir (oak 'some)).

If you evaluate this expression in the usual way, the following will appear in your echo area:

"Here are 3 variables with nil, nil, and some value."

In this example, Emacs binds the symbol birch to the number 3, binds the symbols pine and fir to nil, and binds the symbol oak to the value some.

Note that in the first part of the let, the variables pine and fir stand alone as atoms that are not surrounded by parentheses; this is because they are being bound to nil, the empty list. But oak is bound to some and so is a part of the list (oak 'some). Similarly, birch is bound to the number 3 and so is in a list with that number. (Since a number evaluates to itself, the number does not need to be quoted. Also, the number is printed in the message using a `%d' rather than a `%s'.) The four variables as a group are put into a list to delimit them from the body of the let.

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  Copyright 2003   by The Free Software Foundation     Updated Jun 2003