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From: ksr@vax135.UUCP (Ken Roberts)
Newsgroups: net.emacs
Subject: MLisp local variables in UniPress (Gosling)
Message-ID: <1514@vax135.UUCP>
Date: Tue, 22-Jul-86 12:06:59 EDT
Article-I.D.: vax135.1514
Posted: Tue Jul 22 12:06:59 1986
Date-Received: Wed, 23-Jul-86 07:44:46 EDT
Organization: AT&T Bell Labs, Holmdel, NJ
Lines: 44

There is an interesting subtlety about local variables and parameter
passing in the MLisp of Unipress (Gosling) Emacs.
Suppose I define the functions 'insert-times' and 'my-test':

(defun
    (insert-times   this-string  n
     ; Inserts 'n' (argument 2) copies
     ; of the string given by 'this-string' (argument 1).
        (setq this-string (arg 1))
        (setq n (arg 2))
        (while (> n 0)
            (setq n (- n 1))
            (insert-string this-string)
        )
    )
)

(defun
    (my-test   this-string
        (insert-times "Hello--" 2)
        (setq this-string "Hello--")
        (insert-times this-string 2)
    )
)

If I now invoke the function 'my-test', 
what will be inserted is not "Hello--Hello--Hello--Hello--",
but rather "Hello--Hello00".
This is because the expression for argument 1 is not evaluated
at the time of the invocation of 'insert-times'; but rather when
(arg 1) is reached.  By that time, the name 'this-string' has been
rebound, and initialized to 0 (by the new local declaration in
the function 'insert-times').

Conclusion:  Due to the way the 'arg' function is defined, local
variables are not entirely insulated from other variables
(local or global) that have the same name.

This is a problem, since the major selling point of local variables
is supposed to be that you are free to choose names for them
without bothering to check if those names are already being used
somewhere else.

Ken Roberts