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From: edp@jareth@enet.dec.com (Always mount a scratch monkey.)
Newsgroups: comp.sys.handhelds
Subject: Re: OBGET and EXGET
Message-ID: <12073@shlump.nac.dec.com>
Date: 31 May 90 03:44:28 GMT
Sender: newsdaemon@shlump.nac.dec.com
Organization: Digital Equipment Corporation
Lines: 32

Here's an example of why OBGET and EXGET are insufficient.  Suppose you
are examining an expression and want to know (in a program) what the
arguments to a MOD function are.  You might have an expression of the form
'MOD(B*C,D)' or 'MOD(B,MOD(C,D))'.  Are the arguments to the first MOD function
B*C and D or are they B and MOD(C,D)? 

Well, your program might do OBGETs and EXGETs and try to figure out the
difference.  But the results are nearly identical for the two expressions:
If you use OBGET to get any object from the two expressions, you'll get
the same type of object in each case -- both expressions have operator,
variable, operator, variable, variable, in that order.  If you use EXGET and
look at the size of the result, you'll get the same numbers for both
expressions -- 5, 1, 3, 1, and 1.

Now your program could figure out the difference between B*C and MOD(C,D)
by performing further analysis.  But that's going to be messy.  You'd have
to do a tree traversal to analyze an algebraic expression anyway, but this
is more involved than a straightfoward traversal.  Some analysis routine
calls itself to figure out what 'B*C' or 'MOD(C,D)' looks like, and then it
reports that to its caller, which uses that to figure out whether the other
argument was D or B, except that could also involve analysis, et cetera.  This
isn't a clean algorithm.

OBGET and EXGET don't analyze algebraic expressions the way the expressions
themselves are structured.  OBJ-> does.  Also, having APPLY makes it easier
to put expressions back together.  On the 28, it is necessary to convert to
strings and fiddle with them in order to reconstruct an algebraic without
evaluating it (which could cause unwanted substitution of variables or
premature calculation of functions of constants).


				-- edp