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From: pbiron@weber.ucsd.edu (Paul Biron)
Newsgroups: comp.sys.mac.hypercard
Subject: EvalExpr() callback in XCMDs
Keywords: EvalExpr, XCMDs vs. Scripts vs. Msg
Message-ID: <2608@network.ucsd.edu>
Date: 8 Jul 90 01:41:59 GMT
Sender: news@network.ucsd.edu
Reply-To: pbiron@weber.ucsd.edu (Paul Biron)
Organization: Division of Social Sciences, UCSD
Lines: 50
Nntp-Posting-Host: weber.ucsd.edu

Hi all,

A while back I asked if there was a way to tell if an
arbitrary object existed (e.g. if Exists (cd fld "foo") then ...).

Numerous people pointed me to the Dartmouth XCMDs (vol 3)
available from sumex.  Well, I finally got them (thanx
to all who helped with that), and eagerly jumped into
the source for ObjectExists().

What I found has the rather simple :

	paramBlock^.returnVal =
		EvalExpr (CONCAT ('the name of '), paramBlock^.params[1]) ;

(or the equivalent, I don't have the code in front of
me at the moment).

Why is it that this works in an XCMD and not in a script
or from the message box?  In a script or from the msg
any reference to a non-existant object results in a

	Never heard of ...

error box (even in HC 1.2.2).

In all the references I own (or have looked at) the
description of the EvalExpr() callback is that it does
the same thing that would happen if its argument were
typed in the msg.

In the same vein, why is it that something like

	go card foo   (where foo is a non-existant card)

when typed into the msg will produce the above "Never..."
error box, yet when in a script no such error occurs?

Anybody know of anything else which works differently
when typed in the Msg or called from a script (except
for 'if', 'then', 'repeat', etc which are documented
to *NOT* work when typed in the Msg)?

Any help will be greatly appreciated.


Paul Biron      pbiron@ucsd.edu        (619) 534-5758
Central University Library, Mail Code C-075-R
Social Sciences DataBase Project
University of California, San Diego, La Jolla, Ca. 92093