Path: utzoo!utgpu!water!watmath!clyde!att!ihnp4!ucbvax!LBL.GOV!LEICHTER%Venus.YCC.Yale.EDU%KL.SRI.COM%lbl%sfsu1.hepnet
From: LEICHTER%Venus.YCC.Yale.EDU%KL.SRI.COM%lbl%sfsu1.hepnet@LBL.GOV
Newsgroups: comp.os.vms
Subject: Forcing expression evaluation in DCL
Message-ID: <880529004903.23e038be@LBL.Gov>
Date: 29 May 88 07:49:03 GMT
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Date: Thu, 26 May 88 09:05 EST
From: "Jerry Leichter (LEICHTER-JERRY@CS.YALE.EDU)"
 <LEICHTER@Venus.YCC.Yale.Edu>
Subject: Forcing expression evaluation in DCL
To: INFO-VAX@KL.SRI.COM
X-VMS-To: INFOVAX
 
In a recent message, I said that DCL provided a syntax for specifying SYMBOL
SUBSTITUTION anywhere (the apostrophe), but no mechanism for specifying
GENERAL EXPRESSION EVALUATION.
 
After thinking about it a bit more, I realized that this is false:  Since
symbol substitution includes evaluation of lexicals, and evaluation of
lexicals can cause general expression evaluation, you can get the effect by
using the F$STRING() lexical in an unexpected way.  Thus, suppose X1 and X2
contain integers and, for whatever reason, you want to create a file whose
name is the sum of the two integers.  The direct approach is:
 
	$ X = X1 + X2
	$ CREATE 'X'
 
The DCL hacker's approach is:
 
	$ CREATE 'F$STRING(X1 + X2)'
 
You can use any expression you like as an argument to F$STRING.  If you want
an integer, rather than a string, result, you can use F$INTEGER instead.  I
can't see any way to make this polymorphic (i.e., get back a string or integer
depending on what the expression evaluated to), but I also can't think of
any reason why you'd want to do that.
 
Of course, this works for quoted strings, too:
 
	$ SEARCH/EXACT FILE.TXT "The number is ''F$STRING(X1 + X2)'"
 
							-- Jerry