Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!VENUS.YCC.YALE.EDU!LEICHTER
From: LEICHTER@VENUS.YCC.YALE.EDU ("Jerry Leichter ", LEICHTER-JERRY@CS.YALE.EDU)
Newsgroups: comp.os.vms
Subject: Forcing expression evaluation in DCL
Message-ID: <8805280454.AA02287@ucbvax.Berkeley.EDU>
Date: 26 May 88 14:05:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 28

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