Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!oliveb!pyramid!decwrl!labrea!russell!gandalf
From: gandalf@russell.STANFORD.EDU (Juergen Wagner)
Newsgroups: comp.lang.lisp
Subject: Re: CL question - conditional list elements
Message-ID: <2002@russell.STANFORD.EDU>
Date: 27 Jan 88 23:44:57 GMT
References: <1350005@otter.HP.COM>
Reply-To: gandalf@russell.stanford.edu (Juergen Wagner)
Organization: Center for the Study of Language and Information, Stanford U.
Lines: 23

Conditional elements in lists (which can be determined at compile time)
are easy to write using the backquote syntax:

	`(bit1 bit2
	  ,(and (need-bit3) 'bit3)
	  bit 4 bit5
	  ,(and (need-bit4) 'bit4)
	  bit5 bit6)

I assume, you are aware of this possiblility. However, if you are intending
to sort of tag list elements to restrict their visibility, and to make the
accessibility of list elements context dependent, this should be handled
in an application-dependent way inside the menu handler, or whatsoever used
to interprete these lists. Lists changing from context to context (in 
particular, without external operations being performed) can cause severe
inconsistencies in your program. Imagine taking the (conditional) Car of a
list into a temporary variable. Then you call a function, then another one.
The validity of the isolated Car is questionable after the first call.
Therefore, I suggest to leave such problems up to the application.

-- 
Juergen Wagner,			        gandalf@Russell.Stanford.edu
Center for the Study of Language and Information (CSLI), Stanford CA