when to use #:
in [Lisp]
|
From: Ron Garret on 6 Apr 2010 11:14 In article <a06fc1e0-aa10-43f3-bd8c-d52032178c7c(a)5g2000yqj.googlegroups.com>, Pillsy <pillsbury(a)gmail.com> wrote: > On Apr 6, 5:31 am, Tamas K Papp <tkp...(a)gmail.com> wrote: > > > I am not completely clear on the applications of the #: read macro for > > uninterned symbols. Redshank inserts it everywhere before symbols > > (defsystem, in-package, and other forms), but AFAIK ASDF loading > > happens in a temporary package anyway, so it should not be necessary. > > Nevertheless, I have seen ASDF files peppered with #: all over the place, > > so maybe I am mistaken. > > > When is it "good style"/recommended to use #:? > > I use them exclusively as string designators in DEFPACKAGE forms, > though their advantages over keywords are marginal. I got into the > habit very early on, before I really understood how packages and > symbols worked, because I saw someone else do it and immitated it and > all the problems I was having using symbols interned in whatever > package I evaluated the DEFPACKAGE form went away. > > Another place I'll use them is when I'm using format to build interned > symbols in macros. For example, I'll often have things like > > (defmacro deffrob (name &body stuff) > `(progn > (defun ,(intern (format nil "~A~A" '#:make name)) () > ... > > Again, this is more habit than anything else. That's a pretty bad habit. Why not: (format nil "~A~A" :make name) or (format nil "~A~A" "MAKE" name) or (format nil "MAKE~A" name) or (concatenate 'string "MAKE" (string name)) rg
From: Tim Bradshaw on 6 Apr 2010 11:15 On 2010-04-06 15:38:00 +0100, Ron Garret said: > > Yes, but in those cases you don't need uninterned symbols. Keywords > work just fine. In fact, that is what keywords are for. Some people don't like interning a large number of symbols they don't need for things like package export lists and so on. > > This is exactly why you should never use #: syntax. It is extremely > rare that you actually need an uninterned symbol with a particular name. > If you find yourself thinking that #:FOO is more appropriate than > (MAKE-SYMBOL "FOO") or #.(MAKE-SYMVBOL "FOO") you are almost certainly > doing something wrong. > What you are often doing is saving yourself a lot of typing.
From: Captain Obvious on 6 Apr 2010 11:16 ??>> Um, I don't remember any system definitions which need lots of string ??>> designators. ZB> They're used for the system's name, and for naming :depends-on ZB> components that are other systems, e.g. Well, yep, it needs some of them, but that doesn't qualify as "lots" IMHO.
From: Ron Garret on 6 Apr 2010 11:32 In article <8239z8zkto.fsf(a)netfonds.no>, "Frode V. Fjeld" <frode(a)netfonds.no> wrote: > > In article <87k4skslja.fsf(a)hangup.portland.xach.com>, > > Zach Beane <xach(a)xach.com> wrote: > >> > >> For DEFPACKAGE, IN-PACKAGE, ASDF:DEFSYSTEM, etc., the uninterned > >> symbol is used only for the string it designates, and it is not > >> evaluated. > > Ron Garret <rNOSPAMon(a)flownet.com> writes: > > > Yes, but in those cases you don't need uninterned symbols. Keywords > > work just fine. In fact, that is what keywords are for. > > I don't think that's true. I believe keywords are typically used for > their identity, whereas (literal) uninterned symbols are usually used > for their name, and occationally for their identity, for example like > this: > > (loop for x = (read nil nil '#0=#:eof) > until (eq x '#0#) > collect x) This example is one of the few legitimate uses of an uninterned symbol, and it is legitimate precisely *because* you *want* to break READ-READ consistency to insure that #0# is returned ONLY on end-of-file and under no other circumstances. But there is nothing special about *symbols* for this application. *Any* uninterned object will do. You could just as well have written: (read nil nil '#0=(list 'eof)) or (let ((eof #:eof)) (loop for x = (read nil nil eof) until (eq x eof) ... or (let ((eof (list :eof))) ... or (let ((eof (make-symbol "EOF"))) ... or (let ((eof (make-instance 'end-of-file-marker))) ... Which is better is a matter of taste. Personally I hate #N= reader syntax because I think it's horribly unreadable. But a reasonable person could disagree. rg
From: Tim Bradshaw on 6 Apr 2010 12:17
On 2010-04-06 15:30:14 +0100, Ron Garret said: > > It's better to use keywords because they also meet all these > requirements, and they also introduce less syntactic clutter and don't > break read-read consistency like #: does. I should have said "don't want to intern a symbol in any package" not "whatever package is current". |