about PCL's once-only - comp.lang.lisp

In PCL, chapter 8 - "Macros: Defining Your Own", Peter Seibel presents the macro-writing macro ONCE-ONLY: "Another classic macro-writing macro is once-only, which is used to generate code that evaluates certain macro arguments once only and in a particular order. " My doubt is, given that the pattern ONCE-ONLY abstracts is so common, isn't there an equivalent defined in the standard? Or does everybody use Peter Seibel's ONCE-ONLY?

In article <abb9b34e-0694-4f7d-8953-b71d6159a016@34g2000pru.googlegroups.com>, Simon Ortiz <o.simon@gmail.com> wrote: > My doubt is, given that the pattern ONCE-ONLY abstracts is so common, > isn't there an equivalent defined in the standard? Or does everybody > use Peter Seibel's ONCE-ONLY? No, there's no standard macro for this. I don't recall it even coming up for discussion during standardization. -- Barry Margolin, barmar@alum.mit.edu Arlington, MA *** PLEASE post questions in newsgroups, not directly to me *** *** PLEASE don't copy me on replies, I'll read them in the group ***

Barry Margolin <barmar@alum.mit.edu> wrote: +--------------- | Simon Ortiz <o.simon@gmail.com> wrote: | > My doubt is, given that the pattern ONCE-ONLY abstracts is so common, | > isn't there an equivalent defined in the standard? Or does everybody | > use Peter Seibel's ONCE-ONLY? | | No, there's no standard macro for this. I don't recall it even | coming up for discussion during standardization. +--------------- Though there are certainly variants which predate Peter's, e.g., there's a ONCE-ONLY macro defined in CMUCL ["code/extensions.lisp", line 297 in version 19f] with a slightly different API than Peter's: "Once-Only ({(Var Value-Expression)}*) Form* Create a Let* which evaluates each Value-Expression, binding a temporary variable to the result, and wrapping the Let* around the result of the evaluation of Body. Within the body, each Var is bound to the corresponding temporary variable." -Rob ----- Rob Warnock <rpw3@rpw3.org> 627 26th Avenue <http://rpw3.org/> San Mateo, CA 94403

SO> My doubt is, given that the pattern ONCE-ONLY abstracts is so common, SO> isn't there an equivalent defined in the standard? It is just a conveniece function and is not necessary. There are other ways to do it, e.g. Doug Hoyte describes macro similar to once-only which is even more conventient but funky. SO> Or does everybody use Peter Seibel's ONCE-ONLY? I think everybody should use Alexandria library: http://common-lisp.net/project/alexandria/ It is, basically, a collection of commonly used tools and utilities which are widely used to the point where people think "they should be in the standard". Actually there is a lot of utility libraries (http://www.cliki.net/utilities) which often provide quite similar macros/functions and that proves the fact that "they should be in the standard". Alexandria is meant to stop this madness and provide a common base so people will stop doing this over and over and over again. I'd say ONCE-ONLY is not the most missed macro/function. My personal list (alexandria nomenclature): 1. IF-LET/WHEN-LET 2. ASSOC-VALUE replaces the ugly (CAR (ASSOC ..)) combo 3. ALIST-PLIST/PLIST-ALIST 4. SWITCH 5. STARTS-WITH-SUBSEQ 6. IOTA 7. LASTCAR because 99% of time you write (car (last ...)) 8. ENSURE-CONS I also have a lot of code which looks like: (or (gethash foo bar) (setf (gethash foo bar) (compute-value)) I wish Alexandria had macro for it but unfortunately there is no, so I ended writing my own macro (get-or-init (gethash foo bar) (compute-value)) Alexandria has function ensure-gethash key hash-table &optional default but the problem is that default is always evaluated, even if it is not used. This sucks.

"Captain Obvious" <udodenko@users.sourceforge.net> writes: > SO> My doubt is, given that the pattern ONCE-ONLY abstracts is so common, > SO> isn't there an equivalent defined in the standard? > > It is just a conveniece function and is not necessary. There are other > ways to do it, e.g. Doug Hoyte describes macro similar to once-only > which is even more conventient but funky. > > SO> Or does everybody use Peter Seibel's ONCE-ONLY? > > I think everybody should use Alexandria library: > http://common-lisp.net/project/alexandria/ > It is, basically, a collection of commonly used tools and utilities > which are widely used to the point where people think "they should be > in the standard". > > Actually there is a lot of utility libraries > (http://www.cliki.net/utilities) which often provide quite similar > macros/functions and that proves the fact that "they should be in the > standard". > Alexandria is meant to stop this madness and provide a common base so > people will stop doing this over and over and over again. > > I'd say ONCE-ONLY is not the most missed macro/function. Foremost when you consider the pros and cons: (defmacro mm (x) (let ((vx (gensym))) `(let ((,vx ,x)) (stuff ,vx ,vx)))) Pro: simple to write no overhead simple to read (needs to know only CL), no dependencies Con: you need to know what you're doing and why you need it, and not make an error in realisation. vs. (asdf:load-op :alexandria) (use-package :alexandria) (defmacro mm (x) (once-only (x) `(stuff ,x ,x))) Pro: simple to write you can be more mindless Con: has overhead less simple to read (you now need to know alexandria) has a dependency So unless you have a lot of such macros to write, or you have other needs for a dependency on alexandria (which could occur quite easily), ONCE-ONLY doesn't seem so needed. That said, I have (asdf:load-op :alexandria) in my rc files, along of course with (asdf:load-op :com.informatimago.common-lisp) which loads cesarum ;-) -- __Pascal Bourguignon__ http://www.informatimago.com/ A bad day in () is better than a good day in {}.

On 06/04/2011 06:05, Simon Ortiz wrote: > In PCL, chapter 8 - "Macros: Defining Your Own", Peter Seibel presents > the macro-writing macro ONCE-ONLY: "Another classic macro-writing > macro is once-only, which is used to generate code that evaluates > certain macro arguments once only and in a particular order. " > > My doubt is, given that the pattern ONCE-ONLY abstracts is so common, > isn't there an equivalent defined in the standard? Or does everybody > use Peter Seibel's ONCE-ONLY? LispWorks has 'rebinding which I like a lot. Pascal -- My website: http://p-cos.net Common Lisp Document Repository: http://cdr.eurolisp.org Closer to MOP & ContextL: http://common-lisp.net/project/closer/ The views expressed are my own, and not those of my employer.

On 2011-04-08 16:07:56 -0400, Pascal Costanza said: > LispWorks has 'rebinding which I like a lot. Yes, I use this all the time. It's also available portably as part of arnesi: <http://common-lisp.net/project/bese/docs/arnesi/html/api/macro_005FIT.BESE.ARNESI_003A_003AREBINDING.html> warmest regards, Ralph -- Raffael Cavallaro