pro November 2010

pro@common-lisp.net

11 participants
4 discussions

[pro] When to use SLOT-VALUE...
by Hans Hübner 02 Dec '10

02 Dec '10

Hi, The company I work for has a Common Lisp style guide that generally disallows using SLOT-VALUE. Instead, accessor should be used so that: BEFORE and :AFTER methods are always invoked when accessing a slot. Generally, I think this is a good idea when looking at classes and instances from the outside. Slots should be considered as being an implementation detail, and users (i.e. client code and derived class methods) should not make assumptions about how functionality is implemented. Now, I often have the need for class instances that are constant in some respect, i.e. some properties of the instance that are implemented using slots can't directly be changed. I often declare such slots havin only a: READER in the class definition, which makes the read-only nature of this slot apparent right away. Of course, such slots need to be initialized somehow. An :INITARG sometimes does the trick, but it is more common that the value of such slots is calculated and recalculated during the lifetime of the instance, and as such the slot's value must be set. Now, from the perspective of seeing the class declaration as documenting the visible behavior of instances of a class, it does not seem to be proper to declare an accessor to be used in class-internal code so that the slot's value can be updated. Instead, I think that it is better to use SLOT-VALUE to mess with the guts of an instance from code that is part of the guts itself. Of course, one may want to argue that DEFCLASS forms should not be considered to be an interface definition. Instead, one could call for a series of DEFGENERIC forms to define the external interface of some "module" and make class definitions be internal. From a more practical perspective, though, class definitions in CL serve both as interface and implementation definition, thus it seems to be appropriate using the mechanisms provided by CLOS to support both uses. How do others use or avoid SLOT-VALUE? Is it frowned upon in your company's or project's (verbal) style guide? Thanks for your input, Hans

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[pro] Chains of multiple-value-bind...
by Scott L. Burson 26 Nov '10

26 Nov '10

[Sorry for the dup, Pascal. Meant to send this to the whole list.] On Fri, Nov 26, 2010 at 5:36 AM, Pascal Costanza <pc(a)p-cos.net> wrote: > Did anybody see anything similar before? Any suggestions for improvement? > My `bcond' macro, which works closely with my `nlet' macro, attempts to solve the same problem. `nlet' subsumes `multiple-value-bind': (nlet ((a b (foo))) (bar a b)) Coming up with a good syntax for `bcond' wasn't easy, and I'm not sure what you'll think of my solution, but here it is. It's like `cond', except that if the test subform is an `nlet' form, the scope of the bindings it establishes is magically extended to included the entire clause. E.g.: (bcond ((nlet ((a b (foo))) (test a b)) (do-something a b)) ... more clauses ...) The expansion is careful not to extend the scope of the bound variables over the following clauses. These macros are in my "misc-extensions" project on common-lisp.net, included in LibCL and Quicklisp. As they are upward-compatible with `cl:let' and `cl:cond', the package provides the same macros under the names `let' and `cond', which can be shadowing-imported if desired (I use them this way). `nlet' also subsumes and generalizes `let*'. BTW I wrote these 30 years ago, in ZetaLisp. I've always made heavy use of multiple values in my style, and `multiple-value-bind' is way too inconvenient to use -- the name is too long and it only takes one binding clause at a time. -- Scott

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[pro] Chains of multiple-value-bind...
by Pascal Costanza 26 Nov '10

26 Nov '10

Hi, I just hacked the following macro: (defmacro multiple-value-case (&body clauses) (when clauses (destructuring-bind (first &rest rest) clauses (cond ((member (first first) '(t otherwise)) (when clauses (warn "Dead code: ~S." clauses)) `(progn ,@(rest first))) (t (destructuring-bind ((&rest lambda-list) expression (guard) &rest body) first `(multiple-value-bind ,lambda-list ,expression (if ,guard (progn ,@body) (multiple-value-case ,@rest))))))))) Did anybody see anything similar before? Any suggestions for improvement? Thanks, Pascal -- Pascal Costanza, mailto:pc@p-cos.net, http://p-cos.net Vrije Universiteit Brussel Software Languages Lab Pleinlaan 2, B-1050 Brussel, Belgium

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[pro] Professional uses of Common Lisp...
by Pascal Costanza 11 Nov '10

11 Nov '10

Hi, Dan recently did a survey of existing Common Lisp implementations, which IMO is a very valuable resource. Another valuable resource would be an updated list of current existing professional uses of Common Lisp. The ALU wiki has a list here: http://wiki.alu.org/Industry%20Application - but that is somewhat old by now, as far as I can tell. Any volunteers for doing a survey along those lines? ;) Pascal -- Pascal Costanza, mailto:pc@p-cos.net, http://p-cos.net Vrije Universiteit Brussel Software Languages Lab Pleinlaan 2, B-1050 Brussel, Belgium

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pro November 2010