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Re: [pro] Modularity for subclassing in Common Lisp
by Anton Vodonosov 07 Dec '10

07 Dec '10

I think the idea to use packages to specify module protocol (interface, contract) is right. And also I think this is better than to have a separate public/protected/private language feature for classes. What would be the motivation for a separate language feature? What is the difference for modularity in case of subclassing, from other cases were we apply modularity? I can't find the difference. You provide two examples. One is sublcallsing: > My favorite simple example is an > "output stream" protocol, that has a write-character operation and a > write-string operation. The common base class provides an > implementation of write-string that works by iterating over the > characters of the string and calling write-character. Any output > stream that can write strings in a more efficient way can override > that method. The second example is modularity: the fhash library hides the implementation details of the fhash-table object from the library client under a generic API. I am trying to find an example where modularity is applied to subclassing to explore the specifics of the modularity in this case, but I can't think of a good example which will highlight the specifics. We may note that "protected" is actually public, in the sense that when we specify something as "protected", we specify that the library has external clients which can rely on this protected method or field. Anyone may inherit from our public class, and access the "protected" members, therefore these members are available to public. If we change the protected member, we affect the library clients. This drives me to a conclusion, that a single language feature is sufficient which allows to separate a protocol for interaction with a module from the module part we expect to change without affecting clients. I made observation on many java systems, that addition to packages the public/protected/private for classes has bad influence on the system modularity. Many java programmers tend to hide attributes of every single class behind get/set accessors, even for simple classes which just hold values passed between methods and do not contain any logic; to create hierarchies of interface/abstract class/concrete for various minor classes, employing protected methods. But at the same time, people do not care to create structure at larger level, to divide the system into modules. It is not uncommon to see large systems where all these classes, subclasses in all the packages are public: everything is available to everything. Separate access control for classes obscures the idea of modularity (especially in case of such an OO centric language like java, the programmers are mostly concentrated on classes/object, and often don't even think about packages;). Class is too small entity to form a module. A module API (protocol) usually consists of set of classes, methods, factory functions, maybe some global variables. Another thought is that the interface between base class and its subclasses is not a second interface, but a part of the first interface. As you said: > So we could have one package > for the code that calls into our library, which would have the usual > protocol, which we can call the "external protocol". Then we could > have a *second package*, which presents a second protocol that > subclases would use! What would be the name for the second protocol? I.e. what is the interaction the base class and the subclass perform via this protocol? As you point, it may be reusing of the implementation (derived streams reuse write-string implementation from the base class). Looks like a thing intended for reusing can rightly be called an "external protocol" too. Another view on the interaction in subclassing is that the base class serves as a template of some functionality, and we can customize its logic by overriding some methods. The output stream example may be viewed in this perspective: we plug-in our write-character implementation into the functionality implemented by the base class in order to customise it to work with different character source. The means to parametrize/customize the protocol behavior can also be considered as a part of the protocol. In simple case it's method parameters, in more complex case we provide, for example, custom strategies, and can do it by defining a method for the sublcass. Therefore it looks to me that in many cases we deal with a single interface, not two distinct ones. Sometimes a module can interact with the outside world via several protocols, but it not necessary relates to sublcassing, and it's not necessary that both (all) the protocols are defined by this module. > Now, there's a great thing about CL packages: you can have more than > one defpackage for the *same* symbols. It's a great feature. I think you mean something like when we want to define one protocol as an extension of another protocol. Although in many cases I suppose it's possible instead of extending interface to have a separate interfaces. I.e. if one API is (func1, func2, func3), and another API is (finc1, func2, func3, extra-func), we could have the second API just as (extra-func). A good example is needed. Maybe something like database connection API, where implementation for particular database is provided by a pluggable "driver"; or maybe swank, and swank backends for different lisps; or something simpler. - Anton

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Re: [pro] The Best Examples of "Code is Data"
by Matthew D. Swank 05 Dec '10

05 Dec '10

On 09/06/2010 12:41 PM, *Paul Tarvydas* wrote: > 1. Doug Hoyte's book Let Over Lambda, chapter 6.3 Alet and Finite State > Machines, raised my macrology to a new level. I used his ideas to > implement a full-blown hierarchical state machine macro(s) (roughly 200 > lines of CL, too large to post here). The hierarchical state machine > paradigm is great for programming reactive things, e.g. gui's, hardware > controllers, etc. > I've read his treatment of state machines, and they seemed like simplified co-routines. Or are you just referring to his macro techniques in aid of your own ideas about state machines? Matt

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[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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Re: [pro] Dennis Kucinich
by Daniel Weinreb 29 Oct '10

29 Oct '10

Apologies, the auto-complete in Thunderbird seems to have screwed the address here. Sorry! Yakov Zaytsev wrote: > wtf > > On Fri, Oct 29, 2010 at 7:23 PM, Daniel Weinreb <dlw(a)itasoftware.com > <mailto:dlw@itasoftware.com>> wrote: > > Dennis Kucinich's race is very close. If you want him to win, > send $, see: > > http://kucinich.us/index.php?option=com_content&task=view&id=46&Itemid=58 > <http://kucinich.us/index.php?option=com_content&task=view&id=46&Itemid=58> > > _______________________________________________ > pro mailing list > pro(a)common-lisp.net <mailto:pro@common-lisp.net> > http://common-lisp.net/cgi-bin/mailman/listinfo/pro > >

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[pro] Out of curiosity, ALU membership?
by Peter Seibel 29 Oct '10

29 Oct '10

I'd be interested to know how many people on this list are members of the ALU. If you are or are not and care to share, feel free to email me. And if you're on the board, maybe you could tell me for how long you've been on it. -Peter -- Peter Seibel http://www.codequarterly.com/

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[pro] Dennis Kucinich
by Daniel Weinreb 29 Oct '10

29 Oct '10

Dennis Kucinich's race is very close. If you want him to win, send $, see: http://kucinich.us/index.php?option=com_content&task=view&id=46&Itemid=58

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[pro] Polling CL implementors and maintainers
by Peter Seibel 05 Oct '10

05 Oct '10

Hi folks, I’m working on my talk for the upcoming ILC. If you are the implementor or maintainer of a Common Lisp system, I’d love to know if you were involved in the standardization of Common Lisp either in the pre-ANSI period or on the ANSI committee. If you could drop me an email saying whether you were or were not and what implementation you work on, that’d be a huge help. If you were, please let me know if it was pre-ANSI, ANSI, or both. -Peter -- Peter Seibel http://www.codequarterly.com/

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