Re: Don't get all religious on me

Post #176,076 by JimWeirich 9/24/04 1:11:21 AM Reply	Re: Don't get all religious on me I understand that in Smalltalk everything is descended from a root Object. Yes. Does that mean than any object in Smalltalk is substitutable for every other object? No. Just because two classes inherit from a common base class does not mean instances of those classes are substitutable for each other. But both are substitutable for instances of the common base class. (I.e. Instances of Dog and Cat are both substituable for instances of Animal, but dogs and cats are not interchangable). Or that not every instance of inheritance has to be substitutable? There are certainly reasons to inherit without maintaining substitutability (Meyer's OOSC[1] gives a lot of reasons for using inheritance). This is (one of the reasons) why subclassing and subtyping are not the same thing. But non-substitutable inheritance is the exception rather than the rule. [1] "Object Oriented Software Construction" -- Bertrand Meyer. -- -- Jim Weirich jim@weirichhouse.org [link\|http://onestepback.org\|http://onestepback.org] --------------------------------------------------------------------- "Beware of bugs in the above code; I have only proved it correct, not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)
Post #176,129 by ben_tilly 9/24/04 10:34:42 AM Reply	And that's the key point Just because two classes inherit from a common base class does not mean instances of those classes are substitutable for each other. But both are substitutable for instances of the common base class. Substitution is a one-way street. The specific can substitute for the generic, but the generic cannot substitute for the specific. Nor can one specific substitute for another. For instance code which expects to get an object should work with any object, of whatever type. Code which expects to get a receipt should work with different kinds of receipts. But code that expects to get a receipt should not be expected to work properly if it gets an invoice. Cheers, Ben About the use of language: it is impossible to sharpen a pencil with a blunt axe. It is equally vain to try to do it with ten blunt axes instead. -- Edsger W. Dijkstra
Post #176,248 by drewk 9/24/04 3:18:39 PM Reply	Someday I've got to take a class Substitution is a one-way street. The specific can substitute for the generic, but the generic cannot substitute for the specific. Now it makes sense. Yes, in ours any specific can substiture for a generic. I got the impression that bluke was saying that any two child classes of the same parent had to be substitutable. The [link\|http://c2.com/cgi/wiki?LiskovSubstitutionPrinciple\|c2 wiki] I where I looked up LSP also gave that impression. Along with wide disagreement on whether it was a requirement of OO. So the way we do it -- typically a child class is the parent class with additional functionality layered on top -- still counts as substitution? If so then yes, we have been doing that. === Implicitly condoning stupidity since 2001.
Post #176,252 by deSitter 9/24/04 3:54:21 PM Reply	..when I get my inheritance -drl
Post #176,254 by FuManChu 9/24/04 4:00:08 PM Reply	Yup. No generic substitute for that.
Post #176,261 by admin 9/24/04 4:20:45 PM Reply	Good idea in this instance. After all, OO programming is a members-only club... Regards, -scott anderson "Welcome to Rivendell, Mr. Anderson..."
Post #176,281 by FuManChu 9/24/04 5:44:34 PM Reply	I object; it's part of a general pattern
Post #176,288 by admin 9/24/04 6:17:48 PM Reply	Ah, but I have a cunning strategy... Longtime visitors to this forum should recognize the facade of generalization I am presenting here. Private reflection should instantiate understanding, to wit: construction of a public collection of like-minded individuals (a set of peers, as it were) requires interfacing through a common protocol of thought. Without this self-affirming list of congruencies, members of the group will discover that they have begun to substitute incompatible parameters in their world view (Liskov's rules of order notwithstanding). Without a proper shared language, the chain of command breaks down; proxies are invoked, semantics differ from one person's understanding of the social contract to the next, and we are left with a merely functional proposition of useless orientation. Now, I have compiled a List of charter singletons the properties of which should bear up under inspection: 1) That all people are to be objectified equally (this is in there for the feminists). 2) That sorting people into distinct types for purpose of evolving break-out discussion groups should be performed dynamically, as opinions can change. 3) That static organizations die if not fed new ideas periodically. 4) That members should be private in their speculations, and public in their memberships. 5) That closure in rancorous conversations (private message passing is not to be tolerated) can only be achieved through proper allocation of public discussion blocks in the forums. 6) That in most instances one should scope one's thoughts locally, and allow only global actions. I believe I've encapsulated succinctly the behaviour of our group above. Objectors are kindly asked to make sure that they has a valid argument before submitting it to the Club Secretary Isa, as she gets grumpy when presented with more than a single thing at a time... Regards, -scott anderson "Welcome to Rivendell, Mr. Anderson..."
Post #176,291 by FuManChu 9/24/04 6:26:44 PM Reply	Bah. Overridden. Glad you have more time to compile than I do to interpret.
Post #176,294 by admin 9/24/04 6:31:13 PM Reply	Overloaded your capacity for understanding, eh? Typical. Basic cogitation yields continuing bad results. You replied just in time; I'm off to void someone else's warranty package. Ta Ta For New. Regards, -scott anderson "Welcome to Rivendell, Mr. Anderson..."
Post #176,293 by Another Scott 9/24/04 6:29:07 PM Reply	Ashton, is that you? :-) I've heard that the Club Secretary is a party ooper. Cheers, Scott.
Post #176,296 by admin 9/24/04 6:33:02 PM Reply	Better than the treasurer... He's looking for an assistant, but he's a stickler for proper resumes. He spends all his time counting references... Regards, -scott anderson "Welcome to Rivendell, Mr. Anderson..."
Post #176,301 by ben_tilly 9/24/04 6:59:43 PM Reply	He'll have to wait - I'm collecting garbage It is a circular reference, he won't get it. Cheers, Ben About the use of language: it is impossible to sharpen a pencil with a blunt axe. It is equally vain to try to do it with ten blunt axes instead. -- Edsger W. Dijkstra
Post #176,323 by admin 9/24/04 8:48:18 PM Reply	Coincidentally, there's a fellow around here named Mark... Sweeps up the place after people are done heaping up the trash. He applied for the job, but he and the treasurer don't get along. Too much of a generational gap. Regards, -scott anderson "Welcome to Rivendell, Mr. Anderson..."
Post #176,332 by ben_tilly 9/24/04 9:11:32 PM Reply	I'm sorry but that pointer is dangling Buffer luck next time, you wouldn't want to overflow. Cheers, Ben About the use of language: it is impossible to sharpen a pencil with a blunt axe. It is equally vain to try to do it with ten blunt axes instead. -- Edsger W. Dijkstra
Post #176,389 by static 9/25/04 6:26:13 AM Reply	Some people have too much time on their hands... Something obviously failed; better backtrack and try a different goal Wade. I don't know if icon do much better than that pun...
Post #176,400 by admin 9/25/04 9:35:21 AM Reply	You're a real LALRf riot. At least your grammar is good. You might bison new terms for your lexicon, though, before people start feeling the need to flex their stomachs and yacc. Now, I have to say that I'm glad we're keeping this conversation lighthearted. Otherwise it would seem like a bad Film Bacchus Naur... Regards, -scott anderson "Welcome to Rivendell, Mr. Anderson..."
Post #176,403 by Another Scott 9/25/04 10:12:54 AM Reply	So you sed. Personally, I think it's a Perl in the Blinker.
Post #176,309 by jb4 9/24/04 7:27:18 PM Reply	Uuuuuhh... C'mon, It's Friday evening, ferchrissakes! Can't you save something like this for Monday?!? (Nicely done, dammit!) ;-) jb4 shrub\ufffdbish (Am., from shrub* + rubbish, after the derisive name for America's 43 president; 2003) n. 1. a form of nonsensical political doubletalk wherein the speaker attempts to defend the indefensible by lying, obfuscation, or otherwise misstating the facts; GIBBERISH. 2. any of a collection of utterances from America's putative 43rd president. cf.* BULLSHIT
Post #176,387 by Ashton 9/25/04 3:08:51 AM Reply	The cun.. er, clever linguist suffers Code 715 -715 PROGRAM program-name WITH MARK 56064 release-dependency-mark FAILED BECAUSE IT DEPENDS ON FUNCTIONS OF THE RELEASE FROM WHICH FALLBACK HAS OCCURRED ie A stitch, in time has FAILED TO SAVE 9 //so this is how the neurons twitch hereabouts; it is thought that thoughts think themselves via inherited synapience (until compilations by later scholars of the Twelve Original koans reconstruct a hashish table with passed toke(n)s?) Ummm, methinks Sanskrit is hard, but a -904 UNSUCCESSFUL EXECUTION CAUSED BY 57011 AN UNAVAILABLE RESOURCE. REASON reason-code, TYPE OF RESOURCE resource-type, AND RESOURCE NAME resource-name suggests that This is harder... Resources Are scarce, maybe scarce as REASON ..and the penalty appears to be: EXECUTION! Y.P.B. Aummmm mighty sorry fer y'all The ridiculous way is sometimes also the best way that we have to think about things. The LRPD of Small Gods er, :-\ufffd
Post #176,354 by tuberculosis 9/24/04 10:56:23 PM Reply	Or an interface (rimshot here) Seriously, the practical example is something like you have an abstract class WriteStream. You have concrete implementations FileWriteStream and SocketWriteStream. The idea is that you could hand either a FileWriteStream or a SocketWriteStream to anything that expects a WriteStream. Does that make more sense? That was lovely cheese. --Wallace, The Wrong Trousers
Post #176,438 by drewk 9/25/04 10:04:33 PM Reply	Yes, and that's what we do Although PHP doesn't have a concept of an abstract class. We could probably write that way anyway if we wanted to. === Implicitly condoning stupidity since 2001.
Post #176,440 by tuberculosis 9/25/04 11:47:26 PM Reply	Language doesn't need to support abstract class to have one. Smalltalk doesn't have an abstract class per se either. Its quite possible to construct an instance of Object (and its occasionally done to create non-nil sentinel values). You just decide/document something is abstract and don't instance it anywhere. That was lovely cheese. --Wallace, The Wrong Trousers
Post #176,451 by bluke 9/26/04 2:30:26 AM Reply	That was not what I meant Sorry if I was unclear. What I was trying to say is that if 2 classes implement an interface then they are substitutable for the interface or if 2 classes inherit from the same base class they can substitute for the base class.

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