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Alan Kay disagrees that objects exist to 'encapsulate state'. I agree with him. They also encapulsate behaviour. While it's difficult to get a precise overvi

by chton 12y ago

Alan Kay disagrees that objects exist to 'encapsulate state'. I agree with him. They also encapulsate behaviour. While it's difficult to get a precise overview of Kay's definition (he never gave them explicitly), this page does a good job of collecting the various bits: http://c2.com/cgi/wiki?AlanKaysDefinitionOfObjectOriented http://c2.com/cgi/wiki?AlanKaysDefinitionOfObjectOriented Notable for the "behaviour and state": 3. Objects have their own memory (in terms of objects). 4. Every object is an instance of a class (which must be an object). 5. The class holds the shared behavior for its instances (in the form of objects in a program list) But my point is that I don't agree that an object is "a value that maps names to stuff". I bet even Alan Kay would disagree, seeing as he was a proponent of the Actor model style of working, where objects exchange messages instead of method calls or values. In that case, there are no names to map, there is incoming data that matches a protocol. The object itself determines what behaviour to enact on its own state. Seeing an object as just values and methods is too simplistic, and at the same time too vague to mean anything. And more importantly, as a first sentence to explain what an object is, it's misleading and will raise a lot more questions later.

shriramkmurthi 12y ago
Diving into the middle of a large and complex document and extrapolating from that is always a dangerous venture. The style of the book is to gradually reveal things as it goes along, often even starting with incorrect statements, because we learn best when we see missteps, not only from perfect solutions. Therefore, there's nothing wrong with "starting from" a statement that may not be where we end up. It's meant to be a pedagogic instrument, not a dictionary or other book of definitions. Every chapter is written in this style.
- chton 12y ago
  I agree, my gripes are not with the entire document, or even with the chapter in question. I also don't have a problem with starting from a statement that may be wrong in an informative way, it's a valid approach to learn a subject. My problem is that it's not presented as a starting point or even implied that the definition might be wrong. It categorically states "The simplest notion of an object—pretty much the only thing everyone who talks about objects agrees about— is". If it had said "One notion of an object -one that many people who talk about objects agree on- is", all would be dandy. When you're stating things as fact in a document dedicated to learning, they should be solid. My other gripe is that I've heard too many vague, biased or incomplete answers to the question "what is an object". These are structures people use every second of every day, but they've never learned a proper definition for it?
  - klibertp 12y ago
    > My other gripe is that I've heard too many vague, biased or incomplete answers to the question Well, "behaviour and state" is also incomplete, because it fails to mention anything about how you refer to parts of state and particular behaviours. "a mapping of names to values and functions" on the other hand mentions both state ("values") and behaviour (functions) and also tells you that they have names. > they've never learned a proper definition for it? Well, there are many different valid definitions for objects, and even more for OO. There are people who don't know any of them for sure, but I strongly suspect that you've dismissed many good definitions as "vague, biased, incomplete" on the grounds of them not being your preferred one. You did it at least once just now, when you failed to realize that "a mapping of names to values and functions" is the same thing as "[named] behaviour and state".
    - chton 12y ago
      How you refer to state or behaviour is not strictly relevant to the definition of an object, because it depends on how you implement it. It might fit in the language it was used in, but it doesn't in others. Behaviour might be reactive on state changes, it might be perpetual in a nameless loop, it doesn't matter. In the same vein, referring to state as values and behaviour as functions is too narrow. The lifecycle of the object is part of its state too, an object's construction and destruction is part of its behaviour. That may sound like hair-splitting, but it can seriously trip you up and give you a wrong idea of what an object is in non-conventional settings. An object is more than a map of 'name' to 'value or function'. It's possible that my dismissal of many definitions is partly because they're not my preferred one. I'm human, so it's most likely the case. I like to think my preferred one at least came about through some investigation, and it's the only one that I've seen fit a wide enough variety of cases. For me, at least part of the validation comes from the fact that I was frustrated at the definitions people gave me long before I found one I liked. Too many conversations with college teachers, mentors and colleagues that went "So how would you define/describe an object?" -"..." "OK, but what about [language/edge case/model]?" -"Oh yeah..." I'll gladly have this conversation again, and again, and again, and I hope somebody can poke holes in the definition I prefer, and put me on the other side of it.

by chton 12y ago

shriramkmurthi 12y ago

Diving into the middle of a large and complex document and extrapolating from that is always a dangerous venture. The style of the book is to gradually reveal things as it goes along, often even starting with incorrect statements, because we learn best when we see missteps, not only from perfect solutions. Therefore, there's nothing wrong with "starting from" a statement that may not be where we end up. It's meant to be a pedagogic instrument, not a dictionary or other book of definitions. Every chapter is written in this style.

chton 12y ago
I agree, my gripes are not with the entire document, or even with the chapter in question. I also don't have a problem with starting from a statement that may be wrong in an informative way, it's a valid approach to learn a subject. My problem is that it's not presented as a starting point or even implied that the definition might be wrong. It categorically states "The simplest notion of an object—pretty much the only thing everyone who talks about objects agrees about— is". If it had said "One notion of an object -one that many people who talk about objects agree on- is", all would be dandy. When you're stating things as fact in a document dedicated to learning, they should be solid. My other gripe is that I've heard too many vague, biased or incomplete answers to the question "what is an object". These are structures people use every second of every day, but they've never learned a proper definition for it?
- klibertp 12y ago
  > My other gripe is that I've heard too many vague, biased or incomplete answers to the question Well, "behaviour and state" is also incomplete, because it fails to mention anything about how you refer to parts of state and particular behaviours. "a mapping of names to values and functions" on the other hand mentions both state ("values") and behaviour (functions) and also tells you that they have names. > they've never learned a proper definition for it? Well, there are many different valid definitions for objects, and even more for OO. There are people who don't know any of them for sure, but I strongly suspect that you've dismissed many good definitions as "vague, biased, incomplete" on the grounds of them not being your preferred one. You did it at least once just now, when you failed to realize that "a mapping of names to values and functions" is the same thing as "[named] behaviour and state".
  - chton 12y ago
    How you refer to state or behaviour is not strictly relevant to the definition of an object, because it depends on how you implement it. It might fit in the language it was used in, but it doesn't in others. Behaviour might be reactive on state changes, it might be perpetual in a nameless loop, it doesn't matter. In the same vein, referring to state as values and behaviour as functions is too narrow. The lifecycle of the object is part of its state too, an object's construction and destruction is part of its behaviour. That may sound like hair-splitting, but it can seriously trip you up and give you a wrong idea of what an object is in non-conventional settings. An object is more than a map of 'name' to 'value or function'. It's possible that my dismissal of many definitions is partly because they're not my preferred one. I'm human, so it's most likely the case. I like to think my preferred one at least came about through some investigation, and it's the only one that I've seen fit a wide enough variety of cases. For me, at least part of the validation comes from the fact that I was frustrated at the definitions people gave me long before I found one I liked. Too many conversations with college teachers, mentors and colleagues that went "So how would you define/describe an object?" -"..." "OK, but what about [language/edge case/model]?" -"Oh yeah..." I'll gladly have this conversation again, and again, and again, and I hope somebody can poke holes in the definition I prefer, and put me on the other side of it.