Classes vs. Data Structures
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Classes vs. Data Structures
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Re: Classes vs. Data Structures
#2This conversation reminds me of https://en.wikipedia.org/wiki/Expression_problem .
I don't understand: "but the existence of the data structure implies that some operations must exist."
Grounding it out to a specific data structure, the existence of `List` implies that e.g. `sort` exists?
That direction makes less sense than `sort` implies the existence of e.g. `List`(something to be sorted).
Re: Classes vs. Data Structures
#3The claim that "an object is a set of functions that operate on implied data elements" has a strange corollary because of how in modern OO languages like Java or C#, there is no syntax or popular naming convention to tell the difference between a data structure and an Object. For example, in Java, a LinkedList object is actually not a linked list, it is a set of functions that operate on an implied linked list. If the system needed direct access to the data for whatever reason, we'd need to explicitly have a LinkedList data structure object that only contained the data (the values and their pointers), as well as a second class, the LinkedListOperator, that contains all the functions (add, first, etc.). Likewise, in the author's examples, there'd be a Square class and a SquareOperator class.
I was going to say that Haskell addresses this by putting values in data types and behaviors in "type classes", but then I remembered that functions can be values... which is now making me think that the reality is probably more abstract or complex than the author here is letting on.
Re: Classes vs. Data Structures
#4This conversation reminds me of https://en.wikipedia.org/wiki/Expression_problem . I don't understand: "but the existence of the data structure implies that some operations must exist." Grounding it out to a specific data structure, the existence of `List` implies that e.g. `sort` exists? That direction makes less sense than `sort` implies the existence of e.g. `List`(something to be sorted).
the existence of `List` implies that e.g. `sort` exists?
The existence of List implies that operations must exist to insert an element in a list, access to elements in a list, find out the size of the list, etc.
Edit: BTW 'implies' is the the magic word in the text. It's what creates all the appearance of meaning. Try to replace it what something else. Now I remember why I disliked Plato so much.
Re: Classes vs. Data Structures
#5very interesting and worthwhile! Data has gravity; and data dependancies are more costly than code dependancies, are two lines that are current.
Objects may possibly have broader uses that what is described here ("business data applications") but within the definition given, the description of Object and operations on object make a lot of sense. This post is worth re-reading a few times.
Re: Classes vs. Data Structures
#6This conversation reminds me of https://en.wikipedia.org/wiki/Expression_problem . I don't understand: "but the existence of the data structure implies that some operations must exist." Grounding it out to a specific data structure, the existence of `List` implies that e.g. `sort` exists? That direction makes less sense than `sort` implies the existence of e.g. `List`(something to be sorted).
the existence of `List` implies that e.g. `sort` exists? The existence of List implies that operations must exist to insert an element in a list, access to elements in a list, find out the size of the list, etc. Edit: BTW 'implies' is the the magic word in the text. It's what creates all the appearance of meaning. Try to replace it what something else. Now I remember why I disliked Plato so much.
I thought about using `[]`or `indexOf` as examples of operations, but my question still remains: what is implicit about it? It's part of the public interface of `List`.
Not at all like the private members of an object, which I think was the analogy being made.
Re: Classes vs. Data Structures
#7The first set of points:
> Classes make functions visible while keeping data implied. Data structures make data visible while keeping functions implied. > Classes make it easy to add types but hard to add functions. Data structures make it easy to add functions but hard to add types.
is known as the Expression Problem https://en.wikipedia.org/wiki/Expression_problem.
The last point:
> Data Structures expose callers to recompilation and redeployment. Classes isolate callers from recompilation and redeployment.
is only somewhat true. I suspect it would be more accurate to say that it's a matter of indirection: static dispatch isolates callers from recompilation; static dispatch exposes callers to recompilation; calling a function pointer isolates callers from recompilation; calling a function directly exposes callers to recompilation. (All of this in statically typed languages.) Though this isn't my area of expertise. Perhaps someone else knows more? [Edit: sounds like these "expose" cases often don't cause recompilation either.]
Re: Classes vs. Data Structures
#8Earlier quoted context omitted.
the existence of `List` implies that e.g. `sort` exists? The existence of List implies that operations must exist to insert an element in a list, access to elements in a list, find out the size of the list, etc. Edit: BTW 'implies' is the the magic word in the text. It's what creates all the appearance of meaning. Try to replace it what something else. Now I remember why I disliked Plato so much.
I thought about using `[]`or `indexOf` as examples of operations, but my question still remains: what is implicit about it? It's part of the public interface of `List`. Not at all like the private members of an object, which I think was the analogy being made.
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Re: Classes vs. Data Structures
#9The claim that "an object is a set of functions that operate on implied data elements" has a strange corollary because of how in modern OO languages like Java or C#, there is no syntax or popular naming convention to tell the difference between a data structure and an Object. For example, in Java, a LinkedList object is actually not a linked list, it is a set of functions that operate on an implied linked list. If th…
In a purely oop language, data structures exist as an implementation detail, but you can never access them directly (as in, bypassing the object interface). That's by design.
Re: Classes vs. Data Structures
#10This is really hard to follow and I'm not sure I'm understanding it the way he intended.
Correct me if I'm wrong, but he seems to be saying that, to avoid breaking consumers of your library often by changing the implementation, hide the implementation details behind classes, right?
This seems to be a common reaction to someone who experiments with the "freedom of functional programming" where that freedom means operating on and returning raw data structures that OOP usually hides behind private variables.
That's still bad practice, even in code that heavily uses FP, and good code usually mixes FP and OOP properly, so that you're given functions when you're meant to have functions, and data when you're meant to have data. This is how I've been writing JavaScript for a few years now, and it's not how I've seen Java or Clojure usually written.