Hacker News Reader

Comment by betterunix | Hacker News Reader

"the LCR circuit is the object and you are asking for a method"

I think that you still have the wrong abstraction here. If you are looking for a general solution, it should be one that can handle LCR circuits, mass-spring systems, etc., though that is probably too much for an introductory course. My real point is that there is nothing to be gained by modeling circuit components or even circuits as objects -- you do not improve maintainability, you do not encourage code reuse, and you do increase the amount of code that does nothing to solve the problem.

View on HN

"My real point is that there is nothing to be gained by modeling circuit components or even circuits as objects"

That directly contradicts how SICP teaches it:

Our computational model of a circuit will be composed of objects that correspond to the elementary components from which the circuit is constructed. (p. 273, 2nd ed.)

Elsewhere they say:

If we have been successful in our system organization, then to add a new feature or debug an old one we will have to work on only a localized part of the system. (p. 217)

... which sounds a lot like the benefits you deny. Do you really think SICP is wrong to teach this way? I don't.

I quoted a more general passage where they introduce object modeling here: https://news.ycombinator.com/item?id=6910292. But it seems striking that you would have brought up circuit simulation as when not to do object modeling, as it is the very domain they use to illustrate the strength of the approach.

My real point is that there is nothing to be gained by modeling circuit components or even circuits as objects

Then I still find your point unclear. Why would you choose them as an exercise to introduce classes, which is what we were talking about? I agree, it's odd. Mammals and dogs, however, are a concept people can understand. As are windows and doors.

We were? I thought the question was, "Why shouldn't we teach that objects model real-world things?" The answer is that in many cases, the real-world things should not be represented as objects; I gave an example of such a case. If a student is being told that objects model real-world things and has internalized that, then when they are presented with the problem of simulating an LCR circuit, it is very likely that they will do the wrong thing and create classes for circuit components, and a class for circuits as a whole.

Further, "dogs and mammals" or "windows and doors" are extremely contrived examples. The fact that you have to invoke an artificial problem just to illustrate this point should be an indication that you are doing something wrong. On the other hand, simulating an LCR circuit is a real-world problem and one that students will probably be asked to solve as a homework assignment (e.g. in a differential equations course).

Just because some things should not be represented as objects doesn't mean that nothing should.

And mammals and dogs as a concrete example- a metaphor, if you will- is an example of a powerful way to teach abstract ideas, not an indication that something is wrong.

What I would find enlightening is how the mammal-dog relationship fails, not how there is something out there for which the model may not fit.

The problem with 'dog is a mammal' is that there's basically no obvious scenario in which you'd actually be modeling things this way unless you're trying to build some kind of ecosystem simulation or whatever. If you want to teach people, not only what inheritance is but why it's useful, you need to use a situation where inheritance makes things easier.

Here's an example that's visual: You are putting interaction widgets on the screen- they display some text, perhaps some other visual indicator, and have a button or text area or slider or something on them (there are some of each). Your base class has the setup: the window/panel/whatever-your-language-calls it, the place where the text goes, and the place where the variable widget goes. All of these widgets inherit the base class, and only differ in the user response portion. This makes it very easy to change the interaction style.

Reply on news.ycombinator.com