Name: 講義 4B.ジェネリック演算子 (Lecture 4B: Generic Operators)
Uploaded: 2021-01-14T10:27:06.000Z
Duration: 1 h 23 min 38 s
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[MUSIC-- "JESU, JOY OF MAN'S DESIRING" BY

PROFESSOR: So far in this course we've been talking a

And remember the idea is that we build systems that have

these horizontal barriers in them, these abstraction

barriers that separate use, the way you might use some

data object, from the way you might represent it.

Or another way to think of that is up here you have the

boss who's going to be using some sort of data object.

And down here is George who's implemented it.

Now this notion of separating use from representation so you

can think about these two problems separately is a very,

very powerful programming methodology, data abstraction.

On the other hand, it's not really sufficient for really

complex systems. And the problem with this is George.

Let's suppose there is George, and there's also Martha.

OK, now George and Martha are both working on this system,

They wouldn't cooperate on a representation under any

And the problem is you would like to have some system where

both George and Martha are designing representations, and

yet, if you're above this abstraction barrier you don't

want to have to worry about that, whether something is

Somehow in designing a system, you not only want these

horizontal barriers, but you also want some kind of

vertical barrier to keep George and Martha separate.

Imagine that you're thinking about personnel records for a

large company with a lot of loosely linked divisions that

And imagine even that this company is formed by merging a

whole bunch of companies that already have their personnel

And imagine that once these divisions are all linked in

some kind of very sophisticated satellite

network, and all these databases are put together.

And what you'd like to do is, from any place in the company,

to be able to say things like, oh, what's the name in a

Or, what's the job description in a personnel record?

And not have to worry about the fact that each division

obviously is going to have completely separate

conventions for how you might implement these records.

From this point you don't want to know about that.

One way, of course, is to send down an edict from somewhere

that everybody has to change their format to some fixed

That's what people often try, and of course it never works.

Another thing that you might want to do is somehow arrange

it so you can have these vertical barriers.

So that when you ask for the name of a personnel record,

somehow, whatever format it happens to be, name will

We want name to be, so-called, a generic operator.

Generic operator means what it sort of precisely does depends

on the kind of data that it's looking at.

More than that, you'd like to design the system so that the

next time a new division comes into the company they don't

have to make any big changes in what they're already doing

to link into this system, and the rest of the company

doesn't have to make any big changes to admit their stuff

So that's the problem you should be thinking about.

You want to be able to include new things by

OK, well that's the problem that we'll be

And you should have this sort of distributed personnel

But actually the one I'll be talking about is a problem

that's a little bit more self-contained than that.

that'll bring up the issues, I think, more clearly.

That's the problem of doing a system that does arithmetic on

Just as a little review, there are things

Complex number you can think of as a point in

And you can represent a point either by its real-part and

So if this is z and its real-part is this much, and

its imaginary-part is that much, and you write z

Or another way to represent a complex number is by saying,

what's the distance from the origin, and what's the angle?

So that represents a complex number as its

This one's called-- the original one's called

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講義 4B.ジェネリック演算子 (Lecture 4B: Generic Operators)

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