On Sunday 28 August 2016 15:56, Juan Pablo Romero Méndez wrote:
> 2016-08-27 21:30 GMT-07:00 Steve D'Aprano <steve+pyt...@pearwood.info>:
[...]
>> Now it is true that speaking in full generality, classes and types refer to
>> different things. Or to be perhaps more accurate, *subclassing* and
>> *subtyping* are different things:
>>
>> http://c2.com/cgi/wiki?SubTypingAndSubClassing
>>
>> Many languages treat them the same, but fundamentally they are different.
>
> Oh, I don't think he is thinking in terms of OO "classes", I think he meant
> two different "kinds" or "varieties" of values (although kind has a
> technical meaning)
That's not quite right either. "Two different kinds or varieties" is what both
subclassing and subtyping aim to describe, in different ways: e.g. dogs and
cats are two different kinds of mammal.
What you are describing here:
> In TypeScript terms what he is saying can be described like this:
>
> type Complex =
> { real: number, i: number }
> | { r: number, φ: number}
>
> const c1: Complex = { real: 1, i: 1 }
> const c2: Complex = { r: 1, φ: 0.5 }
>
> You have two values of the same type but different representation.
seems to be more like what is called "variant record" in Pascal.
http://www.freepascal.org/docs-html/ref/refsu16.html
I'm not familiar with TypeScript. How does this work? If I say:
const c1: Complex = {real: 1, imag: 1 }
print c1.r
print c1.φ
what do I get? If r and φ are merely alternative names for "real" and "imag",
that's not good enough. Given real=1, imag=1, then we need to get r=1.414213
and φ = pi/4 without any extra effort on the part of the user.
The point is, the complex number (1, 1) in Cartesian coordinates and (sqrt(2),
pi/4) in polar coordinates aren't two different kinds of things, they are two
different ways of writing the same value. Like writing 1A in hex and 26 in
decimal. Somebody may choose to implement this as two different classes
("CartesianComplex" and "PolarComplex") but that's a limitation of their code,
or of their language, it doesn't reflect a real difference between two
different kinds of things.
Another way to put it:
If you were programming a first person shooter game, you might choose each of
the enemies as an object. Let's say you have an enemy called the Grue. You can
view the Grue from the front or from the back, depending on which way it is
standing when you see it. Would you implement this as two different classes?
GrueSeenFromFront
GrueSeenFromBack
I should hope not. It's the same object, the same Grue, it just looks different
depending on which way you approach it.
--
Steve
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