If Inheritance is so bad, why does everyone use it? (buttondown.email)

submitted 2 months ago by armchair_progamer@programming.dev to c/programming_languages@programming.dev

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This essay says that inheritance is harmful and if possible you should "ban inheritance completely". You see these arguments a lot, as well as things like "prefer composition to inheritance". A lot of these arguments argue that in practice inheritance has problems. But they don't preclude inheritance working in another context, maybe with a better language syntax. And it doesn't explain why inheritance became so popular in the first place. I want to explore what's fundamentally challenging about inheritance and why we all use it anyway.

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[-] mrkeen@mastodon.social 16 points 2 months ago

@onlinepersona @armchair_progamer

A type has a number of 'inhabitants'. 'Sum' indeed corresponds to adding the possible inhabitants together.

A Boolean has two inhabitants - true and false. A byte has 256 inhabitants. A BoolOrByte type has 258 inhabitants.

If you have BoolByte pair, that's a product type - 512 possible inhabitants.

It may make no fucking sense depending on your exposure to Java, where Void (literally 'empty') has an inhabitant, and Boolean has 5.

[-] onlinepersona@programming.dev 2 points 2 months ago

OK, that explains bool | byte, but how is an enum a sum type? And what's a product type? A product of sets is a cartesian product. How does that work with types?

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[-] mrkeen@mastodon.social 3 points 2 months ago

@onlinepersona

An enum is a sum type because the number of inhabitants of the enum is the sum of the inhabitants of its parts.

A product type's number of inhabitants is the product of its parts' inhabitants. So a struct would fit that definition, or a pair, or a tuple.

Looking at the pic on your Cartesian product link:
if A is an enum {x,y,z} and B is an enum {1,2,3}, then a struct AxB has 9 possible inhabitants.

[-] onlinepersona@programming.dev 4 points 2 months ago

OK, I think I'm getting it.

A product is a set that this is the result of an ordered cartesian products.

struct Car {
  make: String,
  model: String,
  seats: u8,
}

Car = String X String x u8.

An enum is a series of "or"s.

enum Animal {
  Dog,
  Cat,
  Giraffe,
  Chimpanzee,
}

can also be thought of as Animal = Dog | Cat | Giraffe | Chimpanzee. Where Dog is a type that only has single value in its set aka Animal = {1} | {2} | {3} | {4}, but it could also be strings, or other objects. Rust however allows more complex objects:

enum ComplexEnum {
    Nothing,
    Something(u32),
    LotsOfThings {
        usual_struct_stuff: bool,
        blah: String,
    }
}

In this case is Something(u32) the equivalent of any "tagged" u32, meaning in memory it's something like a Tag + 32 bits where Tag is a constant string of bits, maybe itself a u32? Wouldn't that make it a product type?
But then LotsOfThings is itself a product type LotsOfThings = bool x String.

So to put it all together ComplexEnum = Nothing | TaggedU32 | (bool x String)? Is that correct?

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[-] arendjr@programming.dev 3 points 2 months ago

So to put it all together ComplexEnum = Nothing | TaggedU32 | (bool x String)? Is that correct?

Pretty much, yeah. But just be aware the tags are effectively unique constants, so each has only one value. For consistency I would write it as:

ComplexEnum = Nothing | Something(u32) | LotsOfThings(bool x String)

In this notation,Something(u32) could also be written as 1 x u32 because tags are constants.

[-] onlinepersona@programming.dev 3 points 2 months ago

OK, so finally I get it. It's pity none of the blogs I've read or wikipedia articles in existence spell it out this way. Instead it's a bunch of math mumbo jumbo.

Thanks for helping me reach understanding 🙏 And thanks to @Kacarott@feddit.de too.

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this post was submitted on 11 Apr 2024

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