Programming Languages

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This is the current Lemmy equivalent of https://www.reddit.com/r/ProgrammingLanguages/.

The content and rules are the same here as they are over there. Taken directly from the /r/ProgrammingLanguages overview:

This community is dedicated to the theory, design and implementation of programming languages.

Be nice to each other. Flame wars and rants are not welcomed. Please also put some effort into your post.

This isn't the right place to ask questions such as "What language should I use for X", "what language should I learn", and "what's your favorite language". Such questions should be posted in /c/learn_programming or /c/programming.

This is the right place for posts like the following:

"Check out this new language I've been working on!"
"Here's a blog post on how I implemented static type checking into this compiler"
"I want to write a compiler, where do I start?"
"How does the Java compiler work? How does it handle forward declarations/imports/targeting multiple platforms/?"
"How should I test my compiler? How are other compilers and interpreters like gcc, Java, and python tested?"
"What are the pros/cons of ?"
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"Confused about the semantics of this language"
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If Inheritance is so bad, why does everyone use it? (buttondown.email)

submitted 7 months ago by [email protected] to c/[email protected]

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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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[–] [email protected] 2 points 7 months ago (1 children)

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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[–] [email protected] 3 points 7 months ago (1 children)

@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.

[–] [email protected] 4 points 7 months ago (1 children)

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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[–] [email protected] 3 points 7 months ago (1 children)

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.

[–] [email protected] 3 points 7 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 @[email protected] too.

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