Comment by randomdata | Hacker News Reader

Comment by randomdata | Hacker News Reader

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> Pascal doesn't require case matching of enumerations to be exhaustive

Normally in Pascal you would not match on the enumeration at all, but rather on the sum types.

    type
       Foo = (Bar, Baz)

    case foo:
       Bar: ...  // Matches type Bar
       Baz: ...  // Matches type Baz

The only reason for enumerations in Pascal (and other languages with similar constructs) is because under the hood the computer needs a binary representation to identify the type, and an incrementing number (an enum) is a convenient source for an identifier. In a theoretical world where the machine is magic you could have the sum types without enums, but in this reality...

Thus, yes, in practice it is possible to go around the type system and get the enumerated value out with Ord(foo), but at that point its just an integer and your chance at exhaustive matching is out the window. It is the type system that allows more flexibility in what the compiler can tell you, not the values generated by the enumeration.

> Go only has enums by convention

"Enums by convention" would be manually typing 1, 2, 3, 4, etc. into the code. Indeed, that too is an enumeration, but not as provided by the language. Go actually has enums as a first-class feature of the language[1]. You even say so yourself later on, so this statement is rather curious. I expect you are confusing enums with sum types again.

[1] Arguably Pascal doesn't even have that, only using enums as an implementation detail to support its sum types. Granted, the difference is inconsequential in practice.

Pascal enums are not sum types, because they are not the sum of multiple types. They are an enumeration of discrete values, which is why they're called enums.

Sum types in Pascal are called variant records:

  type
    FooKind = (Foo, Bar, Baz); (* An enum *)

    FooOrBaz = record (* This is the sum type *)
      case foo: FooKind of
        Foo: (quux: Double);
        Bar: (zot, zap: Double);
        Baz: (xyzzy: String);
      end

Rust conflates the 'enum' keyword with sum types. Pascal does not do this. One of us is confused about what a sum type is. It isn't me.

As for Go, my full opinion on that subject may be found here. If you're... curious. Let's say.

https://news.ycombinator.com/item?id=40224485

> This is the sum type

This is the exact same thing, except in addition to the tag there is also a data component. Yes, this is the more traditional representation of sum types, but having an "undefined" data component is still identifiable as a sum type. It is the tag that makes the union a sum type.

In Typescript terms, which I think illustrates this well, it is conceptually the difference between:

   { kind: 0 } | { kind: 1 }

and

   { kind: 0; data: T } | { kind: 1; data: U }

Which is to say that there is no difference with respect to the discussion here.

> They are an enumeration of discrete values

Yes, the "tag" is populated with an enumerator. There is an enumerator involved, that it is certain, but it is outside of the type system as the user sees it. It's just an integer generated to serve as an identifier – an identifier like seen in the above examples – but provided automatically. The additional information you can gain from it, like exhaustive matching, comes at the type level, not the number itself.

> Rust conflates the 'enum' keyword with sum types.

Right, because it too uses an enumerator to generate the tag value. Like Ord(foo) before, you can access the enumerated in Rust with something like

    mem::discriminant(&foo)

The spoken usage of 'enum' in Rust is ultimately misplaced, I agree. An enumerator is not a type! But it is not wrong in identifying that an enumerator is involved. It conflates 'enum' only in the very same way you have here.

There's absolutely a difference between an enumeration and a sum type.

Exactly. One describes a type, the other a numberer. Very different concepts, even if the latter is often used in the implementation of the former (e.g. in Pascal and Rust). Glad to see you are past your earlier confusion.

You do not understand what a sum type is. At all.

There are innumerable ways you can rectify this lack of understanding if you want to.

There may be innumerable (ironic) ways for someone else to explain what it means, but we are here to understand what you mean. The only logical way to achieve that is for you to tell us.

1. If you have a pet sum type definition, let it be known. But traditionally, a sum type is better known as a discriminated or tagged union. So far, this is what we understand Pascal offers: A union type that discriminates its subtypes by an enumerated value.

2. The tag that discriminates the type within the union (or whatever your explanation above ends ups calling it) is, in implementation, generated by a process that assigns a number to each entity; something also true of Rust. This is undeniable, as proven by the use of Ord and mem::discriminant. If you have another name for that numberer, if not an enumerator, let it be known.

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