1. 14
    Null-tracking, or the difference between union and sum types plt blog.waleedkhan.name
    agent281 avatar via agent281 4 years ago |
    Archive.org Archive.today Ghostarchive
    | 4 comments
    1. 5
      kivikakk avatar kivikakk 4 years ago

      I was kind of thinking “what’s the point” until I hit “Singleton null versus a null for every type”, and then the difference became much more obvious.

      1. 4
        bfiedler avatar bfiedler 4 years ago

        Union types also do not form a category-theoretical coproduct, contrary to sum types.

        1. 1
          nutty-buddy-bars avatar nutty-buddy-bars 4 years ago

          Having never worked in a language with union types but no sum types, I wonder if I am correct in thinking that you would almost always want to use union types with manual tagging to emulate sum types. Anyone with experience in union-type-only languages want to weigh in?

          As a concrete example, I feel like I would almost always prefer

          enum UserKind { Id, Name }
type User = 
    { kind : UserKind.Id, id : Number } 
  | { kind : UserKind.Name, name : String }

          over

          type User = String | Number

          (Apologies for the unchecked TypeScript. I did say I’ve never worked in a language with union types.)

          1. 1
            bitrot avatar bitrot 4 years ago

            I would think that the sum type language version (Haskell for example) would look something more like this:

            data User = UserById { id :: Int }
          | UserByName { name :: String }

            To me, the two are equivalently legible, the sum type might look more elegant with the value constructors. But union types are more flexible, since a value can belong to more than one type:

            type Fruit = 'apple' | 'banana';
type Sphere = 'apple' | 'ball';

const acceptsFruit = (fruit: Fruit) => {
  console.log(fruit);
}

const acceptsSphere = (thing: Sphere) => {
  console.log(thing);
}

const toUpper = (s: string) => s.toUpperCase();

let x: string = 'apple';
toUpper(x); // passes
acceptsFruit(x); // type error
if (x === 'apple') {
  acceptsFruit(x); // passes
  acceptsSphere(x); // passes
}