Tagged Unions Are Overrated

Re:Tagged Unions Are Overrated by mamcx on Friday February 19, 2021 @09:14AM (#26193340)

The problem with unions/enums is that is hard to pick a subset or superset of them, despite it will be not hard to add to any language. Is similar to projection on the relational model (aka:select), sometimes I need:

    enum Value {
        Int32(i32),
        Int64(i32),
        Str(String),    
    }
    fn only_int(of:Value[Int32, Int64]) -> Value.Str

And other times:

    enum ExtraValue:Value {
        Bool(bool), 
    }

This is analog to structs, where eventually is "discovered" that is nice to:

    let user2 = User {
        email: String::from("[email protected]"),
        username: String::from("anotherusername567"),
        active: user1.active,
        sign_in_count: user1.sign_in_count,
    };


    let user2 = User {
        email: String::from("[email protected]"),
        username: String::from("anotherusername567"),
        ..user1
    };

But somehow enums are still considered second-class in the algebraic type space.

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Re:Tagged Unions Are Overrated by Ari_Rahikkala on Friday February 19, 2021 @09:54AM (#26193782)

I used to think that this was only ever going to be possible with some sort of incomprehensible type trickery that I would never be able to understand. Then TypeScript came along and showed me that no, actually, in a structural type system, it's just about as simple as you could imagine:

    type Foo = { kind: 'A', aItem: number } | { kind: 'B', bItem: string } | { kind: 'C' };
    
    type SubsetFoo = Foo & { kind: 'B' | 'C' }
    type SupersetFoo = Foo | { kind: 'D', dItem: boolean };

I'm sure there are imperfections here. For a start, SubsetFoo's normalized form looks rather ugly when you mouse over it in VSCode. But it does get you the niceties of exhaustiveness checking and type-aware suggestions with control flow awareness, etc..

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Re:Tagged Unions Are Overrated by skjfdoslifjeifj on Friday February 19, 2021 @11:44AM (#26194938)

I believe you can get a better normalized form of SubsetFoo by instead doing:

    type SubsetFoo = Extract<Foo, { kind: 'B' | 'C' }>;

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Re:Tagged Unions Are Overrated by colanderman on Friday February 19, 2021 @10:30AM (#26194164)

OCaml has this for unions, see Polymorphic Variant Types here: https://www.ocaml.org/releases/4.11/htmlman/types.html

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Re:Tagged Unions Are Overrated by mpartel on Friday February 19, 2021 @09:50AM (#26193732)

Scala and TypeScript have (partial) answers to this.

In your example, Scala 2 would let Int32 and Int64 inherit a trait that Str doesn't. TypeScript would let you write a union type Int32 | Int64 that doesn't include Str (or add ExtraValue), and I think Scala 3 will have something very similar.

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Re:Tagged Unions Are Overrated by vmchale on Friday February 19, 2021 @09:48AM (#26193700)

There's a hack!

You can do

    data Value a =
      I32 Int32 a
      B Bool

and then

    type Value1 = Value ()
    type Value2 = Value Void

as a hack

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