Question about union/intersection types AST

[fisker]

Flow version: 0.201.0

Currently, Prettier print UnionTypeAnnotation and IntersectionTypeAnnotation differently from TypeScript parser. It's caused by differences between Flow AST and typescript-eslint AST.

1. The range

// typescript-eslint
type A = & B & C
         ^^^^^^^ Range of `TSIntersectionType`

// Flow
type A = & B & C
           ^^^^^ Range of `IntersectionTypeAnnotation`

2. Single type

// typescript-eslint
type A = | B
         ^^^ TSUnionType
           ^ TSTypeReference

// Flow
type A = | B
           ^ GenericTypeAnnotation
           ^ Identifier

Personally, I think the AST shape of typescript-eslint contains more information. Do you think we can align with it?

I'm working on this part, if we decide not to change, we'll have to make TS AST align with Flow (before print), because it's not possible to do the opposite, we don't have information of the leading | and & (unless we use tokens, which we havn't used yet).

[fisker]

Hi, I saw this assigned to @mroch. Are we going to change the AST?

[mroch]

yes, we'll change this.

I also noticed that (B) | C is missing the leading paren in the union's range (similar for intersections)

[mroch]

actually, the single type would be a much more involved change because it would mean there can be single-member unions or intersections, which we'd then have to handle throughout the type system. neither Babel nor Flow currently produce union or intersection ASTs in this case.

i think we'll skip changing that one for now.