[eyelidlessness]

My first thought was ASTs and union narrowing too! Probably because that’s been an area of focus for me lately too. Of course there’s other ways to represent a discriminated union, but type keys are particularly useful and usable for AST tools which are designed to be extensible (unified, estree come to mind).

One significant downside, which I think would better support the article’s point, is that type keys make composition (or inheritance if that’s your thing) more awkward. To represent an intersection type, you need either a nested structure (more flexible) or implicit relationships between types (please do not).