I just started a side project with Typescript, and I like a lot of it, but, ironically, type discovery seems _very_ lacking. The JS keywords like `instanceof` are useless because everything is just an object, and Typescript doesn't give you a way to pattern match on types. For example, in React there is this type that's a union of an array, single object or null. Some kind of catch-all, I guess. So, when you see one…
Good call. I love TypeScript, and wouldn't build a web app of even small scope without a typing solution at least as good as TypeScript, and it's the best.
But, as you say, pattern matching sum types is a weak point in practical use of TypeScript.
Since the types only exist at compile time, maybe there could be a heavier opt-in system that uses something like Symbols to do pattern matching:
matchable type Shape = Circle | Square | Triangle;
const myShape: Shape;
match myShape {
// uses symbol magic under the hood, eg. if (SymbolShapeCircle in o)
case (c: Circle) { ... }
case (other: Triangle | Square) { ... }
}