Disjoint unions without tags

[ghost]

I have this situation and no means to meaningful change the data structure. So I cannot add a tag.
Is there a way to distinguish the types without a tag? I tried ducktyping, but it does not work. See my example

    type Result = Done | Error; // a disjoint union type with two cases
    type Done = { count: number }
    type Error = { message: string }
    
    const doSomethingWithDone = (obj: Done) => {/*...*/}
    const doSomethingWithError = (obj: Error) => {/*...*/}
        
    const f = (result: Result) => {
      if (result.count) {
        doSomethingWithDone(result)
      } else {
        doSomethingWithError(result)
      }
    }

[AugustinLF]

It does make sense, since your typings don't say that a Done cannot have a count property.

Using exact objects types seems to partially work, in the sense that it does refine properly, as you can see in this example. Sadly, you also have to do an explicit check in the else.

[nmote]

I answered this on Stack Overflow: http://stackoverflow.com/a/42281240/901387