alectic

Streamed-response use cases get weird/complicated if you have to return the response.

Jeff here (I work on Flow).

I talked a bit about why we infer unions in this way in my ReactEU talk earlier this week (https://www.youtube.com/watch?v=VEaDsKyDxkY).

Ultimately it boils down to the notion that inference is about understanding the type, and annotations are about expressing it.

If you write a type annotation for a variable, then Flow will of course not infer anything more or less than your annotation. If, however, you use the variable as multiple types (but do so in a way that is clearly safe), Flow infers the union so that it doesn't give you errors for code that is clearly ok.

I think the example from the talk was something like:

  var name = "Jeff";
  name = name.toUpperCase(); // safe
  if (loggedOut) {
    name = null; // safe
  }
  var firstInitial = name ? name[0] : null; // safe

The above code has no errors in it, and because flow infers `name` as a type `null | string`, Flow is able to verify its safety and thus doesn't error.

OTOH, if we use an annotation to express the type of `name` as intended as only `string`, then we would get an error on the null assignment:

thing like:

  var name: string = "Jeff";
  name = name.toUpperCase();
  if (loggedOut) {
    name = null; // Error!
  }
  var firstInitial = name ? name[0] : null;

So in summary: Inference is the means by which Flow understands, annotations are the means by which you express to Flow your intentions.

'..."c" must come from H2, "a" from H4 and that leaves "b" as either flaw in H4 or also a H2..."

^^ false dilemma, no?