Flow type refinement

flow-type-refinement.js

/* @flow */

type A = {
  x: number
};

type B = A & {
  y: string
};

type C = A & {
  z: string
};

function f(o: $Exact<A>|$Exact<B>): ?B {
  if (o.y && o.x > 5)
    return o;
  else
    return null;
}

function g(o: $Exact<B>): A {
  return o;
}

function h(o: ?($Exact<B>|$Exact<C>)) {
  (o: ?(B|C));
  
  if (o)
    (o: $Exact<B>|$Exact<C>);
  
  if (o)
    // A. This type is incompatible with union: `B` | `C`
    (o: B|C);
  
  if (o)
    ((o: $Exact<B>|$Exact<C>): B|C);
}