Wsrtjtyk

I am basically trying to create "checker" function and based on the result of that function, call another function. This another function can only accept single type. The checker function should "filter" unwanted types.

Here is an example:

class A {}



class B {}



const checkB = (entity: A | B): boolean => {

  return (entity instanceof B)

}



const execute = (b: B) => {

  console.log('executed')

}



const start = (entity: A | B) => {

  if (checkB(entity)) {

    execute(entity)

  }

}



const a = new A()

const b = new B()



start(a)

start(b)

However Flowtype does not like this and returns this error:

execute(entity)

                ^ Cannot call `execute` with `entity` bound to `b` because `A` [1] is incompatible with `B` [2].

References:

15: const start = (entity: A | B) => {

                           ^ [1]

11: const execute = (b: B) => {

                        ^ [2]

I was wondering if one can somehow convince flow to work this way. Link to Flowtype REPL:
https://flow.org/try/#0PTAEAEDMBsHsHcBQiDG0CGBnToCCoBvAX2TSxwCFCTVYA7TAF1BQAsBTFAayoF5QAFOzqMAlowCeALjygAPqAoBKGQCNYsaO3R1QvAHyFEoUACd2jAK6ndQkeImhRDRjpTtYkRUsQ0U9JlB2AA9OS0Z2PUFVGWU9QwJjFgDNdgA6OABzAQByELCIgBMcnz8A5iZ0U2Z+OzFJGXwFOIMjE1EvATZOHjqHJSU2kyDQlHD2PskfExIyl1B0KLp2eDwBH3951SWVxXXkSuqBdB9DxgFVHyA

EDIT: My environment is set up with Flow v. 0.72.0 but as you can see in the REPL it doesn't work with 0.86.0 either.

Update Nov 26 2018:

There seems to be undocumented %checks annotation that will work that way: https://github.com/facebook/flow/issues/4723#issuecomment-325157852

// @flow



class A {}



class B {}



const checkB = (entity: A | B): boolean %checks => {

  return (entity instanceof B)

}



const execute = (b: B) => {

  console.log('executed')

}



const start = (entity: A | B) => {

  if (checkB(entity)) {

    execute(entity)

  }

}



const a = new A()

const b = new B()



start(a)

start(b)

This doesn't throw any more errors. However there are some issues with this:

• does not work if the checkB function is in different module / file


• does not work with every version of Flow (yes, I'm using older Flow version because of reasons, the code base is pretty large and currently cannot be changed quickly to upgrade to newer Flow version)


• does not work on class methods (which is actually something I'd need)


• since it's not documented, I'm a bit skeptic on using it, they might as well remove it later on and I'd be where I started again

Looks like it's this opened issue (consider add your case too :)).

However, it works well if you delete method checkB and do refinements exactly in start:

const start = (entity: A | B) => {

  if (entity instanceof B) {

    execute(entity)

  }

}

If you have more complex type refinements and need it to be extracted into separate method, you can change checkB into getB, as null refinements seems to works well:

const getB = (entity: A | B): ?B => {

  return (entity instanceof B) ? entity : null;

}



const execute = (b: B) => {

  console.log('executed')

}



const start = (entity: A | B) => {

  const b = getB(entity);

  if (b) {

    execute(b)

  }

}