Function intersections/unions in $ObjMap don't work

[k15a]

This is a:

• Bug Report

Which concerns:

• flow-runtime

---

What is the current behaviour?

The following code breaks at runtime:

type $DeepShape<O: Object> = Object &
  $Shape<$ObjMap<O, (<V: Object>(V) => $DeepShape<V>) & (<V>(V) => V)>>;

type Thing = {
  a: string,
  b: string,
  c: {
	  a: string,
  }
}

type Hello = $DeepShape<Thing>
  
const hello: Hello = {
	b: 'hello',
}

console.log(hello)

The $DeepShape type is from https://github.com/facebook/flow/issues/5262.

---

What is the expected behaviour?

The code should not break at runtime and should run without type errors.

---

Which package versions are you using?

0.17.0

[phpnode]

This turns out to be due to this part - (<V: Object>(V) => $DeepShape<V>) & (<V>(V) => V) - which is an intersection of two functions. I was surprised to see that Flow supports this because it doesn't appear to be possible to create a real value that satisfies this intersection.

[k15a]

The other option there would be to use a union type instead of an intersection. Then I get the following error:

Mapper must be a function type.

[jedwards1211]

@phpnode

I was surprised to see that Flow supports this because it doesn't appear to be possible to create a real value that satisfies this intersection.

I don't get what you mean, I think any function that deletes some properties of the given object tree when called with an object, or returns a primitive of the same type otherwise, would satisfy that intersection