type with Maybe props doesn't allow type with Non-Maybe props

Question

Why does flow throw errors here?

type MaybeProp = {
  prop: ?number,
}

type DefinetlyProp = {
  prop: number,
}


const requireMaybe = (u: MaybeProp) => console.log(u)
const requireDefinetly = (u: DefinetlyProp) => requireMaybe(u)

https://flow.org/try/#0FDAuE8AcFMAIFkCG4BG0AKAnA9pWBeWAb2Flkh0gC5YB+AOwFcBbNTAGmAF8QIZYAItABmAS3rRQAG3BZcBYqXKUaTVtA7cQwAMbZ6AZ1CxM0AI6NRppKjiEAFIxo20cyAEoCAPlh7D2KWgAOilsAHNHd119IxNzS1MhMQlpcAVHGiTxSRk3T3wfUwsraBdoSKA

It seems like that a function, that can handle maybe properties, should also be able to handle the same properties when they are definetly defined.

Any way to work around this?

Answer 1

This is because objects in JS are mutable. As far as the type system is concerned, requireMaybe could execute u.prop = null . Then, the original caller would be expecting prop to be a number , but it would actually be null . This would break type safety.

You can accomplish what you want to by using property variance :

type MaybeProp = {
  +prop: ?number,
}

type DefinetlyProp = {
  +prop: number,
}


const requireMaybe = (u: MaybeProp) => console.log(u)
const requireDefinetly = (u: DefinetlyProp) => requireMaybe(u)