Disjoint Union Question

[maxsalven]

type Pet = {|
  petId: string
|}

type Car = {|
  carId: string
|}

type Union = Pet | Car

This causes an error:

function getId(arg: Union): string {
  return arg.petId || arg.carId
}

This doesn't work either:

function getId(arg: Union): string {
  let id
  if (arg.petId) {
    id = arg.petId
  } else if (arg.carId) {
    id = arg.carId
  }
  return id
}

This works, but is a very convoluted amount of typing given I have told Flow that there will 100% surely be either a carId or petId attribute of type string.

function getId(arg: Union): string {
  let id
  if (arg.petId) {
    id = arg.petId
  } else if (arg.carId) {
    id = arg.carId
  } else {
    throw new Error('This is pointless')
  }
  return id
}

https://flow.org/try/#0C4TwDgpgBAChxQLxQN4B8BQUqWASQBMAuKAZ2ACcBLAOwHMM0BfDDUSKAYQEMKlVM2AMa9CJctXqMWbcNACqNKgHsa-OAjRderAGYBXGkOAq1deIQAUvOiUWmAlOMq06qLFArx9FNTYB0uIRQaFoBIhSEGDIGRiaqUOb4BNYUtlD2qk5kLvTu2AA28FBUBB5UulCpdIEWBA752CUE-AFBZdhMUBAFpNAVVeGi9Y3Ypa1p-hFRnR5ewD5qpdF6hsamiXXVdkpZzpJuKB5FCMtjldW1yQ1HTc0TNe0eXT19JRdDkSO3TePInzMoC9etAfthgAALCjKADuUBoEDhAFEKNCKJYAOQAFQhVFIJXxYGUtGARVIpAxDmec28vma0SAA

Is there a 'right way' to do this? If not, aren't these disjoint unions very limited in their ability?

[vkurchatkin]

It works if you modify it like this:

function getId(arg: Union): string {
  return arg.petId || arg.carId || '';
}

The thing is that arg.petId could be an empty string. In that case you function would return undefined.

[maxsalven]

Thank you, that makes a lot of sense.

The error messages given by Flow are quite cryptic and didn't help me figure out that what you said might be the reason for the error, unfortunately. Thanks for your help.

12:   return arg.petId || arg.carId
                              ^ property `carId`. Property not found in
12:   return arg.petId || arg.carId
                          ^ object type

[maxsalven]

Actually, the following still returns an error, even though I have explicitly stated the function can return undefined:

function getId(arg: Union): (string | void) {
  return arg.petId || arg.carId
}

[apsavin]

I think, the issue should be reopened. See this:

type Pet = {|
  petId: string
|}

type Car = {|
  carId: string
|}

type Union = Pet | Car

function getId(arg: Union): string {
    return typeof arg.petId === 'string' ? arg.petId : arg.carId
}

I don't think flow should report an error in the code above but it does.