CODEWITHSUNDEEP
flowtype
DTupalov
DTupalov

Reputation: 145

Error in flow type signature. Why?

Here is the exemple:

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVUCuA7AxgFwEs5swBDMACgAcyAnMgWwGcAuMAb2oEZ3n86hbAHMANGGoAmAPx8BQ4QF8AlOwBucQgBNOi9FjxESYAEZVVnHnMEjFnVGDB0Apvkx1SXXmADk-G8I+igDcqHqoavSmAErOzJgw+GAAvKaUyqGoZJQmsfGJGUA

/* @flow */

function a (params: {p1: string, p2?: string}): void {}

function b (): {p1: string} {
  return {p1: 'string'};
} 

var bResult = b();

a(bResult);

And I got an flow error

3: function a (params: {p1: string, p2?: string}): void {}
                       ^ property `p2`. Property not found in
11: a(bResult);
      ^ object type

Why this error happens? Why signature {p1: string} not is part of {p1: string, p2?: string}?

Upvotes: 1

Views: 144

Answers (1)

Nat Mote
Nat Mote

Reputation: 4086

The type {p1: string, p2?: string} means that if a value of this type has a p2 property, it must be a string. Flow's width subtyping means that the b function could return {p1: 'foo', p2: 6}. Basically, if you have something of type {p1: string} you know it has a p1 property but you don't know anything about its other properties.

Because of this, if Flow allowed you to take something of type {p1: string} and pass it to the a function which expects the type {p1: string, p2?: string}, the a function could check if the p2 property exists, and if so it could assume that it is a string. However, it could actually be a number as detailed above. This will break type safety.

Upvotes: 2

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