CODEWITHSUNDEEP
javascriptflowtype
Edo
Edo

Reputation: 3481

Flowtype: Extend Array type parameter

First of all, forgive me if my wording is incorrect, I'm not so familiar with all of the typing jargon.

I have two types: A and B, where B is an extension of A, like so:

type A = { a: string };
type B = { a: string, b: string };

Asserting that a value of type B conforms to type A works fine:

const bar: B = { a: 'hello', b: 'world' };
(bar: A); // No problemo

But when wrapping the value into an array, the assertion goes bad:

const bars: Array<B> = [bar]; // This is fine
(bars: Array<B>); // Also fine
(bars: Array<A>); // Breaks!!

Am I completely misunderstanding something? Is this a Generics problem, an Array problem, or just a bug in Flow?

Flow version: v0.52.0

Check it out yourself on Flow

Upvotes: 0

Views: 263

Answers (1)

Jordan Brown
Jordan Brown

Reputation: 638

That's because the type parameter T in Array<T> is invariant-- meaning it does not accept supertypes or subtypes when considering a subtyping relationship. Check out the docs on variance here for more info on that: https://flow.org/en/docs/lang/variance/#toc-invariance

What you're looking for is a data structure with a covariant type parameter. Specifically, you may want to use $ReadOnlyArray<T> instead.

Try it

Upvotes: 3

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