CODEWITHSUNDEEP
flowtype
Christian Schlensker
Christian Schlensker

Reputation: 22478

Custom type is incompatible with mixed

The problem can be demo'd here

I want to define a function that can take an array of mixed types:

function foo(x: Array<mixed>): string {
  // ... do something
}

Then I try to call it with an array off some custom object type:

type Thing = {
  id: string
}

let array : Array<Thing> = [{id: 'hello'}];

foo(array);

... and I get the following error

Cannot call `foo` with `array` bound to `x` because `Thing` [1] is incompatible with mixed [2] in array element.`

Is there something that I'm not understanding about the mixed type. Why can't a use an array of objects as an argument?

Upvotes: 1

Views: 1364

Answers (1)

Dave Meehan
Dave Meehan

Reputation: 3201

It has to do with a mutability of the array argument. You can use $ReadOnlyArray to satisfy the requirement.

function foo(x: $ReadOnlyArray<mixed>): string {
  // ... do something
  return "hello";
}

type Thing = {
  id: string
}

let array : Array<Thing> = [{id: 'hello'}];

foo(array);

Array's are passed by reference, not by value, so the value contained in the variable array can be modified within foo. e.g.

x.push(1)

array would no longer be an array of Thing only. So using $ReadOnlyArray means that x is immutable and therefore array is safe.

Upvotes: 3

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