Flow: Object literal {} incompatible with $Exact<{}>

[jcready]

type T = $Exact<{}>;
const t: T = {} // flow error

The only work around seems to be to replace $Exact<{}> with { [any]: empty }

type E = { [any]: empty }
const e: E = {}

[mwalkerwells]

Or to simplify without an alias...

const foo: {| bar: 'bar' |} = { bar: 'bar' }

const fooError: {||} = {}

4: const fooError: {||} = {}
                          ^ object literal. Inexact type is incompatible with exact type
4: const fooError: {||} = {}
                   ^ exact type: object type

You're right, this is a surprising behavior.

[mhagmajer]

+1

[hon2a]

I encountered this when trying to assign an empty object to an exact type defined with an indexer.

type Exact = {|
  [KeyType]: ValueType
|};

Seems like this can be resolved by defining the type as

type PossiblyEmptyExact = Exact | {};

[marcianx]

That's not a better workaround since {} is a superclass of _all_ structs. Consequently,

type Foo = {| bar: number |} | {};
const foo: Foo = {bar: "baz"};

is happily accepted by the compiler.

Using a non-exact type would be more type-safe.

[hon2a]

@marcianx: Thanks. Seems like I was so happy it worked, I didn't think it through.

[marcianx]

@hon2a https://github.com/facebook/flow/issues/5688 has two workarounds. I've used the $Shape one successfully for my workaround.

type Foo = $Shape<{bar: number}>;
const foo1: Foo = {bar: "baz"}; // wrong field type
const foo2: Foo = {qux: "baz"}; // property missing

[mhagmajer]

Guys, please beware of this issue with shapes: https://github.com/facebook/flow/issues/5884. The currently accept null and undefined.

[hon2a]

@marcianx: $Shape allows objects with some of the properties missing.

type T = $Shape<{ a: number, b: number }>;
const partial: T = { a: 1 }; // no errors

In my case $Shape is of no use as I need this for maps (defined with an indexer, e.g. type T = {| [KeyType]: ValueType |}).

[marcianx]

@mhagmajer Thanks for pointing this out! The easy workaround for this is to ensure that this is an object via & {}.

type Foo = $Shape<{bar: number}> & {};
const foo: Foo = null; // null is incompatible

@hon2a Thanks for mentioning. I should have explicitly mentioned this -- indeed, this workaround is only for the case where you wish all fields to be optional (which was my case when I ran into this bug).

[AlexanderShushunov]

I think this code does not work for the same reason.

type T =  {|a?: number|};
const t: T = {};

[vkurchatkin]

Closing in favour of https://github.com/facebook/flow/issues/2977