Announcing Import Type
As of Flow 0.3.0, it’s now possible to import types from another module. So, for example, if you’re only importing a class for purposes of referencing it in a type annotation, you can now use the new import type
syntax to do this.
Motivation#
Has this ever happened to you:
// @flow
// Post-transformation lint error: Unused variable 'URI'
import URI from "URI";
// But if you delete the require you get a Flow error:
// identifier URI - Unknown global name
module.exports = function(x: URI): URI {
return x;
}
Now you have an out! To solve this problem (and with an eye toward a near future with ES6 module syntax), we’ve added the new import type
syntax. With import type
, you can convey what you really mean here — that you want to import the type of the class and not really the class itself.
Enter Import Type#
So instead of the above code, you can now write this:
// @flow
import type URI from 'URI';
module.exports = function(x: URI): URI {
return x;
};
If you have a module that exports multiple classes (like, say, a Crayon and a Marker class), you can import the type for each of them together or separately like this:
// @flow
import type {Crayon, Marker} from 'WritingUtensils';
module.exports = function junkDrawer(x: Crayon, y: Marker): void {}
Transformations#
Like type annotations and other Flow features, import type
need to be transformed away before the code can be run. The transforms will be available in react-tools 0.13.0
when it is published soon, but for now they’re available in 0.13.0-beta.2
, which you can install with
npm install react-tools@0.13.0-beta.2
Anticipatory Q&A#
Wait, but what happens at runtime after I’ve added an import type
declaration?#
Nothing! All import type
declarations get stripped away just like other flow syntax.
Can I use import type
to pull in type aliases from another module, too?#
Not quite yet…but soon! There are a few other moving parts that we need to build first, but we’re working on it.
EDIT: Yes! As of Flow 0.10 you can use the export type MyType = ... ;
syntax to compliment the import type
syntax. Here’s a trivial example:
// @flow
// MyTypes.js
export type UserID = number;
export type User = {
id: UserID,
firstName: string,
lastName: string
};
// @flow
// User.js
import type {UserID, User} from "MyTypes";
function getUserID(user: User): UserID {
return user.id;
}
Note that we only support the explicit named-export statements for now (i.e. export type UserID = number;
). In a future version we can add support for latent named-export statements (i.e. type UserID = number; export {UserID};
) and default type exports (i.e. export default type MyType = ... ;
)…but for now these forms aren’t yet supported for type exports.