Runtime Introspection of Flow Types in JS (opens in new tab)

Cool article. I'm the author of https://github.com/codemix/babel-plugin-typecheck mentioned and the next version of the plugin (WIP) takes this kind of approach, so that you can reference a type at runtime:

    type User = {name: string, isAdmin: boolean};

    // compiles to
    const User = t.object(
      t.property('name', t.string()),
      t.property('isAdmin', t.boolean())
    );

    // which you can then use with
    User.check({name: 'Alice'}); // boolean
    User.assert({nom: false}); // throws

Ultimately it should be possible to use this to implement pattern matching in JS based on flow types, for example:

    type User = {
      name: string;
      roles: Role[];
    };

    type Role = {
      id: number;
      name: string;
      users: User[]
    };

    const url = match(someInput, 
      (user: User) => `/users/${user.name}`,
      (role: Role) => `/roles/${role.id}`,
      (_) => `/` // default URL
    );

The really hard thing is to do this whilst maintaining perfect compatibility with flow.

This is really neat! You may want to also look at https://github.com/gcanti/babel-plugin-tcomb for some inspiration- I've used tcomb extensively for runtime ADT and typing, but using just Flow would be an interesting challenge!