Missing type annotation for A. A is a type parameter declared in function type

Question

I have the following code:

import { combineReducers } from 'redux';
import planeReducer from './plane/reducer';

export default combineReducers({
    planes: planeReducer
});

which runs properly when running:

> expo start

Then, when I run flow with the following command:

>  npm run flow

I get the following flow error:

Missing type annotation for A. A is a type parameter declared in function type [1] and was implicitly instantiated at
call of combineReducers [2].

     src/store/index.js
      1| import { combineReducers } from 'redux';
      2| import planeReducer from './plane/reducer';
      3|
 [2]  4| export default combineReducers({
      5|        planes: planeReducer
      6| });
      7|

     flow-typed/npm/redux_v4.x.x.js
 [1] 56|   declare export function combineReducers<O: Object, A>(reducers: O): CombinedReducer<$ObjMap<O, <S>(r: Reducer<S, any>) => S>, A>;

Then, when I modify the code above by adding: <any, any> as follows:

import { combineReducers } from 'redux';
import planeReducer from './plane/reducer';

export default combineReducers<any, any>({
    planes: planeReducer
});

when I run flow again as before, the flow error goes away, but if I run again:

> expo start

I get the following runtime error:

[01:30:16] Your app is running at exp://192.168.1.194:19000
Logs for your project will appear below. Press Ctrl+C to exit.
[01:30:16] SyntaxError: D:\react-navigation-header-issue\src\store\index.js: Unexpected token, expected ";" (4:34)
[01:30:16]   2 | import planeReducer from './plane/reducer';
[01:30:16]   3 |
[01:30:16] > 4 | export default combineReducers<any, any>({
[01:30:16]     |                                   ^
[01:30:16]   5 |        planes: planeReducer
[01:30:16]   6 | });
[01:30:16]   7 |

Any idea on how to modify the code properly in order fix that flow error and at the same time keep the applicatioin running with no errors?

Thanks!

Answer 1

Please try adding @flow pragma comment in the first line of the file. It's probably related to the babel issue: https://github.com/babel/babel/issues/9240 .

Edit : There's undocumented all option in flow-strip-types ( babel-preset-expo makes use of it internally).

You need to overwrite it in your babel config:

overrides: [{
    plugins: [
        ['@babel/plugin-transform-flow-strip-types', {all: true}],
    ]
}]