Currently, GHC-8.8.4 is not allowed:

➜ stack build

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for fcm-client-0.2.0.0:
    base-4.13.0.0 from stack configuration does not match >=4.10.1.0 && <4.13  (latest matching version
                  is 4.12.0.0)
needed since fcm-client is a build target.

Creating an issue so I can take a look later.

Ref: https://blogiestools.com/firebase-legacy-fcm-apis-discontinued/

We use a legacy endpoint here and perhaps should migrate.
Instructions at https://firebase.google.com/docs/cloud-messaging/migrate-v1

See https://firebase.google.com/docs/cloud-messaging/http-server-ref

The mentioned fields have been removed from the http protocol spec.

Since delay_while_idle is not supported anymore, no point in having it in the code.

There is a bug with generating the JSON request of the field fcmRegistrationIDs

Bug: It serializes to JSON field name: "registration_i_ds" (Instead of "registration_ids")

This causes the FCM HTTP request to fail with error:

FCMErrorResponseInvalidJSON {_fcmErrorMessage = "to\n"}

(Because the FCM server expects a JSON object either with a value for "to" or a value for "registration_ids", and we gave neither, so it defaults to reporting missing "to" field)

I am using Aeson 1.4.2.0