horriblecallbacks.java

gitHubService.fetchUsers().enqueue(new Callback<List<GithubUser>>() {
            @Override
            public void onResponse(Response<List<GithubUser>> response) {
                for(GithubUser user : response.body()){
                    gitHubService.fetchDetails(user.getId())
                            .enqueue(new Callback<GithubUserDescription>() {
                                @Override
                                public void onResponse(Response<GithubUserDescription> response) {
                                    //do things
                                }

                                @Override
                                public void onFailure(Throwable throwable) {

                                }
                            });
                }
            }

            @Override
            public void onFailure(Throwable throwable) {

            }
        });