people module for my blog post

people.erl

-module(people).

-export([count/2]).
-export([test/0]).

-type user() :: binary().
-type country() :: atom().
-type group() :: #{ country => country()
                  , members => [user()]
                  }.

-spec count(country(), [group()]) -> non_neg_integer().
count(Country, Groups) ->
  lists:sum(
    lists:map(
      fun(Group) ->
        case Group of
          #{country := Country, members := Members} -> length(Members);
          _ -> 0
        end
      end, Groups)).

test() ->
  Groups = [ #{country => ar, members => [<<"m1">>, <<"m2">>]}
           , #{country => ar, members => [<<"m3">>]}
           , #{country => uk, members => [<<"m4">>, <<"m5">>]}
           ],
  0 = people:count(us, Groups),
  2 = people:count(uk, Groups),
  3 = people:count(ar, Groups),
  ok.