Erlang:creating list of tuples within lists:foreach - foreach

Question

I query the list of users from Mnesia Database in Chicagoboss. I'm getting the error when I try to add the Lists within lists:foreach with ++ operator. My aim is, based on userid I will do ets:lookup to my cache and create a List like - [{{<<"name">>,<<"Batman">>}, {<<"steps">>,2552}, {<<"distance">>,2050}}].
For each user I'll create this list and add with the previous List. So that ultimately I can can sort on <<"steps">> and convert the binary list by json encoding and send it to the client via Websockets.
I'm getting the error at this line:
Reading1 = Reading2 ++ Currentlist
as I've decalred Reading1 as an Empty list.
My question is how can I manipulate the lists within the lists:foreach and then send the result List via websocket?
BelugaUsers = boss_db:find(users, [{accesstoken, 'not_equals', ''}]),
Reading1 = [],
Reading2 = [],
lists:foreach(fun(X) ->
{_,_,BEmail,BName,_,_,BAccessToken,_} = X,
UserKey = BEmail ++ "-" ++ ?MYAPICALL1,
io:format("UserKey for Leader Board: ~n~p~n",[UserKey]),
[Reading] = ets:lookup(myapi_cache, list_to_binary(UserKey)),
{_,Result} = Reading,
ActivitySummary = proplists:get_value(<<"activitySummary">>, Result),
%Print ActivitySummary for the user ....printing fine
io:format("ActivitySummary ==========: ~n~p~n",[ActivitySummary]),
%Create a list of the format
%[{{<<"name">>,<<"Batman">>}, {<<"steps">>,2552}, {<<"distance">>,2050}}]
Currentlist = [{{<<"name">>, list_to_binary(BName)}, {<<"steps">>, proplists:get_value(<<"steps">>, ActivitySummary)}, {<<"distance">>, proplists:get_value(<<"distance">>, ActivitySummary)}}],
%% HERE I'M GETTING error%%
Reading1 = Reading2 ++ Currentlist
end, BelugaUsers),
%sort the list
Reading3 = lists:keysort(2, Reading1),
%reverse the list
Reading4 = lists:reverse(Reading3),
WebSocketId ! {text, jsx:encode(Reading4)},

Answer 1

Erlang variables are single-assignment; once bound to a value, they can't be re-bound to a different value.
The lists:foreach/2 function is not useful for this problem because it can't create a new value and return it to its caller. You should instead use lists:map/2, perhaps like this:
BelugaUsers = boss_db:find(users, [{accesstoken, 'not_equals', ''}]),
Reading = lists:map(
fun(X) ->
{_,_,BEmail,BName,_,_,BAccessToken,_} = X,
UserKey = BEmail ++ "-" ++ ?MYAPICALL1,
io:format("UserKey for Leader Board: ~n~p~n",[UserKey]),
{_,Result} = hd(ets:lookup(myapi_cache, list_to_binary(UserKey))),
ActivitySummary = proplists:get_value(<<"activitySummary">>, Result),
%%Print ActivitySummary for the user ....printing fine
io:format("ActivitySummary ==========: ~n~p~n",[ActivitySummary]),
%%Create a tuple of the format
%%{{<<"name">>,<<"Batman">>}, {<<"steps">>,2552}, {<<"distance">>,2050}}
{{<<"name">>, list_to_binary(BName)},
{<<"steps">>, proplists:get_value(<<"steps">>, ActivitySummary)},
{<<"distance">>, proplists:get_value(<<"distance">>, ActivitySummary)}}
end, BelugaUsers),
%%sort the list
Reading2 = lists:keysort(2, Reading),
%%reverse the list
Reading3 = lists:reverse(Reading2),
WebSocketId ! {text, jsx:encode(Reading3)}.
The lists:map/2 function applies a function to each value in a list to a produce a potentially different value and returns a new list consisting of those new values. This is essentially what you were trying to do with lists:foreach/2 and your attempt to use imperative assignment to add each element to an already-existing list.
You could alternatively use a list comprehension but I think lists:map/2 is clearer in this situation.