count_chars example from Programming Erlang 2nd ed that works

count_chars.erl

-module(count_chars).
-export([count_chars/1, count_characters/1]).


count_characters(Str) ->
    count_characters(Str, #{}).

%% maps module functions cannot be used as guards (release 17)
%% or you'll get "illegal guard expression" error
count_characters([H|T], X) ->
    case maps:get(H,X,no_key) of
        no_key -> count_characters(T, maps:put(H,1,X));
        Cnt    -> count_characters(T, maps:update(H,Cnt+1,X))
    end;
count_characters([], X) ->
    X.



%% Only difference from count_characters/1 is that maps:is_key/2 is used
%% instad of maps:get/3
count_chars(L) -> c(L,#{}).

c([],M) -> M;
c([H|T],M) ->
    case maps:is_key(H,M) of
        % maps:put      =>
        % maps:update   :=
        true  -> c(T, maps:update(H, maps:get(H,M)+1, M));
        false -> c(T, maps:put(H, 1, M))
    end.