相关疑难解决方法(0)

代码如下:

-module(map_demo).
-export([count_characters/1]).

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

count_characters([H|T], #{ H => N } = X) ->
    count_characters(T, X#{ H := N+1 });
count_characters([H|T], X) ->
    count_characters(T, X#{ H => 1});
count_characters([], X) ->
    X.

在Erlang shell中编译代码时,它报告了以下错误:

1> c(map_demo).
map_demo.erl:7: illegal pattern
map_demo.erl:8: variable 'N' is unbound
map_demo.erl:10: illegal use of variable 'H' in map
map_demo.erl:7: Warning: variable 'H' is unused
error

我是Erlang的新手,我自己找不到任何错误.怎么纠正呢？