Erlang地图中的非法格局 [英] illegal pattern in map of Erlang

问题描述

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.

当编译Erlang shell中的代码时，它报告了以下错误：

when compiling the code in the Erlang shell, it reported the following errors:

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

I 在Erlang新的，只是找不到任何错误我自己。如何纠正？

I'm new in Erlang, and just can't find anything wrong by myself. How to correct it?

推荐答案

IRC的答案（＃erlang @ freenode）：

The answers from IRC (#erlang@freenode):

1. 变量作为匹配中的键不受支持（版本17.0）


2. 更一般的问题影响函数的匹配参数：第7行 H 匹配2次;或者一次，然后用于匹配N。 （此问题也出现在二进制文件中）

1. variables as keys in matches are not supported yet (release 17.0)
2. A more general issue affects matching arguments of a function: line 7's H is matched 2 times; or once and used to match N then. (This issue also appears with binaries)

这个应该在下一个版本中解决。

This should be solved in the coming releases.

从版本17 此起的作品：

As of release 17 this works:

-module(count_chars).
-export([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:is_key(H,X) of
        false -> count_characters(T, maps:put(H,1,X));
        true  -> Count = maps:get(H,X),
                         count_characters(T, maps:update(H,Count+1,X))
    end;
count_characters([], X) ->
        X.

这是另一个版本（仅在18年测试），稍微更相似在书中的一个：

Here is another version (only tested on 18) that is slightly more similar to the one in the book:

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

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

count_characters([H|T], X) ->
    case maps:is_key(H,X) of
        false -> count_characters(T, X#{ H => 1 });
        true  -> #{ H := Count } = X,
                 count_characters(T, X#{ H := Count+1 })
    end;
count_characters([], X) ->
        X.

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