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最新推荐文章于 2018-02-06 22:55:00 发布

Lu_ming

最新推荐文章于 2018-02-06 22:55:00 发布

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文章标签： underscore compiler variables debugging file io

本文链接：https://blog.youkuaiyun.com/Lu_ming/article/details/4831080

本文探讨了Erlang编程语言中下划线变量的用途，包括标记不打算使用的变量以提高代码可读性及调试时避免编译警告。

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Underscore Variables

There’s one more thing to say about variables. The special syntax _VarName is used for a normal variable, not an anonymous variable. Normally the compiler will generate a warning if a variable is used only once in a clause since this is usually the sign of an error. If the variable is used only once but starts with an underscore, the warning message will not be generated.

There are two main uses of underscore variables:

To name a variable that we don’t intend to use. That is, writing open(File, _Mode) makes the program more readable than writing open(File, _).
For debugging purposes. For example, suppose we write this:

          some_func(X) ->
             {P, Q} = some_other_func(X),
             io:format("Q = ~p~n", [Q]),
             P.
         This compiles without an error message. Now comment out the format statement:

         some_func(X) ->
            {P, Q} = some_other_func(X),
            %% io:format("Q = ~p~n", [Q]),
            P.
        If we compile this, the compiler will issue a warning that the variable Q is not used.

If we rewrite the function like this:

        some_func(X) ->
           {P, _Q} = some_other_func(X),
           io:format("_Q = ~p~n", [_Q]),
           P.
       then we can comment out the format statement, and the compiler will not complain.