如何写“ a == b”？ X：Y”；在Erlang中，换句话说，如何编写C风格的三元运算符？ [英] How to write "a==b ? X : Y" in Erlang, in other words how to write a C-style ternary operator?

问题描述

在Erlang中是否有编写这种代码的好方法？

Is there a good way to write code like this in Erlang?

A == B ? X : Y

以下是红宝石风格的代码。这也称为三元运算符。

below is ruby-style code. This is also known as a ternary operator.

推荐答案

说明

三元运算符 _的原因？ _：_ 之所以存在于多种语言中，是因为它们具有两个语法类：语句和表达式。由于if-then-else构造通常属于语句类，因此当您输入表达式时，没有任何方法可以使其正常工作。因此，您添加 _吗？ _：_ 表达式类的运算符。

Explanation

The reason the ternary operator _ ? _ : _ exists in many languages is due to the fact that they have two syntactic classes: Statements and Expressions. Since if-then-else constructions usually belong the the statement-class, there is no way to get that working for when you are entering an expression. Hence you add the _ ? _ : _ operator to the expression class.

如另一篇文章所述，您可以使用 a == b？ true：false 并只写 a == b ，但这不能解释我们可能拥有 a的一般情况== b？ X：Y 用于任意表达式 X 和 Y 。另请注意，在Erlang中， a == b 始终为 false ，因此您可以说，真正要做的是用 false 替换整个表达式。

As another post states, you can take a == b ? true : false and just write a == b, but that does not explain the general case where we may have a == b ? X : Y for arbitrary expressions X and Y. Also note that a == b is always false in Erlang, so you could argue that the real thing to do is to replace the whole expression with false.

幸运的是，Erlang与大多数函数式语言一样，拥有仅一个语法类，即表达式。因此，您可以在的情况下使用X =-b的情况-> ...; Y-> ...在函数中的任何位置结束，包括其他表达式。换句话说，三元 _？ _：_ 运算符在Erlang中是多余的，因为情况已经有效。

Luckily, Erlang, as is the case for most functional languages, have one syntactic class only, expressions. Hence you can use case a == b of X -> ...; Y -> ... end in any place in a function, other expressions included. In other words, the ternary _ ? _ : _ operator is redundant in Erlang since the case already works.

假设我们要返回一个简单的属性列表，并且需要做一些计算

Suppose we are to return a simple proplist and we have some computation we need to do

  f() ->
    case a == b of
          true -> 
           [{a, 3},
            {b, <<"YE">>},
            {c, 7}];
          false ->
           [{a, 3},
            {b, <<"YE">>},
            {c, "HELLO!!!"}];
    end.

但是由于 case 构造是一个表达式，我们可以内联它：

But since the case construction is an expression, we can just inline it:

  f() ->
    [{a, 3},
     {b, <<"YE">>},
     {c, case a == b of
          true -> 7;
          false -> "HELLO!!!"
         end}].

然后用这个东西完成。

在Erlang中<.c $ c> if .. end 的构造通常不是什么你要。在这种情况下，您要检查值 a == b ，它可以产生两个输出 true 或 false 。在那种情况下， case 表达式更为直接。如果必须检查多个不同的测试并选择第一个匹配项，则最好使用 if ，而我们在这里只有一个测试。

the if .. end construction in Erlang is usually not what you want. You want to scrutinize a value a == b in this case and it can yield one of two outputs true or false. In that case the case-expression is more direct. The if is better used if you have to check for multiple different tests and pick the first matching, whereas we only have a single test to make here.

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