具有多个子句的命名函数的Elixir默认参数

问题描述

我很难理解默认参数如何与命名函数中的多个子句交互.归结为以下代码段为何起作用?

I have trouble understanding how default parameters interact with multiple clauses in named functions. It boils down to, why does the following snippet work?

defmodule Lists do

  def sum([], total \\ 0), do: total
  def sum([h|t], total), do: h + sum(t, total)

end

据我了解，编译器将其扩展为:

From my understanding this gets expanded by the compiler into:

defmodule Lists do

  def sum([]), do: sum([], 0)
  def sum([], total), do: total
  def sum([h|t], total), do: h + sum(t, total)

end

所以我希望会发生以下情况:

So I would expect the following to happen:

iex(1)> Lists.sum [1,2,3,4]
** (FunctionClauseError) no function clause matching in Lists.sum/1

相反，它可以正常工作:

instead it works:

iex(1)> Lists.sum [1,2,3,4]
10

使用 Elixir 0.12.4 .

推荐答案

实际上，def sum([], total \\ 0), do: total将定义一个类似于def sum(list), do: sum(list, 0)的函数子句.因此，我绝对可以看到您的困惑.我将保证在将来的版本中针对此类情况发出警告.谢谢！

Actually, def sum([], total \\ 0), do: total will define a function clause that looks like def sum(list), do: sum(list, 0). So I can definitely see your confusion. I will guarantee we emit a warning for such cases in future releases. Thank you!

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