长生不老药中的双反斜杠是什么意思？

Question

我在网上查了一下双反斜杠在 elixir 中的含义。我遇到了使用它的代码。但我仍然无法理解双反斜杠在长生不老药中的作用。示例代码如下。

defmodule Concat do
    def join(a, b, sep \ " ") do
        a <> sep <> b
    end
end

IO.puts Concat.join("Hello", "world")      #=> Hello world
IO.puts Concat.join("Hello", "world", "_") #=> Hello_world

Answer 1

简而言之 - 它表示参数的“默认值”。然而，由于 Erlang 的实现方式，实际上它意味着“创建新函数并省略此参数，该函数将使用给定值调用此函数来代替此参数”。因此您的示例将扩展为：

defmodule Concat do
  def join(a, b), do: join(a, b, " ")
  def join(a, b, sep), do: a <> sep <> b
end

Answer 2

这在documentation中有解释：

Default arguments

\ is used to specify a default value for a parameter of a function. For example:
defmodule MyMath do
  def multiply_by(number, factor \ 2) do
    number * factor
  end
end

MyMath.multiply_by(4, 3)
#=> 12

MyMath.multiply_by(4)
#=> 8
The compiler translates this into multiple functions with different arities, here MyMath.multiply_by/1 and MyMath.multiply_by/2, that represent cases when arguments for parameters with default values are passed or not passed.

When defining a function with default arguments as well as multiple explicitly declared clauses, you must write a function head that declares the defaults. For example:
defmodule MyString do
  def join(string1, string2 \ nil, separator \ " ")

  def join(string1, nil, _separator) do
    string1
  end

  def join(string1, string2, separator) do
    string1 <> separator <> string2
  end
end
Note that \ can't be used with anonymous functions because they can only have a sole arity.

长生不老药中的双反斜杠是什么意思？

What does double backslash mean in elixir?

elixir

Default arguments