elixir - Elixir 函数名称中的问号和 "is_"

Question

我正在阅读 Elixir 的命名约定。它指出:

The names of predicate functions that cannot be used within guards should have a trailing question mark (?) rather than the is_ (or similar) prefix.

那么，Guard 中不能使用哪些功能呢？

Answer 1

所有这三个函数都包含在守卫中有效的表达式；只是自定义守卫必须写成宏，而不是普通函数:

defmodule User do
  defstruct age: 0

  defmacro kid?(age) do
    quote do
      6 < unquote(age) and unquote(age) < 12
    end
  end

  defmacro teen?(age) do
    quote do
      12 < unquote(age) and unquote(age) < 18
    end
  end

  defmacro elder?(age) do
    quote do
      60 < unquote(age)
    end
  end
end

defmodule Greeting do
  import User

  def greet(%{age: age}) when kid?(age), do: "Hiya"
  def greet(%{age: age}) when teen?(age), do: "Whatever"
  def greet(%{age: age}) when elder?(age), do: "You kids get off my lawn"
  def greet(_), do: "Hello"
end

for age <- [0, 5, 10, 15, 20, 90] do
  IO.inspect {age, Greeting.greet(%{age: age})}
end

输出:

{0, "Hello"}
{5, "Hello"}
{10, "Hiya"}
{15, "Whatever"}
{20, "Hello"}
{90, "You kids get off my lawn"}