Elixir - 调用/调用宏 - UndefinedFunctionError

Question

为什么 Elixir 在 .exs 文件中使用 Module.macroName 语法调用宏时会报告 UndefinedFunctionError？只有当我有另一个调用宏的函数并且我调用函数而不是宏时，我似乎才能调用宏。

下面的代码演示了这一点：

defmodule Sample do
  defmacro doIt(expression) do
    quote do
      IO.puts unquote(expression)
    end
  end

  def doFunc(e), do: doIt(e)
end

Sample.doFunc "Hello World via Function" # Works fine
Sample.doIt "Hello World from Macro!" # Gives error

输出

Hello World via Function
** (UndefinedFunctionError) undefined function: Sample.doIt/1
    Sample.doIt("Hello World from Macro!")
    (elixir) lib/code.ex:307: Code.require_file/2

Elixir documentation 示例使用iex，而不是在.exs 文件中调用宏。即使是上面的代码，如果我们删除对Sample.doIt 的调用并将其加载到iex，然后调用Sample.doIt 也可以正常工作。

E:\elixir>iex hello.exs
Hello World via Function
Interactive Elixir (1.0.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> require Sample
nil
iex(2)> Sample.doIt "Hello"
Hello
:ok
iex(3)>

如果我在上面的文件中尝试require Sample，如下所示

defmodule Sample
  ... rest of stuff as shown above ...
end

require Sample

Sample.doFunc "Hello World via Function"
Sample.doIt "Hello World from Macro!"

我收到错误

** (CompileError) hello.exs:11: module Sample is not loaded but was defined. This happens because you are trying to use a module in the same context it is defined. Try defining the module outside the context that requires it.
    (stdlib) lists.erl:1352: :lists.mapfoldl/3
    (stdlib) lists.erl:1353: :lists.mapfoldl/3

Answer 1

像往常一样，您需要在使用其宏之前需要一个模块，以向编译器表明模块的编译顺序：

defmodule Other do
  def run do
    require Sample

    Sample.doFunc "Hello World via Function"
    Sample.doIt "Hello World from Macro!"
  end
end

Other.run # => Hello World via Function
          #    Hello World from Macro!