Fix scope warning without adding extra code

Questions / Help
21

This is not really true, right? We aren’t using the return value of the if block.

22

As I said the else is implied

23

Whenever you assign to a variable inside if, you actually return it, although not in the usual sense, but implicitly via scope.

1 Like
24

Just for fun, I built this.


defmodule ImplicitElse do
  defmacro ifeq(x, eq_value \\ nil, do: block) do
    quote do
      if unquote(x) == unquote(eq_value) do
        unquote(block)
      else
        unquote(x)
      end
    end
  end

  def ifnil(x, do: block) do
    ifeq(x, nil, do: block)
  end

  def ifzero(x, do: block) do
    ifeq(x, 0, do: block)
  end
end

import ImplicitElse

x = nil
x = ifnil x, do: 10
# x now is 10
x = ifnil x, do: 99
# x still is 10
2 Likes
25

Or if you want to be really evil, you can use an unhygienic macro, and override an operator. (disclaimer: If you use this in production code, your coworkers will probably lynch you in your sleep)

defmodule ImplicitElse do
  defmacro x <<~ default do
    quote do
      var!(unquote(x)) = 
        if unquote(x) == nil do
          unquote(default)
        else
          unquote(x)
        end
    end
  end
end
import ImplicitElse

bar = nil
bar <<~ 42
# now bar is 42
bar <<~ 33
# bar still is 42
3 Likes
26

That is beautifully evil indeed, hadn’t thought of that. ^.^

1 Like