weird "if" behaviour in macro's "quote"

[Serge Smetana]

Hello all,

Can someone explain me what's wrong with this code?

test.exs:

defmodule Test do
  defmacro test({:==, _, [left, right]} = _) do
    quote do
      left = unquote(left)
      right = unquote(right)
      if left == right do
        IO.puts "Equal!"
      else
        IO.puts "Not Equal!"
      end
    end
  end


  def run do
    test 1 == 1
    test 1 == 2
  end
end


Test.run

When I run it I get guards and clause warnings

~>elixir test.exs
test.exs:15: warning: this clause cannot match because a previous clause at line 15 always matches
test.exs:16: warning: this check/guard will always yield the same result
Equal!
Not Equal!

If I rewrite "if" in quote to "case" all works fine

test1.exs

defmodule Test do
  defmacro test({:==, _, [left, right]} = _) do
    quote do
      left = unquote(left)
      right = unquote(right)
      case (left == right) do
        true ->
          IO.puts "Equal!"
        _ ->
          IO.puts "Not Equal!"
      end
    end
  end


  def run do
    test 1 == 1
    test 1 == 2
  end
end


Test.run

Result

>elixir test1.exs
Equal!
Not Equal!

[José Valim]

The compiler tries to warn on cases where things will be obviously always the same or dead clauses. It is a best effort thing, so those warnings are not guaranteed. Therefore what is happening is that the compiler is seeing those cases:

      if 1 == 1 do

        # Always true

      else

        # Never invoked

      end

and:

      if 1 == 2 do

        # Never true

      else

        # Always invoked

      end

And therefore warning on it. The other cases the compiler can't see in this version but may as well see in the future.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[hen...@nyh.se]

A way to avoid those warnings, then, could be to do the comparison at compile time and have the macro only produce the "IO.puts …" bit. That should also be slightly faster at runtime.

[Serge Smetana]

Got it. Thank you.