Need help about quote and unquote

dokuzbir

This code is from metaprograming elixir book. I dont understand when should use unquote. In first function there is no unqote to execute expression but in second function there is unquote why ?

defmodule ControlFlow do
 # Why there is no unquote here ?
  defmacro my_if(expr, do: if_block), do: if(expr, do: if_block, else: nil) 
 
# Why there is unquote here ?
  defmacro my_if(expr, do: if_block, else: else_block) do
    quote do
      case unquote(expr) do
        result when result in [false, nil] -> unquote(else_block)
        _ -> unquote(if_block)
      end
    end
  end
end

8 comments

#metaprogramming #book-metaprogramming-elixir #troubleshooting

1 1838 8

2018-06-14 01:29:57 UTC

Showing Posts 1 to 8

Show Best Posts
Show All (oldest first)
Show All (newest first)

gregvaughn

I suspect there’s a typo there. The first my_if clause should be:

defmacro my_if(expr, do: if_block), do: my_if(expr, do: if_block, else: nil)

so it delegates to the 2nd clause

Post #1

dokuzbir

But it works when it is if.

Post #2

gregvaughn

It’s calling Kernel.if/2 which is a macro that handles the quoting itself. But if you’re going to rely on Kernel.if then why write my_if? That’s why I think it’s a typo.

Post #3

dokuzbir

So in a macro we can use another macro without quote. Thanks i got it

Post #4

dokuzbir

I tried a smilar approach for while as first if function, but my_second_while function freezes iex. my_while function is working as expected

defmodule Loop do
	defmacro my_while(expression, do: block) do
		quote do
			for _ <- Stream.cycle([:ok]) do
				if unquote(expression) do
					unquote(block)
				else
				# break out of loop
				end
			end
		end
	end

    #Same as first if function 
	defmacro my_second_while(expression, do: block) do
		my_while(expression, do: block)
	end
end

iex

require Loop
Loop.my_second_while true do
IO.puts "looping!"
end

Post #5

OvermindDL1

Should be:

quote(do: my_while(expression, do: block))

Also, your Stream.cycle bit is not ever going to end, it will loop forever?

Post #6

dokuzbir

Yes i guess so but why first my_if function works without unquote? what is difference? That example from book yes never ends

Post #7

aethereus

Your no-quote version of my_if does not work. I tested it with the following code:

ControlFlow.my_if 1 > 2, do: IO.puts("This should not be printed.")

And I saw that the sentence “This should not be printed.” is actually printed. This is because when you pass 1 > 2 to expr, it gets automatically quoted (becomes AST), and the AST is {:>, [context: Elixir, import: Kernel], [1, 2]} and is not false or nil, so the test of if(expr, ...) passes, and the if_block (which is also an AST) gets returned by the macro my_if.

If you test it with

ControlFlow.my_if false, do: IO.puts("This should not be printed.")

You get the correct behavior by accident because false is an atom, and atoms are “Elixir literals” that have exactly the same form in code and in AST.

Post #8

— All posts loaded —