Macro expansion bug?

[yrashk]

iex> expr = quote do
...> if !a, do: oneway, else: anotherway
...> end
{:if,0,[{:"!",0,[{:a,0,:quoted}]},[{:do,{:oneway,0,:quoted}},{:else,{:anotherway,0,:quoted}}]]}
iex> IO.puts Macro.to_binary(Macro.expand(expr, __ENV__))  
case(!!a) do
  false ->
    oneway
  true ->
    anotherway
end
:ok
iex> Macro.expand(expr, __ENV__)
{:case,0,[{:"!",0,[{:"!",0,[{:a,0,:quoted}]}]},[{:do,{:"->",0,[{[false],{:oneway,0,:quoted}},{[true],{:anotherway,0,:quoted}}]}}]]}

[josevalim]

What is supposed to be the bug here? Notice that Macro.expand does not expand recursively.

[yrashk]

I guess what I was looking for is recursive macro expansion... anyway, why does it expand ! to !! ?

[josevalim]

if actually expands to case(!...), that's why you get two !!. Btw, if you can probably loop through the expressions if you want to expand it recursively. I have updated the docs to be clear about it.

[yrashk]

No way we can support optional recursive expansion? Doesn't make much sense?

[josevalim]

Maybe with another function that does it explicitly, like expand_all. But I
haven't needed it myself, so I don't have plans to work on it.

[jechol]

I expected Macro.expand to expand recursively, so that I had to do googling a bit of time to find this issue.
Seems a little bit confusing because there's Macro.expand_once.

[jechol]

In Macro.expand_once doc,

Notice that expand_once/2 performs the expansion just once and it is not recursive.
Check expand/2 for expansion until the node can no longer be expanded.

It says Macro.expand_once is not recursive and recommend to check Macro.expand for such a case. It sounds like Macro.expand would expand recursively...

[josevalim]

Macro.expand recurses but it does not traverse.

[OvermindDL1]

For anyone that needs to recursively expand, I use this all over the place in my macro libraries (it is surprising just how often I need it actually...):

expanded_ast = Macro.prewalk(ast, &Macro.expand(&1, env))

But yeah, as seen it is easy enough to write it that it is not really needed to add it to the Macro module.