Why `var!`? - Difference between the two defmacros.

[Jiafeng Cao]

Hi, I've been reading through about macros, and came across the `var!`. I thought I got the idea of it until I wrote some code like this gist.

I know I can use `def unquote(name)(var!(args)), do: ...` to make it work. I just cannot get it. 

What's I missing here? Many thanks.

[Eric Meadows-Jönsson]

It's documented under "Hygiene for variables" here http://elixir-lang.org/docs/stable/elixir/Kernel.SpecialForms.html#quote/2.

The reasoning for this behaviour is that variables from different contexts shouldn't leak into each others contexts without making it explicit with `var!`.

Your code doesn't work because `args` is used in a different context than where it was defined.

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Eric Meadows-Jönsson

[Jiafeng Cao]

I have read it several times. From my little experience, I think you mean:

- the `args` is defined inside the macro quote context, and used in the caller context where the quote expands to?

- and that's why var! comes to rescue?

在 2016年7月29日星期五 UTC+8上午2:23:41，Eric Meadows-Jönsson写道：

[Eric Meadows-Jönsson]

Yes, that is correct. =)

To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-talk/2f17fe5e-6db7-4b3d-858c-41a6fd50aa0c%40googlegroups.com.

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Eric Meadows-Jönsson