Support to unquote fragments pushed to master

[José Valim]

Hello everyone,

We have exciting news to share. I have pushed to master a couple fixes that allow:

1. Macros to define macros

2. Easy dynamic generation of functions

I will detail those two briefly.

In previous versions, Elixir always escaped unquoted contents, so if you had a macro that had to define another macro, like this:

    defmacro new_macro do

      quote do

        defmacro sample(a) do

          quote do

            unquote(a) + 1

          end

        end

      end

    end

The example above would fail because "unquote(a)" would be expanded when evaluating new_macro and since "a" is not available in there, it would generate an exception. On master, the code above works because "unquote(a)" is just expanded when sample is called. In other words, the unquote now always binds to the direct parent quote. This explains 1).

The second builds on top of this feature to allow easy function generation. Now we can do this:

    Enum.each [foo: 1, bar: 2, baz: 3], fn { k, v } ->

      def unquote(k)(arg) do

        unquote(v) + arg

      end

    end

This works because def/2, defp/2 and friends now delegate to quote underneath so it supports unquote fragments on its definition.

Here is an example of how this helps cleaning up code:

https://github.com/elixir-lang/elixir/commit/f10171bfd2d7b4417a08ecffd5c2becaa69e554b

Here is a commit that shows how we made defdelegate support the same feature:

https://github.com/elixir-lang/elixir/commit/ecb23a4b423f7f3c36c2305877afd3ef5730e0b8

In a way, this simulates free variables. It would be impractical to have free variables in Elixir per se, however by using this feature, we can get a similar result. Here is HashDict using this feature as if accessing variables defined in the module body as constants:

https://github.com/elixir-lang/elixir/commit/238e7f4bc23a597d09a2e3db345acc21477f2b29

For more background info, feel free to check issue #880 on Github:

https://github.com/elixir-lang/elixir/issues/880

Please give it a try, there are surely some rough edges to handle.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

[jer...@jeremyhuffman.com]

This all makes a lot of sense, but what is the advantage of the "constants" over using attributes? It seems more verbose:

-      expand_on: size * @node_size, contract_on: contract_on * @node_size) +      expand_on: size * unquote(node_size), contract_on: contract_on * unquote(node_size))

[José Valim]

Hello Jeremy,

For now, it is indeed more verbose but adding an operator for unquote is not out of question in the future.

The advantage so far is simply to avoid naming conflicts since @attributes are also used for declaring documentation, annotating the module and more. Therefore, it looks to me that using variables feels more natural. And if it does become a common idiom, we can try to make it shorter. :)

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

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