How to insert an AST as the code for a function body ?

[Kip Cole]

I have a macro to define a function where one of the parameters is the AST of the function body.  But I can't work out how to insert the AST. Example:

      defmacro define_do_cardinal(rules, locale) do

        quote do

          defp do_cardinal(unquote(locale), n, i, v, w, f, t), do: unquote(rules)

        end

      end

  

unquote(rules) won't do it since unquoting doesn't add the code to the function it just inserts the AST as a literal.  I could, I suppose, hand craft the function header AST and insert the function body AST but that seems more work than I would expect.

Is there a way to insert an AST as the code for a function body?

[José Valim]

Well, quoted expressions are meant to represent the code unless it is escaped, then you get the AST literal. So maybe the question is where the AST is being escaped?

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José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

[Kip Cole]

José, thank you as always for your engagement.

I am basically hand-crafting the AST, mostly via leex/yecc with a few after-parsing transformations.  So I’m left with an AST in literal form (i.e. as a list of tuples - is that the right expression?).  What I’ve worked on in the mean time that does what I want  is:

  # Generate the functions to process plural rules

  @spec do_cardinal(binary, number, number, number, number, number, number) 

    :: :one | :two | :few | :many | :other

  Enum.each Cldr.known_locales, fn (locale) ->

    function_body = cardinal_rules[locale] |> rules_to_condition_statement(__MODULE__)

    function = quote do

      defp do_cardinal(unquote(locale), n, i, v, w, f, t), do: unquote(function_body)

    end

    Code.eval_quoted(function, [], __ENV__)

  end

In this case there’s no macro involved but I feel a little “unclean” about doing the Code.eval_quoted/3 part.

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[José Valim]

Elixir supports something called unquote fragments, which means you can get rid of quote and the eval and it should just work. If not, please provide a sample file and I can take a look when j get back home.

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[Kip Cole]

José, thanks, I’ll put a simple case together.  I have tried the unquote fragment approach first.  Like:

  Enum.each Cldr.known_locales, fn (locale) ->
    function_body = cardinal_rules[locale] |> rules_to_condition_statement(__MODULE__)

    defp do_cardinal(unquote(locale), n, i, v, w, f, t), do: unquote(function_body)

  end

But in that case the `unquote(body)` became an AST literal for the function body rather than the AST being merged into the function definition.

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