Macro.escape_once

[Andrea Leopardi]

Today, we have Macro.expand and Macro.expand_once, but we only have Macro.escape. I've recently found myself in need for something like Macro.escape_once.

Say you want to build a map to be pattern matched agains, so that the result of the macro is the following code:

%{foo: foo, bar: bar} = something

If I do something like this to build the map:

for var <- [:foo, :bar], into: %{} do

  {var, Macro.var(var, __MODULE__)

end

#=> %{foo: {:foo, [], Elixir}, bar: {:bar, [], Elixir}}

and then escape it in order to insert it into a quote, it becomes

{:%{}, [], [foo: {:{}, ...}, ...]}

since the variables {:foo, [], Elixir} and {:bar, [], Elixir} will be escaped (recursively). In order to solve this, I have to manually do:

kv = for var <- [:foo, :bar] do

  {var, Macro.var(var, __MODULE__)

end

{:%{}, [], kv}

It's very very possible that I'm missing something here, so I apologise in advance for that :). However, in the remote case I'm not missing anything, what do you think about adding a Macro.escape_once function?

[José Valim]

I am not sure if we can escape things once since {:{}, ...} can appear any time in an AST and it would be impossible to know if it is a previously escaped result or something that we want to further escape.

FWIW, we had the same issue in Ecto and adopted the same solution.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

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[Andrea Leopardi]

I had to face this today and after implementing with this solution (manually using {:%{}, ...}) I remembered I already saw this pattern used for Ecto queries.

I'm afraid that it's a very "mysterious" pattern though. It's also true that the use cases where you want to do this are probably not common at all.

I'm not sure I understood the reason why this would be hard to do, but I'll trust your judgement :).