Where in the Elixir source does unquoting happen?

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1

I’d like to read the code that actually implements the action of unquote. It seems like it should be somewhere in the Erlang source, in elixir_parser.yrl, elixir_compile.erl, elixir_expand.erl, or elixir_erl_pass.erl, but I can’t find it.

It probably doesn’t help that I don’t know Erlang.

2

It seems unquote is not a function itself, but rather a special call handled here:

In essence, quote takes the ast and handles calls(as in ast call) to unquote differently depending if the :unquote option is true or false

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3

Thanks. I now think I understand. For people interested, I offer the following. Let me know if I’ve gotten things wrong.

Given this: quote do: inspect(1 + a), compilation works as follows.

Quote processing is handled by do_quote. The syntax tree for the body of the quote looks like this:

{:inspect,
 [context: MacroExamples.ExpansionViewer, import: Kernel],
 [{:unquote, [],
  [
    {:+, [context: MacroExamples.ExpansionViewer, import: Kernel],
      [1, {:a, [], MacroExamples.ExpansionViewer}]}]}]}

That matches a different do_quote, whose signature is: do_quote({Name, Meta, Args}. In Erlang, variables are uppercased. There are guards that check that Name is an atom and Args is a list. There’s a bit of rearrangement that ends with a call to do_quote_tuple, which looks like this:

do_quote_tuple(Left, Meta, Right, Q, E) ->
  TLeft = do_quote(Left, Q, E),
  TRight = do_quote(Right, Q, E),
  {'{}', [], [TLeft, meta(Meta, Q), TRight]}.

Left is :inspect and Right is:

[
  {:unquote, [],
   [
    {:+, [context: MacroExamples.ExpansionViewer, import: Kernel],
      [1, {:a, [], MacroExamples.ExpansionViewer}]}]}]

There’s a recursive descent into the list, then the do_quote on line 288 matches the single element:

do_quote({unquote, _Meta, [Expr]}, #elixir_quote{unquote=true}, _) ->
  Expr;

Here unquote is the way Erlang spells atoms. In an Elixir function, the signature would be do_quote({:unquote, _meta, [expr]}

All this version of do_quote does is strip and return the single argument:

{:+, [context: MacroExamples.ExpansionViewer, import: Kernel],
      [1, {:a, [], MacroExamples.ExpansionViewer}]}

… which may puzzle you as it did me. But let’s see how that three-tuple is used. Recall that it was calculated as part of this:

do_quote_tuple(Left, Meta, Right, Q, E) ->
  TLeft = do_quote(Left, Q, E),
  TRight = do_quote(Right, Q, E),
  {'{}', [], [TLeft, meta(Meta, Q), TRight]}.

The interesting bit that the first three arguments came from a three-tuple of the form {:inspect, meta, [{:unquote, ..., ...}]}

Because of the last line, that’s transformed into:

{:{}, [], 
 [
   :inspect, [...],
   [{:+, [...], [1, {:a, [], MacroExamples.ExpansionViewer}]}}]]}

That is:

  1. Because of the quote, the original {:inspect, ..., ...} three-tuple was not quoted verbatim, but rather converted into a different three-tuple, with the shape {:{}, ..., ...}. The compiler interprets that shape as meaning "emit code to call this particular function. (Hold that thought.)
  2. Because of the unquote, the original {:+, ..., ...} three-tuple was quoted verbatim: was left as an instruction to the compiler to create a function call.

It’s important to remember that the structure we’ve been building is destined to be compiled and then executed. The compiler is going to take that tree and turn it into a linear set of virtual machine (“BEAM”) instructions. Assuming a pretty simple virtual machine, that set of instructions will look something like this:

  1. From {:a, [], MacroExamples.ExpansionViewer}, the compiler produces instructions that say “Look up the value for a in the compilation context and push it on the stack.”
  2. The enclosing {:+, [import: Kernel], [1, {...above...}]} produces: “use the function :+ in Kernel and apply it to 1 and the top element on the stack. Put the result on top of the stack.”
  3. The enclosing {:{}, ..., [:inspect, ..metadata.., [...above...]]} says “create a three-tuple with the first element :inspect, the second ..metadata.., and the top of the stack.”

I extracted my example from this code:

defmodule MacroExamples.Inspect do
  a = 10
  IO.inspect(quote do: inspect(1 + a))

Therefore, the result of compiling and executing the quote expression is {:inspect, ..., 6}. Which, since it’s a three-tuple in “here’s a function call, dear compiler” format, could be compiled again, had it been a return value from a defmacro.

“I’ve taught you much, my little droogies.” … Maybe?

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