Clear definition of unquote fragments

Hi there,

After reading @ericmj response here (Using unquote outside of quote block) about “unquote fragments”, I still can’t completely grasp the concept.

I’ve read the documentation many times now, besides some other resources (including McCord’s Metaprogramming Elixir), and I can’t reach to a concrete definition of what an unquote fragment is.

I’ve read in some places that “unquote fragments” is the usage of unquote that lets us dynamically define functions and nested macros ( After reading all this, I think both unquote/1 calls on the documentation example are “unquote fragments”, since they will be injected on the caller context and won’t have an explicit quote block enclosing them. Am I right?:

    defmacro defkv(kv) do
      quote bind_quoted: [kv: kv] do
        Enum.each kv, fn {k, v} ->
          def unquote(k)(), do: unquote(v)

And consider this example please:

defmodule Foo do
  alias Baz

  fun_definition = {:bar, [], [{:arg, [], nil}]}
  body = {:__block__, [], [{:*, [], [{:arg, [], nil}, {:arg, [], nil}]}]}
  function = :bar
  arity = 1

  def(unquote(fun_definition)) do
    mfa = Baz.mfa(__MODULE__, unquote(function), unquote(arity)) # returns "Module.function/arity", fn -> unquote(body) end)

Here I think the unquote(fun_definition) is definitely an unquote fragment because is allowing us to dynamically define the function. But I am not so sure regarding the other unquote/1 calls for function, arity and body. Since they are contributing for the dynamic definition of the function body, are they also unquote fragments?

Thanks in advance,
And thank you all for the amazing community!

P.S.: Given the different opinions and definitions I’ve read, IMO the documentation could be updated to include an explicit unquote fragment definition. This way those like me who are starting to look at Elixir would be able to clearly distinguish between a “normal” unquote inside an explicit quote block and an unquote fragment.

P.S.2: I know that we can use the unquote/1 calls inside the def/2 macro arguments because the def/2 arguments, like any macro, are being quoted for us.

Many thanks!


I think this post has a good summary: Using unquote outside of quote block

1 Like

Hi @kip, thanks for the reply. I’ve mentioned that post on my initial message. I understand why we can do it, it may seem at first glance there’s no quote block wrapping the unquote call, but there is one in fact due to the macro arguments being quoted for us.

What I didn’t understand and think it isn’t completely clear on the documentation is the concept of unquote fragment. Saying that it is what we use to dynamically define a function IMO gives room for interpretation.