Elixir: dynamic parameter in use statement - module

Question

How could I pass adapter parameter dynamically to Nebulex.Cache module initialization?
defmodule MyApp.RedisCache do
use Nebulex.Cache,
otp_app: :nebulex,
adapter: if true do NebulexRedisAdapter else Nebulex.Adapters.Nil end
end

Answer 1

The easiest way would be to move the conditional one level up.
defmodule MyApp.RedisCache do
if true do
use Nebulex.Cache,
otp_app: :nebulex,
adapter: NebulexRedisAdapter
else
use Nebulex.Cache,
otp_app: :nebulex,
adapter: Nebulex.Adapters.Nil
end
...
end
Another way would be to create a private macro with a parameter, or, alternatively, create a module attribute storing the adapter and pass it to use (assuming Nebulex.Cache does Macro.expand/2 arguments in the __CALLER__ context.)
The reason is that the macro receives the AST, so what will __using__/1 receive, is quote do: if .... which won’t be even attempted to compile at the first compilation stage.
The below code might shed a light on how the AST is passed between calls and what Macro.expand/2 does.
defmodule M do
#param if(true, do: M1, else: M2)
defmacrop t1(arg: args) do
IO.inspect(args, label: "#1")
end
defmacrop t2(arg: args) do
IO.inspect(Macro.expand(args, __CALLER__), label: "#2")
end
def test do
t1(arg: if(true, do: M1, else: M2))
t2(arg: if(true, do: M1, else: M2))
t1(arg: #param)
t2(arg: #param)
end
end
It results in:
#1: {:if, [line: 19],
...
#2: {:case, [optimize_boolean: true],
...
#1: {:#, [line: 21], [{:param, [line: 21], nil}]}
#2: M1
Note, that if was expanded to case, and the module attribute was not expanded unless the explicit Macro.expand/2 was called on it.