** (ArgumentError) unknown registry: Req.Finch on a simple http request using Req library

== Compilation error in file lib/cry.ex ==
** (ArgumentError) unknown registry: Req.Finch
    (elixir 1.16.3) lib/registry.ex:1400: Registry.key_info!/1
    (elixir 1.16.3) lib/registry.ex:590: Registry.lookup/2
    (finch 0.18.0) lib/finch/pool_manager.ex:46: Finch.PoolManager.lookup_pool/2
    (finch 0.18.0) lib/finch/pool_manager.ex:34: Finch.PoolManager.get_pool/3
    (finch 0.18.0) lib/finch.ex:428: Finch.__stream__/5
    (finch 0.18.0) lib/finch.ex:472: anonymous fn/4 in Finch.request/3
    (telemetry 1.2.1) c:/Users/pedro/OneDrive/Área de Trabalho/testes_pay/deps/telemetry/src/telemetry.erl:321: :telemetry.span/3
    (req 0.5.1) lib/req/steps.ex:977: Req.Steps.run_finch_request/3

the full compilation error in case it helps

== Compilation error in file lib/cry.ex ==

Oh, you’re using Req at compile-time, i.e. in module body and similar. In that case it seems you need to explicitly start dependencies:

defmodule Foo do
  {:ok, _} = Application.ensure_all_started(:req)
  Req.get(...)
end

Thanks for you help, it worked. Application its a built in module in elixir/mix and ensure_all_started all is another module (or method, I don’t know how things are exactly referenced in lixir) that ensures all dependencies a module uses are loaded before the module itself?
I dont quite get the ''using req at compile-time", there is another way os using it?

compile-time vs runtime is a pretty broad topic so to just scratch the surface when you have a module:

def Foo do
  IO.puts :at_compile_time

  def foo do
    IO.puts :at_runtime
  end
end

the first IO.puts is executed while the module is being compiled. When it is being used at runtime, that IO.puts will not be executed. On the other hand, the second IO.puts is NOT executed when the module is being compiled. It is only executed when you call Foo.foo(). Doing things at compile-time is definitely an Elixir super power and for this particular use case you need that Application.ensure_all_started. (The reason is Req library starts a supervision tree in its application callback module. If it didn’t, ensure_all_started would not be necessary.) In vast majority of day to day Elixir programming, you don’t need ensure_all_started, it is done for you.

No, not at all. Modules are loaded just fine but there is the concept of applications that start various OTP processes which is achieved by the applications themselves being “started” (“application” being a useful abstraction here).

Are you sure you want to be using Req at compile time? I suspect you want to move your use of Req into a function so that you can use it while your application is running

what do you mean by that? I have no idea what i’m doing to be real, i’m learning elixir and the lib now, the docs aren’t helping much, i’m stuck in pretty basic things

What does your “main file” look like?

Others will correct me if I’m wrong, but I think, based on @wojtekmach’s answer, it’s because your function calls are not themselves in a function, so they call functions at compile time instead of run time:

Try putting those function calls in a function:

defmodule Main do
  def go do
    Hello.say_hi()
    GetPayloadInfo.doReq()
  end
end

Then (without the Application.ensure_all_started), run the Main.go function in iex (in the directory containing your Mix project):

$ iex -S mix
iex> Main.go

I think we have a winner: Tweak Finch supervisor children strartup order by elfenlaid · Pull Request #289 · sneako/finch · GitHub

For those finding this thread after running into this error when using Req in a function within a Mix.Task module: This problem is resolved by adding @requirements ["app.start"] at the module level. This ensures that Req and its dependencies are available for use when you execute the Mix task.

More information can be found in the documentation here.