The idea is to support :packer option in pack/2 that defaults to Msgpax.Packer.
This way any default packing can be overwritten with:

defmodule MyPacker do
  use Msgpax.Packer

  def pack(value) when is_float(value) do
    # pack into 32-bit format
  end
end

Where use Msgpax.Packer expands to @behaviour Msgpax.Packer and also defines the @before_compile callback that should inject Msgpax.Packer.pack for catch-all clause.

We still need to think how to efficiently pass the user-provided packer into Msgpax.Packer for usage when unpacking collections.