defmodule TestContract do
  use Drops.Contract

  schema do
    %{
      required(:name) => type(:string, [:filled?])
    }
  end

  rule(:unique?, [{:ok, {:name, value}}]) do
    case value do
      "John" -> {:error, {:taken, :name, value}}
      _ -> :ok
    end
  end
end

TestContract.conform(%{name: "Jane"})
# {:ok, %{name: "Jane"}}

TestContract.conform(%{name: "John"})
# {:error, [error: {:taken, :name, "John"}]}