If adding mix deps.add means you have to list your dependencies in a separate file or have to structure them in a special way, then it’s not a good solution. One feature in the language shouldn’t limit other features.

As @LostKobrakai already said mix hex.info shows the configuration you can copy-paste into mix.exs:

> mix hex.info jason
A blazing fast JSON parser and generator in pure Elixir.

Config: {:jason, "~> 1.2"}
Releases: 1.2.2, 1.2.1, 1.2.0, 1.1.2, 1.1.1, 1.1.0, 1.0.1, 1.0.0, ...

Licenses: Apache-2.0
Links:
  GitHub: https://github.com/michalmuskala/jason

The single extra step of pasting the dependency config into mix.exs is not worth losing the flexibility of being able to configure dependencies using code.

Few examples of mix.exs for you to deduce how you would like to add the new dependency:

defmodule Foo.Mixfile do
  use Mix.Project

  def project, do: [app: :foo, version: "0.0.1"]
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [app: :foo, version: "0.0.1"]
  end
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [app: :foo, version: "0.0.1", deps: [{:mydep, ">= 0.0.0"}]]
  end
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [
      app: :foo,
      version: "0.0.1",
      deps: foos()
    ]
  end

  defp foos, do: []
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [
      app: :foo,
      version: "0.0.1",
      deps: deps(Mix.env())
    ]
  end

  defp deps(:dev) do
    []
  end

  defp deps(:test) do
    [{:benchee, ">= 0.0.0"}]
  end
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [
      app: :foo,
      version: "0.0.1",
      deps: deps(System.get_env("BENCH"))
    ]
  end

  defp deps(nil) do
    []
  end

  defp deps(_) do
    [{:benchee, ">= 0.0.0"}]
  end
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [
      app: :foo,
      version: "0.0.1",
      deps: deps()
    ]
  end

  defp deps do
    [{:foo, ">= 0.0.0"} | more_deps()]
  end

  defp more_deps() do
    [{:bar, ">= 0.0.0"}]
  end
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [
      app: :foo,
      version: "0.0.1",
      deps: deps(System.get_env("BENCH"))
    ]
  end

  defp deps(nil) do
    []
  end

  defp deps(_) do
    [{:benchee, ">= 0.0.0"}]
  end
end

defmodule Foo.Mixfile do
  use Mix.Project

  @deps [{:foo, ">= 0.0.0"}]

  def project do
    [
      app: :foo,
      version: "0.0.1",
      deps: @deps
    ]
  end
end

defmodule WhyNot do
  def deps, do: [{:foo, ">= 0.0.0"}]
end

defmodule Foo.Mixfile do
  use Mix.Project

  def project do
    [
      app: :foo,
      version: "0.0.1",
      deps: WhyNot.deps()
    ]
  end
end

Let’s not go down the route of node.js. I absolutely hate it when npm install change my package.json file and mess up the indentation.

Having a file that regularly needs to be modified by human and machine is asking for trouble.