mok

setup

Mix.install([
  {:mok, git: "https://github.com/jechol/mok", ref: "cc1159e"}
])

defmodule Target do
  require Mok

  def call(mocks) do
    # 1
    a = [1, 2, 3] |> List.first() |> Mok.inject(mocks, "unmatched")
    # if mocks contains %{{&List.first/1, "matched"} => 10} will be 10
    b = [1, 2, 3] |> List.first() |> Mok.inject(mocks, "matched")
    # 1
    c = [1, 2, 3] |> List.first() |> Mok.inject(mocks, nil)
    # 100
    d = [1, 2, 3] |> List.first() |> Mok.inject(mocks)

    a + b + c + d
  end
end

Target.call(%{})

Mok.mock(%{
  {&List.first/1, "matched"} => 10,
  {&List.first/1, nil} => 10
})
|> Target.call()