functional-programming - 如何在函数式编程语言中使用函数而不是提前返回

Question

我最近开始学习Elixir我真的很喜欢它，但它是我使用过的第一种函数式编程语言。我面临的问题来自于我在 LearnElixir 上阅读教程和观看截屏视频的内容。是你应该尽量避免使用 IF 类型的语句。

但我发现自己经常嵌套 cond 或 case

我会用 Golang 或 Javascript 等其他语言来解决这些解决方案，方法是仅使用带有提前返回的 if 语句，这样超出该条件的代码就不会运行，这使我在 99% 的情况下不必嵌套条件语句只是检查虚假值并返回。

那么在 Elixir(或其他函数式编程语言)中，您将如何在不使用嵌套的情况下以正确的方式编写如下所示的内容，并利用该语言的功能。

def loginPost(conn, %{"user" => user_params}) do
    # Look for user in database
    query = from u in User,
      where: u.email == ^user_params["email"],
      select: [u.email, u.username, u.password]

    data = Repo.all(query)

    # Check to see if a user had been found
    case (length data) == 0 do
      true -> # No user was found, send an error message
        conn
        |> json(%{ success: false, errors: ["Wrong username or password"]})
      false -> # A user was found, compare password
        [[email, username, db_password]] = data
        case Comeonin.Bcrypt.checkpw(user_params["password"], db_password) do
          true -> # Password was correct, set session and return a json response
            conn
            |> put_session(:authenticated, true)
            |> put_session(:username, username)
            |> put_session(:email, email)
            |> json(%{success: true}) # Send json response and redirect on client side
          false -> # Password was incorrect, send an error message
            conn
            |> json(%{success: false, errors: ["Wrong username or password"]})
        end
    end
  end
end

Answer 1

一种方法是使用with。您可以创建单独的函数，看起来像这样:

def authenticate(email, password) do
  with {:ok, user} <- find_user(email),
    {:ok, user} <- validate_password(user, password),
    {:ok, user} <- validate_preconditions(user)
  do: {:ok, user}
end

defp find_user(email) do
  # return {:ok, user} if user is found, return {:error, :user_not_found} otherwise
end

defp validate_password(user, password) do
  # return {:ok, user} if password is correct, return {:error, :invalid_password} otherwise
end

defp validate_preconditions(user) do
  # return {:ok, user} if user is not banned or whatever, return {:error, :cant_be_logged_in} otherwise
end

然后你可以像这样在你的 Controller 函数中使用它:

def loginPost(conn, %{"user" => user_params}) do
  case authenticate(user_params["email"], user_params["password"]) do
    {:ok, user} -> # start session
    {:error, error_type} -> # handle error
  end
end

这个例子可能会更好，但你明白了。

您还可以阅读此 question 中的答案