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2 Comments

  1. McKinnin Lloyd on January 27, 2020 at 10:04 pm

    I have a question. Why in the function get_name it will throw an error if you do %User{name: name} and not %{name: name}, but in the create_greeting function it is opposite. In that function it wants you to do %User{name: name}?

    • brainlid on January 28, 2020 at 6:20 am

      That’s a good question! In the get_name example, we want the pattern to be less specific. By not specifying the struct type, we match on any map data type (including structs) but only if they have a key called :name. For get_name, it would also be valid to write this:

      def get_name(%Customer{name: name}), do: {:ok, name}
      def get_name(%User{name: name}), do: {:ok, name}
      def get_name(_other), do: {:error, "Doesn't have a name"}

      This is more specific. We give an explicit pattern match on a customer and user struct. However, the results of the functions are identical. It doesn’t actually matter that it’s a Customer or a User struct type. It is better to match more generally to only the data that we actually care about for getting the :name from the data.

      Then in the create_greeting function we want to be more specific with our match because we want a different result based on the data type.

      Another way to describe the behavior in get_name is like this: “if the data has a key called :name then return the value”. It doesn’t care about what kind of struct. Then create_greeting is saying “if it is a customer then greet this way. If it is a user then greet this other way.” The best way to tell them apart is by their struct types.

      Did I answer your question?

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