Elixir: Protocol.UndefinedError

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defmodule User do
  defstruct [:id, :name, :email]
end

# String.Chars プロトコルを実装
defimpl String.Chars, for: User do
  def to_string(%User{name: name, email: email}) do
    "#{name} <#{email}>"
  end
end

# 使用
user = %User{id: 1, name: "John", email: "john@example.com"}
"#{user}"  # => "John <john@example.com>"

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defmodule User do
  @derive {Inspect, only: [:name, :email]}  # :id を隠す
  @derive Jason.Encoder  # JSON エンコード可能に
  defstruct [:id, :name, :email]
end

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defmodule CustomRange do
  defstruct [:start, :stop]
end

defimpl Enumerable, for: CustomRange do
  def count(%CustomRange{start: start, stop: stop}) do
    {:ok, stop - start + 1}
  end

  def member?(%CustomRange{start: start, stop: stop}, value) do
    {:ok, value >= start and value <= stop}
  end

  def reduce(%CustomRange{start: start, stop: stop}, acc, fun) do
    reduce_range(start, stop, acc, fun)
  end

  defp reduce_range(current, stop, {:cont, acc}, fun) when current <= stop do
    reduce_range(current + 1, stop, fun.(current, acc), fun)
  end
  defp reduce_range(_, _, {:halt, acc}, _fun), do: {:halted, acc}
  defp reduce_range(_, _, {:suspend, acc}, fun), do: {:suspended, acc, &reduce_range(&1, &2, &3, fun)}
  defp reduce_range(_, _, acc, _fun), do: {:done, acc}

  def slice(_), do: {:error, __MODULE__}
end

# 使用
range = %CustomRange{start: 1, stop: 5}
Enum.to_list(range)  # => [1, 2, 3, 4, 5]

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defmodule Event do
  defstruct [:id, :name, :timestamp]
end

defimpl Jason.Encoder, for: Event do
  def encode(%Event{id: id, name: name, timestamp: ts}, opts) do
    Jason.Encode.map(%{
      id: id,
      name: name,
      timestamp: DateTime.to_iso8601(ts)
    }, opts)
  end
end

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defmodule Password do
  defstruct [:value]
end

defimpl Inspect, for: Password do
  def inspect(_password, _opts) do
    "#Password<***>"  # 値を隠す
  end
end

# 使用
password = %Password{value: "secret123"}
inspect(password)  # => "#Password<***>"

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defprotocol Describable do
  @fallback_to_any true
  def describe(value)
end

defimpl Describable, for: Any do
  def describe(value) do
    "Unknown: #{inspect(value)}"
  end
end

defimpl Describable, for: User do
  def describe(%User{name: name}) do
    "User: #{name}"
  end
end

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# マップを文字列に変換したい場合
def map_to_string(map) when is_map(map) do
  map
  |> Enum.map(fn {k, v} -> "#{k}: #{v}" end)
  |> Enum.join(", ")
end

# または Kernel.inspect/1 を使用
inspect(%{key: "value"})  # => "%{key: \"value\"}"