[Proposal] Implement String.Chars for Exceptions

[Allen Madsen]

Hi all,

I was trying to log an exception and noticed that this does not work: Logger.error(exception). Logger could handle this case itself, but I think it would be better to handle it more generically.

So, I propose when a module uses the defexception macro, that Elixir automatically define the String.Chars implementation for that module. Inside the defexception macro, the implementation of String.Chars would be the following:

defimpl String.Chars, for: env.module do

  def to_string(error), do: Exception.format(:error, error)

end

[Louis Pop]

What's the reason for not using inspect?

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[José Valim]

Please check the functions on the Exception module for formatting exceptions into messages using different formats and arguments.

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[Allen Madsen]

Louis,

It will generate the string for structs rather than the formatted exception with the stacktrace. See the example below:

require Logger

defmodule X do
  defexception [:message]

  defimpl String.Chars, for: X do
    def to_string(x), do: Exception.format(:error, x)
  end
end

try do
  raise X, "x"
rescue error ->
  Logger.error(error)
end

# 08:17:59.812 [error] ** (X) x
#     (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6
#     (stdlib) erl_eval.erl:904: :erl_eval.try_clauses/8
#     (elixir) src/elixir.erl:223: :elixir.erl_eval/3
#     (elixir) src/elixir.erl:211: :elixir.eval_forms/4
#     (iex) lib/iex/evaluator.ex:135: IEx.Evaluator.handle_eval/6
#     (iex) lib/iex/evaluator.ex:128: IEx.Evaluator.do_eval/4
#     (iex) lib/iex/evaluator.ex:108: IEx.Evaluator.eval/4
#     (iex) lib/iex/evaluator.ex:27: IEx.Evaluator.loop/3

defmodule Y do
  defexception [:message]

  defimpl String.Chars, for: Y do
    def to_string(y), do: inspect(y)
  end
end

try do
  raise Y, "y"
rescue error ->
  Logger.error(error)
end

# 08:18:01.364 [error] %Y{message: "y"}

Allen Madsen
http://www.allenmadsen.com

On Wed, Dec 7, 2016 at 4:03 AM, José Valim <jose....@plataformatec.com.br> wrote:

Please check the functions on the Exception module for formatting exceptions into messages using different formats and arguments.

On Wed, Dec 7, 2016 at 09:18 Louis Pop <louisp...@gmail.com> wrote:

What's the reason for not using inspect?

On Wed, 7 Dec 2016, 02:59 Allen Madsen, <bla...@gmail.com> wrote:

Hi all,

I was trying to log an exception and noticed that this does not work: Logger.error(exception). Logger could handle this case itself, but I think it would be better to handle it more generically.

So, I propose when a module uses the defexception macro, that Elixir automatically define the String.Chars implementation for that module. Inside the defexception macro, the implementation of String.Chars would be the following:

defimpl String.Chars, for: env.module do

  def to_string(error), do: Exception.format(:error, error)

end

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