errors

Error handling helpers for Elixir

Description

Errors is an Elixir package that adds debugging context to error reasons. It is meant to be used in the tagged tuple style of error handling, where a function may return {:ok, result} or {:error, reason}.

Monthly Downloads: 14

Programming language: Elixir

License: MIT License

Tags: Errors and Exception Handling     Errors     Exceptions    

README

Errors is an Elixir package that adds debugging context to error reasons. It is meant to be used in the tagged tuple style of error handling, where a function may return {:ok, result} or {:error, reason}.

Motivation

To illustrate why this might be useful, consider the following code snippet that fetches the contents of a file from a GitHub repo:

defmodule HTTP do
  def get(url) do
    HTTPoison.get(url)
  end
end

defmodule GitHub do
  def file({user, repo, file}) do
    HTTP.get("https://raw.githubusercontent.com/#{user}/#{repo}/master/#{file}")
  end
end

defmodule Repo do
  def read(path) do
    GitHub.file({"nucleartide", "errors", path})
  end
end

# iex> Repo.read("notafile.txt")
# ...> {:error, %HTTPoison.Error{id: nil, reason: :closed}}

Here, we see that the Repo.read/1 operation failed, but the error reason isn't particularly helpful. Where does the HTTPoison.Error come from? What was :closed?

This error is even more opaque if Repo.read/1 comes from a third-party library.

Solution

Errors clarifies error reasons by annotating reasons with a message and stack trace. Here's how you would refactor the code above to provide additional debugging context:

defmodule HTTP do
  def get(url) do
    with res = {:ok, _} <- HTTPoison.get(url) do
      res
    else
      {:error, e} -> {:error, Errors.wrap(e, "http request failed")}
    end
  end
end

defmodule GitHub do
  defp url({user, repo, file}) do
    "https://raw.githubusercontent.com/#{user}/#{repo}/master/#{file}"
  end

  def file(github_file) do
    with res = {:ok, _} <- HTTP.get(github_file |> url()) do
      res
    else
      {:error, e} -> {:error, Errors.wrap(e, "couldn't fetch github file")}
    end
  end
end

defmodule Repo do
  def read(path) do
    with res = {:ok, _} <- GitHub.file({"nucleartide", "errors", path}) do
      res
    else
      {:error, e} -> {:error, Errors.wrap(e, "couldn't read from #{path}")}
    end
  end
end

# iex> {:error, err} = Repo.read("notafile.txt")
# ...> {:error, %Errors.WrappedError{...}}

We can print this new Errors.WrappedError to retrieve our annotated messages:

iex> IO.puts(err)
couldn't read from notafile.txt: couldn't fetch github file: http request failed: closed

Or inspect the error for a stack trace:

iex> IO.inspect(err)
** (Errors.WrappedError) couldn't read from notafile.txt
    example.exs:27: HTTP.get/1
** (Errors.WrappedError) couldn't fetch github file
    example.exs:17: GitHub.file/1
** (Errors.WrappedError) http request failed
    example.exs:6: Repo.read/1
** (HTTPoison.Error) closed

Install

Add :errors to your deps in mix.exs:

def deps do
  [
    {:errors, "~> 0.1.0"}
  ]
end

API

Errors exposes three primary macros/functions:

Errors.wrap/2

@spec wrap(error :: Exception.t, message :: String.t) :: Macro.t
defmacro wrap(error, message \\ "")

Use Errors.wrap/2 to "wrap" tagged errors received from other functions:

defmodule YourModule do
  require Errors

  def get() do
    with res = {:ok, _} <- HTTPoison.get("https://www.google.com/") do
      res
    else
      {:error, e} -> {:error, Errors.wrap(e, "can't ping google")}
    end
  end
end

Errors.new/1

@spec new(message :: String.t) :: Macro.t
defmacro new(message \\ "")

Use Errors.new/1 as a general replacement for custom Exception structs:

defmodule YourModule do
  require Errors

  def transform("https://" <> rest_of_link),
    do: {:ok, rest_of_link}
  def transform(_),
    do: {:error, Errors.new("missing https:// prefix")}
end

Errors.cause/1

@spec cause(error :: Errors.Cause.t | any) :: Exception.t | any
def cause(error)

Errors.cause/1 returns the unwrapped "cause" of an error.

The passed-in error should implement the Errors.Cause protocol; errors returned by Errors.wrap/2 and Errors.new/1 implement Errors.Cause already:

require Errors

%RuntimeError{message: "this is an error"}
|> Errors.wrap("uh-oh")
|> Errors.cause() # => %RuntimeError{message: "this is an error"}

WrappedError

Errors.wrap/2 and Errors.new/1 return a WrappedError. WrappedError implements the String.Chars and Inspect protocols, so you can freely print and inspect:

e = %RuntimeError{} |> Errors.wrap("uh-oh")
IO.puts(e)    # will print "uh-oh: runtime error"
IO.inspect(e) # will print a stack trace

Hexdocs

See the hexdocs for more details and links to source code.

Feedback, issues, concerns

Please open an issue!

Links

• Introduction blog post

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Jason Tu · GitHub @nucleartide · Twitter @nucleartide