Exceptional: Helpers for Elixir exceptions

Table of Contents

Installation

Add exceptional to your list of dependencies in mix.exs:

def deps do
  [{:exceptional, "~> 1.0"}]
end

Prior Art

Tagged Status

The tagged status pattern ({:ok, _}, {:error, _}, etc)has been the bread and butter of Erlang since the beginning. While this makes it very easy to track the meaning of an expression, two things can happen:

  1. The tag becomes out of sync
  • ex. {:ok, "and yet not ok"}
  1. Pattern matching becomes challenging when different lengths exist
  • ex. {:error, "oopsie"}, {:error, "oopsie", %{original: :data, for: "handling"}}

Optimistic Flow

The other alternative is to be optimistic returns, generally seen with bang patterns. Ex. doc = File.read! path instead of {:ok, doc} = File.read path". This is more convenient, but will raise, robbing the caller of control without try/catch.

Error Monad

Currently a very undersused pattern in the Erlang/Elixir ecosystem, this is probably “the right way” to do general error handling (or at last the most theoretically pure one). Essentially, wrap your computation in an ADT struct, paired with a binding function (super-powered |>), that escapes the pipe flow if it encounters an Exception.

The downside is of course that people are generally afraid of introducing monads into their Elixir code, as understanding it requires some theoretical understanding.

Exceptional

Exceptional takes a hybrid approach. The aim is to behave similar to an error monad, but in a more Elixir-y way. This is less powerful than the monad solution, but simpler to understand fully, and cleaner than optimistic flow, and arguably more convenient than the classic tagged status.

Examples

Escape Hatch

[1,2,3] ~> Enum.sum
#=> 6

Enum.OutOfBoundsError.exception("exception") ~> Enum.sum
#=> %Enum.OutOfBoundsError{message: "exception"}

[1,2,3]
|> hypothetical_returns_exception
~> fn would_be_list ->
  would_be_list
  |> Enum.map(fn x -> x + 1 end)
  |> Enum.sum
end.()
#=> %Enum.OutOfBoundsError{message: "exception"}

0..10
|> Enum.take(3)
~> fn list ->
  list
  |> Enum.map(fn x -> x + 1 end)
  |> Enum.sum
end.()
#=> 6

Back to Tagged Status

[1,2,3]
|> hypothetical_returns_exception
~> fn would_be_list ->
would_be_list
|> Enum.map(fn x -> x + 1 end)
end.()
#=>  {:error, "exception"}

0..10
|> Enum.take(3)
~> fn list ->
list
|> Enum.map(fn x -> x + 1 end)
|< Enum.sum
end.()
|> to_tagged_status
#=> {:ok, 6}

Finally Raise

[1,2,3] >>> Enum.sum
#=> 2

%ArgumentError{message: "raise me"} >>> Enum.sum
#=> ** (ArgumentError) raise me

Manually Branch

Exceptional.Control.branch 1,
  value_do: fn v -> v + 1 end.(),
  exception_do: fn %{message: msg} -> msg end.()
#=> 2

ArgumentError.exception("error message"),
|> Exceptional.Control.branch(value_do: fn v -> v end.(), exception_do: fn %{message: msg} -> msg end.())
#=> "error message"