Exceptional: Helpers for Elixir exceptions
Table of Contents
Installation
Add exceptional to your list of dependencies in mix.exs:
def deps do
[{:exceptional, "~> 2.1"}]
endAbout
Exceptional is an Elixir library providing helpers for working with exceptions. It aims to make working with plain old (unwrapped) Elixir values more convenient, and to give full control back to calling functions.
See the Medium article for more
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:
- The tag becomes out of sync
- ex.
{:ok, "and yet not ok"}
- 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.
This is a classic inversion of control, and allows for very flexible patterns.
For example, using >>> (ie: raise if exception, otherwise continue) sidesteps
the need for separate bang functions.
Just like the classic FP wisdom: if it doubt, pass it back to the caller to handle.
Examples
Make Safe
A simple way to declaw a function that normally raises. (Does not change the behaviour of functions that don't raise).
toothless_fetch = safe(&Enum.fetch!/2)
[1,2,3] |> toothless_fetch.(1)
#=> 2
toothless = safe(&Enum.fetch!/2)
[1,2,3] |> toothless.(999)
#=> %Enum.OutOfBoundsError{message: "out of bounds error"}
safe(&Enum.fetch!/2).([1,2,3], 999)
#=> %Enum.OutOfBoundsError{message: "out of bounds error"}Escape Hatch
[1,2,3] ~> Enum.sum()
#=> 6
Enum.OutOfBoundsError.exception("exception") ~> Enum.sum
#=> %Enum.OutOfBoundsError{message: "exception"}
[1,2,3]
|> hypothetical_returns_exception()
~> Enum.map(fn x -> x + 1 end)
~> Enum.sum()
#=> %Enum.OutOfBoundsError{message: "exception"}
0..10
|> Enum.take(3)
~> Enum.map(fn x -> x + 1 end)
~> Enum.sum()
#=> 6Normalize Errors
Elixir and Erlang interoperate, but represent errors differently. normalize normalizes values into exceptions or plain values (no {:error, _} tuples).
This can be seen as the opposite of the functions that convert back to tagged status.
Some error types may not be detected; but you may pass a custom converter (see examples below).
normalize(42)
#=> 42
normalize(%Enum.OutOfBoundsError{message: "out of bounds error"})
#=> %Enum.OutOfBoundsError{message: "out of bounds error"}
normalize(:error)
#=> %ErlangError{original: nil}
normalize({:error, "boom"})
#=> %ErlangError{original: "boom"}
normalize({:error, {1, 2, 3}})
#=> %ErlangError{original: {1, 2, 3}}
normalize({:error, "boom with stacktrace", ["trace"]})
#=> %ErlangError{original: "boom with stacktrace"}
normalize({:good, "tuple", ["value"]})
#=> {:good, "tuple", ["value"]}
{:oh_no, {"something bad happened", %{bad: :thing}}}
|> normalize(fn
{:oh_no, {message, _}} -> %File.Error{reason: message} # This case
{:bang, message} -> %File.CopyError{reason: message}
otherwise -> otherwise
end)
#=> %File.Error{message: msg}
{:oh_yes, {1, 2, 3}}
|> normalize(fn
{:oh_no, {message, _}} -> %File.Error{reason: message}
{:bang, message} -> %File.CopyError{reason: message}
otherwise -> otherwise # This case
end)
#=> {:oh_yes, {1, 2, 3}}Back to Tagged Status
[1,2,3]
|> hypothetical_returns_exception()
~> Enum.map(fn x -> x + 1 end)
~> Enum.sum()
#=> {:error, "exception"}
0..10
|> Enum.take(3)
~> Enum.map(fn x -> x + 1 end)
~> Enum.sum()
|> to_tagged_status()
#=> {:ok, 6}
0..10
|> hypothetical_returns_exception()
~> Enum.map(fn x -> x + 1 end)
~> Enum.sum()
|> ok()
#=> {:error, "exception"}
maybe_sum =
0..10
|> hypothetical_returns_exception()
~> Enum.map(fn x -> x + 1 end)
~> Enum.sum()
~~~maybe_sum
#=> {:error, "exception"}Finally Raise
Note that this does away with the need for separate foo and foo! functions,
thanks to the inversion of control.
[1,2,3] >>> Enum.sum()
#=> 6
%ArgumentError{message: "raise me"} >>> Enum.sum()
#=> ** (ArgumentError) raise me
ensure!([1, 2, 3])
#=> [1, 2, 3]
ensure!(%ArgumentError{message: "raise me"})
#=> ** (ArgumentError) raise me
defmodule Foo do
use Exceptional
def! foo(a), do: a
end
Foo.foo([1, 2, 3])
#=> [1, 2, 3]
Foo.foo(%ArgumentError{message: "raise me"})
#=> %ArgumentError{message: "raise me"}
Foo.foo!([1, 2, 3])
#=> [1, 2, 3]
Foo.foo!(%ArgumentError{message: "raise me"})
#=> ** (ArgumentError) raise meManually 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"
if_exception 1, do: fn %{message: msg} -> msg end.(), else: fn v -> v + 1 end.(),
#=> 2
ArgumentError.exception("error message")
|> if_exception do
fn %{message: msg} -> msg end.())
else
fn v -> v end.()
end
#=> "error message"