Protocol for unwrapping exceptions in Throw.try_catch/1.

When an exception is caught inside a computation and processed by Throw.try_catch/1, this protocol determines how the exception is converted to an error value.

Default Behavior

By default (via the Any fallback), exceptions are returned as-is:

raise ArgumentError, "bad input"
# try_catch returns: {:error, %ArgumentError{message: "bad input"}}

Custom Unwrapping

Domain exceptions can implement this protocol to provide cleaner error values suitable for pattern matching:

defmodule MyApp.NotFoundError do
  defexception [:entity, :id]

  @impl true
  def message(%{entity: entity, id: id}) do
    "#{entity} not found: #{id}"
  end
end

defimpl Skuld.Comp.IThrowable, for: MyApp.NotFoundError do
  def unwrap(%{entity: entity, id: id}), do: {:not_found, entity, id}
end

Now when code raises this exception:

raise MyApp.NotFoundError, entity: :user, id: 123
# try_catch returns: {:error, {:not_found, :user, 123}}

This enables clean pattern matching on domain errors:

case result do
  {:ok, user} -> handle_user(user)
  {:error, {:not_found, :user, id}} -> handle_not_found(id)
  {:error, %ArgumentError{}} -> handle_bad_input()
end

When to Implement

Implement IThrowable for exceptions that represent domain errors - expected failures that are part of your business logic. Examples:

• Validation failures
• Resource not found
• Permission denied
• Business rule violations

Leave the default behavior for unexpected errors - bugs or system failures where you want the full exception for debugging:

• ArgumentError, KeyError, etc.
• Database connection errors
• External service failures