Justified - add missing causes to exceptions

01 Jun 2013 Tags:

I've just published a new gem called justified which is adding missing causes to Ruby exceptions. When an exception is risen because of another one it is stored in the new one, and backtrace is appended with following: 

from caused by: (AnError) an ugly bug
from justified.rb:83:in `bad_code'
from     ... skipped 4 lines

Example

Let's have following snippet:

class AnError < StadardError; end

def bad_code
  raise AnError, 'an ugly bug'
end

def handle_error(error)
  raise AnError, 'something went wrong'
end

def do_something
  bad_code
rescue => error
  handle_error error
end

do_something

When called it will produce:

justified.rb:93:in `handle_error': something went wrong (AnError)
    from justified.rb:89:in `rescue in do_something'
    from justified.rb:87:in `do_something'
    from justified.rb:96:in `<top (required)>'
    from -e:1:in `load'
    from -e:1:in `<main>'

The real problem an ugly bug is hidden. What will happen when justified is used?

require 'justified/stadard_error'

# ... rest of the snipper

It will produce:

justified.rb:93:in `handle_error': something went wrong (AnError)
    from justified.rb:89:in `rescue in do_something'
    from justified.rb:87:in `do_something'
    from justified.rb:96:in `<top (required)>'
    from -e:1:in `load'
    from -e:1:in `<main>'
    from caused by: (AnError) an ugly bug
    from justified.rb:83:in `bad_code'
    from     ... skipped 4 lines

Of course causes can be concatenated.

Usage

  • require 'justified' to include Justified::Error to any exception you need manually
  • require 'justified/standard_error' to have causes in all exceptions which are kind of StandardError

Behavior

When an exception is risen inside rescue block a cause is automatically recorded.

e = begin
      raise 'bug'
    rescue => error
      raise 'this does not work'
    end rescue $!
e.cause.message == 'bug' # => true

Cause can be set explicitly.

e = begin
      raise 'bug'
    rescue => error
      raise StandardError.new('this does not work', error)
    end rescue $!
e.cause.message == 'bug' # => true

Or if signature of .new is changed cause can be set with a setter #cause=

class InspectingError < StandardError
  def initialize(object)
    super object.inspect
  end
end

e = begin
      raise 'bug'
    rescue => error
      raise InspectingError.new(a: 'b').tap { |e| e.cause = error }
    end rescue $!
e.cause.message == 'bug' # => true

and

e = begin
      raise 'bug'
    rescue => error
      raise InspectingError.new(a: 'b')
    end rescue $!
e.cause.message == 'bug' # => true

will work as well.

Do you wish to know more?