Exceptional Failure

In this short season, we are going through EXceptional Ruby by Advi Grimm

What is a failure?

Let's talk about some definitions first:

1. Exception - the occurrence of an abnormal condition during the execution of a software element.
2. Failure - the inability of a software element to satisfy its purpose
3. Error - the presence in the software of some element not satisfying its specification

"Failures cause exceptions which are due to errors"

• Failures - methods are thought to be designed to fulfill a contract. when they don't fulfill the contract, they fail

The life-cycle of an exception

• Exceptions are raised with either the raise or fail methods
• In Ruby, we signal failures with exceptions

🗣 Discussion: When do you find yourself using raise?

🗣 Discussion: When do you find yourself using rescue?

• The rescue clause should be a short sequence of simple instructions designed to bring the object back to a stable state and to either retry the operation or terminate with a failure

syntax

• Starts with the rescue keyword, followed by one or more classes or modules to match, then a hash rocket and the variable name to which the matching exception will be assigned

• Subsequent lines of code up to an end keyword will be executed if the rescue is matched

  rescue IOError => e
  puts "Error while writing to file: #{e.message}"
  end

Moar Code

begin
  raise RuntimeError, 'specific error'
rescue StandardError => error
  puts "Generic error handler: #{error.inspect}"
rescue RuntimeError => error
  puts "runtime error handler: #{error.inspect}"
end

• Order matters when stacking multiple rescue clauses. the RuntimeError rescue block will never execute!

🗣 Discussion: Have you ever used retry?

tries = 0

begin
  tries += 1
  puts "trying #{tries}"
  raise
rescue
  retry if tries < 3
  puts 'I give up'
end

• Ruby gives you the power to retry
• I can see how this might be useful for making API calls
• Be super careful that your 'giving up' criteria will eventually be met
• Retry is nice for things that are unreliable

Picks:

JP: Overcast https://overcast.fm/

John: Apple Refurbished

Episode source