adding pres about exceptions in Ruby

2015-07-31/dario_ruby_exceptions.rb

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+# ERROR HANDLING AND EXCEPTIONS
+#
+# What is an exception?
+# -> unacceptable behaviour in a program
+#
+# Exception:
+#  -> is an object (a special kind)
+#  -> instance of the class Exception, or a class of one of its descendants
+#
+# Raising (an exception):
+#  -> means stopping normal execution of the program, and either dealing with an exception or exiting
+#     the entire program
+#  -> we deal with exceptions by providing a program with a |rescue| clause
+#
+# -> common exceptions:
+#  * RuntimeError - default error raise using the method |raise|
+#  * NoMethodError - object can't resolve a name of the message it receives to a method.
+#  * NameError - interpreter comes across an identifier it can't resolve. It can be variable name,
+#     method name.
+#  * IOError - error caused by trying to read a closed file, write to a read-only file etc.
+#  * TypeError - method receives an argument with the wrong type.
+#  * ArgumentError - method receives incorrect number of arugments.
+# -> they are all descendants of the class Exception
+#
+# Rescuing with a *rescue block*:
+#  -> begin > rescue > end
+
+# 9 - creating custom error classes
+#class HeroError < Exception; end
+HeroError = Class.new(Exception)
+
+class SuperHero
+  attr_accessor :special_power, :name
+
+  def initialize(name)
+    @name = name
+    @special_power = true
+  end
+
+  def save_person(name)
+    puts "I have saved #{name}."
+  end
+
+  # 2 - rescue in method definition
+  def try_to_kill_villain(name)
+    kill_villain(name)
+  rescue 
+    puts "I don't kill people."
+  end
+
+  # 3 - raise keyword
+  def have_a_special_power?
+    raise RuntimeError, "#{name} doesn't have a special power!" unless special_power
+    raise HeroError, "#{name} doesn't have a special power!" unless special_power
+  end
+end
+sh = SuperHero.new("Batman")
+
+# 1 - usual rescuing
+#begin
+  #sh.save_person("John Doe")
+  #sh.kill_villain("Bane")
+#rescue
+  #puts "True superheroes don't kill people."
+#end
+
+# 2 - rescue in method definition
+#sh.try_to_kill_villain("Joker")
+
+# 3 - explicitly raising an exception
+# 4 - raising with arguments
+#sh.special_power = false
+#sh.have_a_special_power?
+
+# 5 - capturing an exception in an object
+#  - it can respond to messages, it is an object
+#begin
+  #nil + 5
+#rescue NoMethodError => ex
+  ## gives an array of strings representing the call stack
+  #puts "Here comes backtrace:"
+  #p ex.class
+  #puts ex.backtrace
+#end
+
+# 6 - What gets raised, an exception or an exception class?
+# 7 - reraise an exception
+# 8 - ensure clause
+# When writing, class names are used [raise NoMethodError], but what gets raised is an instance
+# of that class [NoMethodError.new]
+#begin
+  #sh.some_method
+  #sh.have_a_special_power?
+#rescue => ex
+  #p ex.class, ex
+  # 7 - reraise
+  #puts "do something..."
+#ensure
+  #puts "WILL BE RUN"
+#end
+
+# 9 - namespacing an exception class is a good idea to avoid name clashes