Ruby - Exceptions

The execution and the exception always go together. If you are opening a file, which does not exist, then if you did not handle this situation properly, then your program is considered to be of bad quality.
The program stops if an exception occurs. So exceptions are used to handle various type of errors, which may occur during a program execution and take appropriate action instead of halting program completely.
Ruby provide a nice mechanism to handle exceptions. We enclose the code that could raise an exception in a begin/end block and use rescue clauses to tell Ruby the types of exceptions we want to handle.
begin  
# -  
rescue OneTypeOfException  
# -  
rescue AnotherTypeOfException  
# -  
else  
# Other exceptions
ensure
# Always will be executed
end

Everything from begin to rescue is protected. If an exception occurs during the execution of this block of code, control is passed to the block between rescue and end.
For each rescue clause in the begin block, Ruby compares the raised Exception against each of the parameters in turn. The match will succeed if the exception named in the rescue clause is the same as the type of the currently thrown exception, or is a superclass of that exception.
In an event that an exception does not match any of the error types specified, we are allowed to use an else clause after all the rescue clauses.
#!/usr/bin/ruby

begin
   file = open("/unexistant_file")
   if file
      puts "File opened successfully"
   end
rescue
      file = STDIN
end
print file, "==", STDIN, "\n"
This will produce the following result. You can see that STDIN is substituted to file because open failed.
#<IO:0xb7d16f84>==#<IO:0xb7d16f84>

You can capture an exception using rescue block and then use retry statement to execute begin block from the beginning.
begin
   # Exceptions raised by this code will 
   # be caught by the following rescue clause
rescue
   # This block will capture all types of exceptions
   retry  # This will move control to the beginning of begin
end

#!/usr/bin/ruby

begin
   file = open("/unexistant_file")
   if file
      puts "File opened successfully"
   end
rescue
   fname = "existant_file"
   retry
end
The following is the flow of the process −
An exception occurred at open.
Went to rescue. fname was re-assigned.
By retry went to the beginning of the begin.
This time file opens successfully.
Continued the essential process.
NOTE − Notice that if the file of re-substituted name does not exist this example code retries infinitely. Be careful if you use retry for an exception process.
You can use raise statement to raise an exception. The following method raises an exception whenever it's called. It's second message will be printed.
raise 

OR

raise "Error Message" 

OR

raise ExceptionType, "Error Message"

OR

raise ExceptionType, "Error Message" condition

The first form simply re-raises the current exception (or a RuntimeError if there is no current exception). This is used in exception handlers that need to intercept an exception before passing it on.
The second form creates a new RuntimeError exception, setting its message to the given string. This exception is then raised up the call stack.
The third form uses the first argument to create an exception and then sets the associated message to the second argument.
The fourth form is similar to the third form but you can add any conditional statement like unless to raise an exception.
#!/usr/bin/ruby

begin  
   puts 'I am before the raise.'  
   raise 'An error has occurred.'  
   puts 'I am after the raise.'  
rescue  
   puts 'I am rescued.'  
end  
puts 'I am after the begin block.'  
This will produce the following result −
I am before the raise.  
I am rescued.  
I am after the begin block.  

One more example showing the usage of raise −
#!/usr/bin/ruby

begin  
   raise 'A test exception.'  
rescue Exception => e  
   puts e.message  
   puts e.backtrace.inspect  
end  
This will produce the following result −
A test exception.
["main.rb:4"]

Sometimes, you need to guarantee that some processing is done at the end of a block of code, regardless of whether an exception was raised. For example, you may have a file open on entry to the block and you need to make sure it gets closed as the block exits.
The ensure clause does just this. ensure goes after the last rescue clause and contains a chunk of code that will always be executed as the block terminates. It doesn't matter if the block exits normally, if it raises and rescues an exception, or if it is terminated by an uncaught exception, the ensure block will get run.
begin 
   #.. process 
   #..raise exception
rescue 
   #.. handle error 
ensure 
   #.. finally ensure execution
   #.. This will always execute.
end

begin
   raise 'A test exception.'
rescue Exception => e
   puts e.message
   puts e.backtrace.inspect
ensure
   puts "Ensuring execution"
end
This will produce the following result −
A test exception.
["main.rb:4"]
Ensuring execution

If the else clause is present, it goes after the rescue clauses and before any ensure.
The body of an else clause is executed only if no exceptions are raised by the main body of code.
begin 
   #.. process 
   #..raise exception
rescue 
   # .. handle error
else
   #.. executes if there is no exception
ensure 
   #.. finally ensure execution
   #.. This will always execute.
end

begin
   # raise 'A test exception.'
   puts "I'm not raising exception"
rescue Exception => e
   puts e.message
   puts e.backtrace.inspect
else
   puts "Congratulations-- no errors!"
ensure
   puts "Ensuring execution"
end
This will produce the following result −
I'm not raising exception
Congratulations-- no errors!
Ensuring execution

Raised error message can be captured using $! variable.
While the exception mechanism of raise and rescue is great for abandoning the execution when things go wrong, it's sometimes nice to be able to jump out of some deeply nested construct during normal processing. This is where catch and throw come in handy.
The catch defines a block that is labeled with the given name (which may be a Symbol or a String). The block is executed normally until a throw is encountered.
throw :lablename
#.. this will not be executed
catch :lablename do
#.. matching catch will be executed after a throw is encountered.
end

OR

throw :lablename condition
#.. this will not be executed
catch :lablename do
#.. matching catch will be executed after a throw is encountered.
end

The following example uses a throw to terminate interaction with the user if '!' is typed in response to any prompt.
def promptAndGet(prompt)
   print prompt
   res = readline.chomp
   throw :quitRequested if res == "!"
   return res
end

catch :quitRequested do
   name = promptAndGet("Name: ")
   age = promptAndGet("Age: ")
   sex = promptAndGet("Sex: ")
   # ..
   # process information
end
promptAndGet("Name:")
You should try the above program on your machine because it needs manual interaction. This will produce the following result −
Name: Ruby on Rails
Age: 3
Sex: !
Name:Just Ruby

Ruby's standard classes and modules raise exceptions. All the exception classes form a hierarchy, with the class Exception at the top. The next level contains seven different types −
Interrupt
NoMemoryError
SignalException
ScriptError
StandardError
SystemExit
There is one other exception at this level, Fatal, but the Ruby interpreter only uses this internally.
Both ScriptError and StandardError have a number of subclasses, but we do not need to go into the details here. The important thing is that if we create our own exception classes, they need to be subclasses of either class Exception or one of its descendants.
Let's look at an example −
class FileSaveError < StandardError
   attr_reader :reason
   def initialize(reason)
      @reason = reason
   end
end
Now, look at the following example, which will use this exception −
File.open(path, "w") do |file|
begin
   # Write out the data ...
rescue
   # Something went wrong!
   raise FileSaveError.new($!)
end
end
The important line here is raise FileSaveError.new($!). We call raise to signal that an exception has occurred, passing it a new instance of FileSaveError, with the reason being that specific exception caused the writing of the data to fail.

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