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# 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.

## Syntax :

    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.

## Example:

    #!/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 following result. You can see that STDIN is substituted to file because open failed.

    #<IO:0xb7d16f84>==#<IO:0xb7d16f84>

# Using retry Statement:

You can capture an exception using rescue block and then use retry statement to execute begin block from the beginning.

## Syntax:

    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

## Example:

    #!/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.

# Using raise Statement:

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. Program

## Syntax:

    raise

    OR

    raise "Error Message"

    OR

    raise ExceptionType, "Error Message"

    OR

    raise ExceptionType, "Error Message" condition

The first form simply reraises 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 third form but you can add any conditional statement like unless to raise an exception.

## Example:

    #!/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 following result:

    I am before the raise.
    I am rescued.
    I am after the begin block.

One more example showing 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 following result:

    A test exception.
    ["test.rb:4"]

# Using ensure Statement:

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.

## Syntax:

    begin
       #.. process
       #..raise exception
    rescue
       #.. handle error
    ensure
       #.. finally ensure execution
       #.. This will always execute.
    end

## Example:

    begin
      raise 'A test exception.'
    rescue Exception => e
      puts e.message
      puts e.backtrace.inspect
    ensure
      puts "Ensuring execution"
    end

This will produce following result:

    A test exception.
    ["test.rb:4"]
    Ensuring execution

# Using else Statement:

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.

## Syntax:

    begin
       #.. process
       #..raise exception
    rescue
       # .. handle error
    else
       #.. executes if there is no exception
    ensure
       #.. finally ensure execution
       #.. This will always execute.
    end

## Example:

    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 following result:

    I'm not raising exception
    Congratulations-- no errors!
    Ensuring execution

Raised error message can be captured using $! variable.

# Catch and Throw:

While the exception mechanism of raise and rescue is great for abandoning 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.

## Syntax:

    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

## Example:

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:")

This will produce following result:

    Name: Ruby on Rails
    Age: 3
    Sex: !
    Name:Just Ruby

# Class Exception:

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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