Syntax of Ruby-Exceptions

Question

I'm trying to understand the syntax of Ruby-exceptions.

I found this example:

begin
    puts "Running with b=#{ b }"
    exception_if(b)
    puts "After possible exception"
  rescue ArgumentError => e
    puts "An error occured: #{ e }!"
  ensure
    puts "Always excuted, no matter what."
  end

Source: Wiki-Books

"ArgumentError" is the expection-type, which the rescue-branch shall catch? "e" is the reference-variable?

I'm I right there?

Then I found this snippet and that confused me then completely:

begin
  @product = Product.find(params[:id])
rescue => e
  redirect_to root_path
end

Where's the first part before the arrow (=>)?

Can someone explain me, how the two snippets are meant?

Answer 1

Edit: The second snippet is rescuing errors that are instances of StandardError, in a console:

begin
  raise NoMethodError
rescue => e
  puts e.inspect
end
#<NoMethodError: NoMethodError>

The exception object is assigned to the e variable. This can also read

rescue StandardError => e

The first snippet is initially rescue-ing one specific error, if another type of error is raised the ensure block will always be executed and then the method is exited.

As a general rule of thumb, you should only capture exceptions your program can recover from. If the program experiences an unknown exception crash it and fix the issue.