From: Brian Candler <b.candler@...>
Date: 2008-11-26T23:53:15+09:00
Subject: Re: ruby wish list item: more standard rescues

Roger Pack wrote:
> which always surprises me.  Any preferences over always rescuing
> Exception?

It looks pretty consistent to me. The default is to catch StandardError, 
and most "normal" errors are subclasses of this. Only gross system-level 
errors are outside of this, and you probably didn't want to catch them 
anyway.

There was talk of ruby-1.9 moving NoMethodError outside of StandardError 
- or at least, (x rescue y) not catching it - but a test suggests that 
hasn't been done.

Anyway, here's a program you can use to demonstrate how consistent it 
is. If you can show something inconsistent, please update the code to 
show it.

require 'timeout'

RAISERS = [
  ["timeout error", lambda { raise Timeout::Error }],
  ["bad method name", lambda { zxkjcnvkj }],
  ["divide by zero", lambda { 1/0 }],
  ["Exception", lambda { raise Exception }],
]

CATCHERS = [
  ["rescue blank", lambda { |r|
    begin
      r.call
    rescue
    end
  }],
  ["rescue => e", lambda { |r|
    begin
      r.call
    rescue => e
    end
  }],
  ["rescue StandardError", lambda { |r|
    begin
      r.call
    rescue StandardError
    end
  }],
  ["expr rescue expr", lambda { |r|
    r.call rescue nil
  }],
  ["rescue Exception", lambda { |r|
    begin
      r.call
    rescue Exception
    end
  }],
]

CATCHERS.each do |c|
  puts "*** For #{c.first}"
  RAISERS.each do |r|
    begin
      c[1].call(r[1])
    rescue Exception
      puts "   Didn't catch #{r[0]}"
    else
      puts "   Caught #{r[0]}"
    end
  end
end

$ ruby raiser.rb
*** For rescue blank
   Caught timeout error
   Caught bad method name
   Caught divide by zero
   Didn't catch Exception
*** For rescue => e
   Caught timeout error
   Caught bad method name
   Caught divide by zero
   Didn't catch Exception
*** For rescue StandardError
   Caught timeout error
   Caught bad method name
   Caught divide by zero
   Didn't catch Exception
*** For expr rescue expr
   Caught timeout error
   Caught bad method name
   Caught divide by zero
   Didn't catch Exception
*** For rescue Exception
   Caught timeout error
   Caught bad method name
   Caught divide by zero
   Caught Exception
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