Using inline "rescue" with error class


#1

Hi, instead of:


begin
IPAddr.new(value)
rescue ArgumentError
puts “Invalid IP”
end

I would like to just write:
IPAddr.new(value) rescue ArgumentError puts “Invalid IP”

Unfortunatelly it doesn’t work when the rescue action occurs:

Invalid IP
NoMethodError: undefined method `ArgumentError’ for main:Object

Do I miss some way to set the error class when using inline rescue?

Thanks a lot.


#2

On Tue, Mar 3, 2009 at 5:58 AM, Iñaki Baz C. removed_email_address@domain.invalid wrote:

I would like to just write:
IPAddr.new(value) rescue ArgumentError puts “Invalid IP”

Unfortunatelly it doesn’t work when the rescue action occurs:

Invalid IP
NoMethodError: undefined method `ArgumentError’ for main:Object

The rescue statement modifier doesn’t take an exception parameter.
You’d have to make do with:
IPAddr.new(value) rescue puts “Invalid IP”

This of course makes the construct much less appealing.
There has been a very recent thread related to this.

Solidarity,
lasitha


#3

lasitha wrote:

On Tue, Mar 3, 2009 at 5:58 AM, I�aki Baz C. removed_email_address@domain.invalid wrote:

I would like to just write:
�IPAddr.new(value) rescue ArgumentError puts “Invalid IP”

Unfortunatelly it doesn’t work when the rescue action occurs:

Invalid IP
NoMethodError: undefined method `ArgumentError’ for main:Object

The rescue statement modifier doesn’t take an exception parameter.
You’d have to make do with:
IPAddr.new(value) rescue puts “Invalid IP”

… which rescues StandardError and all subclasses.

However you probably wouldn’t want to do this:

addr = IPAddr.new(value) rescue puts “Invalid IP”
do_something_with(addr)

because the second line would get addr equal to nil, and further errors
would occur. One solution is just to let the ArgumentError propagate
upwards, because the exception message already says “invalid address”:

irb(main):002:0> IPAddr.new(“x”)
ArgumentError: invalid address

Having said that, sometimes I do like to include the offending value in
error messages, so I can end up writing stuff like

begin
addr = IPAddr.new(value)
rescue ArgumentError
raise ArgumentError, “Invalid IP address #{value.inspect}”
end

The risk here is that if the user provides an arbitrarily long string,
you’ll get an arbitrarily long exception message too.

Another option is to stick rescue clause(s) at the end of a method body,
because you don’t need a ‘begin’.

def foo(value)
addr = IPAddr.new(value)
do_something_with(addr)
rescue ArgumentError
puts “Invalid IP address”
raise # or not, it’s up to you
end

On the plus side, this prevents the nil addr being used. On the minus
side, any ArgumentError in the body of foo or any of the methods it
calls will be reported as “Invalid IP address”

Regards,

Brian.


#4

El Martes, 3 de Marzo de 2009, Brian C. escribió:

begin

calls will be reported as “Invalid IP address”
Thanks for so good explanation.