Forum: Ruby Idiomatic ruby

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unknown (Guest)
on 2006-02-13 02:03
(Received via mailing list)
Very often I have a question method, which, in some cases, the caller
would want to know why as well.

Examples:

def valid?
end

def abort?
end

Ruby does not allow subclassing true and false, so, if these methods
return one of those, they can't return any additional info.  But
sometimes the caller needs additional info, as in:

if !valid? logger.warn "Not valid: #{why not?}"

What is the best way to handle this?  I could have those methods set
@instance_variables, but this seems a little hackish, and could
introduce race conditions.

Is there anyway to return false, "reason", or something of that sort?
What is the preferred, idiomatic way of doing this?
unknown (Guest)
on 2006-02-13 02:18
(Received via mailing list)
I can think of two directions you could go real quickly.

You could go the Rails validations way of your question method having a
side-effect that populates some other member. eg:

flash[:warning] = user.errors.get_full_messages unless user.valid?

Of you could go with the Perlish way the Regex library works:

"Cows are Cool" =~ /(\w+)/
puts $1

I prefer the side-effect method.
unknown (Guest)
on 2006-02-13 03:49
(Received via mailing list)
On Mon, 13 Feb 2006 removed_email_address@domain.invalid wrote:

>
> Is there anyway to return false, "reason", or something of that sort?
> What is the preferred, idiomatic way of doing this?

use a block to return info:

     harp:~ > cat a.rb
     class C
       def initialize(x) @x = x end
       def valid?()
         if @x == 42
           true
         else
           yield "x is not 42" rescue nil
           false
         end
       end
     end

     c = C::new 43
     unless c.valid?{|reason| warn "not valid : #{ reason }" }
       # do something
     end

     c = C::new 42
     c.valid?{|reason| warn "not valid : #{ reason }" }


     harp:~ > ruby a.rb
     not valid : x is not 42



or invert your logic so info can be added:

   harp:~ > cat a.rb
   class C
     def initialize(x) @x = x end
     def invalid?() @x == 42 ? false : "x is not 42" end
   end

   c = C::new 43
   if((reason = c.invalid?))
     puts reason
   end

   c = C::new 42
   if((reason = c.invalid?))
     puts reason
   end


   harp:~ > ruby a.rb
   x is not 42


hth.

-a
William J. (Guest)
on 2006-02-13 08:43
(Received via mailing list)
removed_email_address@domain.invalid wrote:
>
> Is there anyway to return false, "reason", or something of that sort?
> What is the preferred, idiomatic way of doing this?

def valid?( n )
  return  n%2==0, "It's odd."
end

f, why = valid? 9
Adam P. Jenkins (Guest)
on 2006-02-13 23:01
(Received via mailing list)
removed_email_address@domain.invalid wrote:

>       end
>
>     harp:~ > ruby a.rb
>     not valid : x is not 42

I like this solution the best.  Unlike the solutions involving
exceptions or multiple return values, it still allows valid? to be used
as an ordinary boolean function when you don't care about the reason why
something's not valid, yet doesn't have the race conditions involved in
the side-effect solutions.

That said, I think part of the reason there's no agreed upon idiom for
this is because it would have been more idiomatic in Ruby to not even
have the valid? method.  Instead an exception would have been thrown as
soon as the object became invalid, and the exception would have
contained the reason.  Having your class have a valid? method seems like
how you'd design things in a language which doesn't have exceptions.

Adam
unknown (Guest)
on 2006-02-13 23:15
(Received via mailing list)
On Tue, 14 Feb 2006, Adam P. Jenkins wrote:

>>           false
>>     c.valid?{|reason| warn "not valid : #{ reason }" }
>
> That said, I think part of the reason there's no agreed upon idiom for this
> is because it would have been more idiomatic in Ruby to not even have the
> valid? method.  Instead an exception would have been thrown as soon as the
> object became invalid, and the exception would have contained the reason.
> Having your class have a valid? method seems like how you'd design things in
> a language which doesn't have exceptions.

still - some people prefer not to do control flow with exceptions.  as
with
most things, both things can be good.  i tend to use things like

   def validate!
     raise unless valid?
   end

and use either/both where it makes sense.

regards.

-a
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