Idiomatic ruby


#1

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?


#2

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.


#3

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


#4

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


#5

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


#6

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