Date class method `once` wraps original call in Array. Why?

[Matt H]

I'm reading the Pickaxe and on page 391, there is a code listing for
the class method `once` from the Date module in Ruby:

def once(*ids) #:nodoc:
for id in ids
module_eval <<-"end:"
alias_method :__#{id.to_i}__, :#{id.to_s}
private :__#{id.to_i}__
def #{id.to_s}(*args, &block)
puts "Inside once: \#{args.inspect}"
(@__#{id.to_i}__ ||= [__#{id.to_i}__(*args, &block)])[0]
end
end:
end
end

I was wondering why the original method called is wrapped in an Array.
So I started digging the the source history hoping to find an
explanation. I found the commit that changed it to use the Array but
no explanation for the change. The commit where it was modified is
here:

<http://svn.ruby-lang.org/cgi-bin/viewvc.cgi/trunk/lib/date2.rb?r1=710&r2=709&pathrev=710>

I created a simple class and use the code with and without the Array
and it seems to work the same for my simple test. Just curious if I'm
not seeing something obvious.

 

[Aryk Grosz]

They wrap it in an array so you that they can cache "nil" and "false"
return values:

result ||= expensive_computation


will still run the expensive computation if it returned nil or false,
however, if you did:

(result ||= [expensive_computation])[0],

you could cache nil as a return value and it wouldn't actually run the
expensive computation.

I'm reading the Pickaxe and on page 391, there is a code listing for
the class method `once` from the Date module in Ruby:

def once(*ids) #:nodoc:
for id in ids
module_eval <<-"end:"
alias_method :__#{id.to_i}__, :#{id.to_s}
private :__#{id.to_i}__
def #{id.to_s}(*args, &block)
puts "Inside once: \#{args.inspect}"
(@__#{id.to_i}__ ||= [__#{id.to_i}__(*args, &block)])[0]
end
end:
end
end

I was wondering why the original method called is wrapped in an Array.
So I started digging the the source history hoping to find an
explanation. I found the commit that changed it to use the Array but
no explanation for the change. The commit where it was modified is
here:

<http://svn.ruby-lang.org/cgi-bin/viewvc.cgi/trunk/lib/date2.rb?r1=710&r2=709&pathrev=710>

I created a simple class and use the code with and without the Array
and it seems to work the same for my simple test. Just curious if I'm
not seeing something obvious.