Array.extend versus instance.extend

ruby-talk
1

Incidentally, you can make your class more general-purpose by using ===,
noting that klass1 === obj is true if obj.is_a?(klass1)

i.e.
case obj
when klass1
# this is executed if klass1 === obj
end

I’m not sure exactly what you’d call it (I seem to remember this naming
problem occured for Test::Unit as well), but you’d have something like:

module ArrayMisc
def shift_until_match(caseitem)
res = []
loop do
break if caseitem === self.first
res << self.shift
end
res
end
end

Then you can do:

class Array; include ArrayMisc; end

a = [0,1,“two”,“three”,“four”,“five”]
b = a.shift_until_match(String)
p a,b
#>> [“two”, “three”, “four”, “five”] [0, 1]

b = a.shift_until_match(/^f/)
p a,b
#>> [“four”, “five”] [“two”, “three”]

Regards,

Brian.

2

Oops, infinite loop if no match! Should be:

		break if length == 0 or caseitem === first

Cheers,

Brian.

···

On Thu, May 29, 2003 at 05:35:05PM +0900, Brian Candler wrote:

                    break if caseitem === self.first
3

Thanks… Brian you have lead me to the solution.

My problem was that ‘Array.extend ArrayMisc’ was located in
a method-scope. Moving the following piece of code out into the
global scope, then everything works.

#Array.extend ArrayMisc
class Array; include ArrayMisc; end

AFAIK. I should be able to do an ‘Array.extend’ as above.
But it does not work. Why ?
What is the difference between these 2 lines ?

···

–
Simon Strandgaard

4

a = Array.new – a is an instance of Array
a.extend ArrayMisc – adds the methods of ArrayMisc into a’s singleton
class (i.e. they become instance methods of ‘a’)

class Array
include ArrayMisc – adds the methods of ArrayMisc as instance methods
of the class Array
end

Now, class Array does have a singleton class, but it’s not what you want
to add your method into. The singleton class of a Class is where the ‘class
methods’ go:

module Foo
def foo
puts “hello”
end
end
Array.extend Foo # puts methods of Foo into singleton class of Array

Array.foo
#>> “hello”

In other words, you have created a method which belongs to class Array, not
which belongs to instances of class Array.

Hope this makes sense… there are several pages about this on the wiki,
including http://www.rubygarden.org/ruby?SingletonTutorial

Regards,

Brian.

···

On Thu, May 29, 2003 at 07:20:00PM +0900, Simon Strandgaard wrote:

Thanks… Brian you have lead me to the solution.

My problem was that ‘Array.extend ArrayMisc’ was located in
a method-scope. Moving the following piece of code out into the
global scope, then everything works.

#Array.extend ArrayMisc
class Array; include ArrayMisc; end

AFAIK. I should be able to do an ‘Array.extend’ as above.
But it does not work. Why ?
What is the difference between these 2 lines ?

5

OK… I see. I wasn’t aware of this. Now I ‘think’ I understand :slight_smile:

BTW: the case equality trick you showed me is really nice.

···

On Thu, 29 May 2003 20:34:54 +0900, Brian Candler wrote:

In other words, you have created a method which belongs to class Array, not
which belongs to instances of class Array.

–
Simon Strandgaard