From: "Mauricio Fernández" <batsman.geo@...>
Date: 2002-08-24T22:29:40+09:00
Subject: Re: eval in class method

On Sat, Aug 24, 2002 at 09:22:41PM +0900, michael libby wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Hi. I'm not seeing what I thought would happen with some eval code, here is 
> a sample via irb:
> 
>   chandler@greyhound:~$ irb
>   irb(main):001:0> class FooBar
>   irb(main):002:1> def baz
>   irb(main):003:2> puts "foobar:baz"
>   irb(main):004:2> end
>   irb(main):005:1> end
>   nil
>   irb(main):006:0> f = FooBar.new 
>   #<FooBar:0x4027e828>
>   irb(main):007:0> f.baz
>   foobar:baz
>   nil
> 
> OK. So far so good, simply made a test class and ran a method from it.
> 
>   irb(main):009:0> eval("class FooBar\ndef baz\nputs 'foobar:new
>     baz'\nend\nend")
>   nil
>   irb(main):010:0> f.baz
>   foobar:new baz
>   nil
> 
> OK. So far even better. Use eval to redefine method.
> 
>   irb(main):011:0> class FooBar
>   irb(main):012:1> def rebaz
>   irb(main):013:2> eval("class FooBar\ndef baz\nputs
>     'foobar:rebaz'\nend\nend")
>   irb(main):014:2> end
>   irb(main):015:1> end
>   nil
>   irb(main):016:0> f.rebaz
>   nil
> 
> So far nothing seems amiss. Instead of doing eval from main script, wrote 
> rebaz method to do eval as instance method.
> 
>   irb(main):017:0> f.baz
>   foobar:new baz
>   nil
> 
> Sigh. After calling #rebaz which I thought would have changed #baz, #baz 
> still acts the same-- even though, except for scope/context, the eval 
> statement was the same exact statement as I used before, where #baz did 
> change.
> 
> How can I write an instance method that will get the eval done the way I 
> thought it was going to work? I don't want to eval dependent on FooBar 
> class, the eval might want to change a method in Gizmo or Widget class 
> instead. Eval just needs to happen from FooBar since that is the class 
> that will have the method to handle evaluation of new code. Any 
> suggestions?
> 


batsman@kodos:~/src/rubylang$ cat class_eval.rb

class FooBar
 def baz
	puts "FooBar::baz"
 end

 def on_me
	FooBar.class_eval <<-EOF
		def baz
			puts "Redefined FooBar::baz"
		end
	EOF
 end

 def on_another( aclass )
	aclass.class_eval <<-EOF
		def baz
			puts "Redefined #{aclass.to_s}::baz"
		end
	EOF
 end
end

class Bar
 def baz
	puts "Bar::baz"
 end
end

f = FooBar.new
b = Bar.new
f.baz
b.baz
f.on_me
f.baz
f.on_another Bar
b.baz
batsman@kodos:~/src/rubylang$ ruby class_eval.rb
FooBar::baz
Bar::baz
Redefined FooBar::baz
Redefined Bar::baz
batsman@kodos:~/src/rubylang$

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