According to the Pickaxe:
···
new Proc.new [{| | block } ] -> aProc
Creates a new Proc object, bound to the current context. It may be called
without a block only within a method with an attached block, in which case
that block is converted to the Proc object.
But if i try to convert the block into a proc and rebind it “to the current
context” it fails. In the following snippet, i wish that the proc that is
defined in main context, be rebound into a Room’s context.
def method_missing(action)
puts "#{action}…in the city"
end
class Room
def method_missing(action)
puts "#{action}…in the the privacy of your own room"
end
def live(&block)
instance_eval { Proc.new.call }
end
def initialize
put_feet_on_table
snore
end
end
walk
go_to_work
room = Room.new
room.live do
walk_around_naked!
end
output:
walk…in the city
go_to_work…in the city
put_feet_on_table…in the the privacy of your own room
snore…in the the privacy of your own room
walk_around_naked!..in the city
oh dear, how embarassing! 
Please explain, kind sirs. I think i’m understanding the word "context"
wrong. Also, could a proc rebinding possibly be done?
regards
Peter
repeatr wrote:
def live(&block)
instance_eval { Proc.new.call }
Why not just
instance_eval( &block )
which appears to work fine for me. The variable block points to a Proc
of the external block already due to the &-prefix.
Or are you getting at something more complex?
As for why it your original code doesn’t work:
instance_eval { Proc.new.call }
instance_eval only changes the context of the given block (which
contains Proc.new.call), but Proc.new.call fetches another block/Proc
(&block) from the outside of the method, and that Proc does NOT get
its context changed by instance_eval. Why should it? It would make
recursive use of Procs a nightmare.
HTH
···
–
([ Kent Dahl ]/)_ ~ [ http://www.stud.ntnu.no/~kentda/ ]/~
))_student/(( _d L b_/ NTNU - graduate engineering - 5. year )
( __õ|õ// ) )Industrial economics and technological management(
_/ö____/ (_engineering.discipline=Computer::Technology)
def method_missing(action)
puts “#{action}…in the city”
end
class Room
def method_missing(action)
puts “#{action}…in the the privacy of your own room”
end
def live(&block)
instance_eval { Proc.new.call }
Proc.new reifies the passed block, which was already bound to the
external context. Use instance_eval(&block). Note however that only the
value of self is substituted, ie.:
batsman@tux-chan:/tmp$ expand -t2 ag.rb
class Foo
def bar(&block)
a = “inner a”
instance_eval(&block)
end
end
a = “external a”
o = Foo.new
o.bar do
puts “self.id: #{self.id}”
puts “a is #{a.inspect}”
end
puts “o.id: #{o.id}”
batsman@tux-chan:/tmp$ ruby ag.rb
self.id: 537725500
a is “external a”
o.id: 537725500
end
def initialize
put_feet_on_table
snore
end
end
walk
go_to_work
room = Room.new
room.live do
walk_around_naked!
end
output:
walk…in the city
go_to_work…in the city
put_feet_on_table…in the the privacy of your own room
snore…in the the privacy of your own room
walk_around_naked!..in the city
LOL
We have to put this in some tutorial or documentation!
···
On Sat, May 17, 2003 at 02:43:57AM +0900, repeatr wrote:
–
_ _
__ __ | | ___ _ __ ___ __ _ _ __
'_ \ / | __/ __| '_ _ \ / ` | ’ \
) | (| | |__ \ | | | | | (| | | | |
.__/ _,|_|/| || ||_,|| |_|
Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com
- DDD no longer requires the librx library. Consequently, librx
errors can no more cause DDD to crash.
– DDD
From: Kent Dahl [mailto:kentda@stud.ntnu.no]
Sent: 16 May 2003 08:21
To: ruby-talk ML
Subject: Re: procs and context
repeatr wrote:
def live(&block)
instance_eval { Proc.new.call }
Why not just
instance_eval( &block )
which appears to work fine for me. The variable block points to a Proc
of the external block already due to the &-prefix.
Ah that’s it! That one flew straight by underneath the radar.
Even for my supposedly accustomed ruby mind, things fudge up. 
Or are you getting at something more complex?
As for why it your original code doesn’t work:
instance_eval { Proc.new.call }
instance_eval only changes the context of the given block (which
contains Proc.new.call), but Proc.new.call fetches another block/Proc
(&block) from the outside of the method, and that Proc does NOT get
its context changed by instance_eval. Why should it? It would make
recursive use of Procs a nightmare.
Rightly so.
Thank you kindly for the explanation and fix.
with regards
Peter
···
-----Original Message-----
From: Mauricio Fernandez [mailto:batsman.geo@yahoo.com]
Sent: 16 May 2003 09:01
To: ruby-talk ML
Subject: Re: procs and context
def method_missing(action)
puts “#{action}…in the city”
end
class Room
def method_missing(action)
puts “#{action}…in the the privacy of your own room”
end
def live(&block)
instance_eval { Proc.new.call }
Proc.new reifies the passed block, which was already bound to the
external context. Use instance_eval(&block). Note however that only the
value of self is substituted, ie.:
batsman@tux-chan:/tmp$ expand -t2 ag.rb
class Foo
def bar(&block)
a = “inner a”
instance_eval(&block)
end
end
a = “external a”
o = Foo.new
o.bar do
puts “self.id: #{self.id}”
puts “a is #{a.inspect}”
end
puts “o.id: #{o.id}”
batsman@tux-chan:/tmp$ ruby ag.rb
self.id: 537725500
a is “external a”
o.id: 537725500
Zoi that is enlightening, with this you’ve cleansed my mind of all the moth
flutterings and dust wisps.
If I may talk myself into understanding, thus with:
instance_eval(&Proc.new)
the proc is converted into a block with its sense of “self” converted.
while with:
instance_eval { Proc.new.call }
a block is created with a self pointing to this object (in this case
redundantly so), while the original proc yonder the mountains is executed
untouched.
end
def initialize
put_feet_on_table
snore
end
end
walk
go_to_work
room = Room.new
room.live do
walk_around_naked!
end
output:
walk…in the city
go_to_work…in the city
put_feet_on_table…in the the privacy of your own room
snore…in the the privacy of your own room
walk_around_naked!..in the city
LOL
We have to put this in some tutorial or documentation!
I hope it doesn’t cause any bad dreams though 
a tip of the hat
Peter
···
-----Original Message-----
On Sat, May 17, 2003 at 02:43:57AM +0900, repeatr wrote: