From: Kent Dahl <kentda@...>
Date: 2002-07-22T19:24:33+09:00
Subject: Re: adding an argument to a proc

Tom Sawyer wrote:
> 
> is there a way to add an argument to a proc after its creation?

Don't think so. Sounds like it would wreak a havoc of ambiguity on
variable scope
 
> ex-
> 
> p = Proc.new { x + 5 }
> px = Proc.new { |x| p.call(5) }
> 
> this won't work

You could perhaps be nasty and run eval on the binding of the block and
setting the scope local variable explicitly, before calling the proc.

<CODE>
x = nil # Has to exist in scope of block definition
p1 = Proc.new { x + 5 }

# Define a wrapper Proc that sets the x to a number
p2 = Proc.new {|y_uNiQuE_VarName| 
  eval "old_x_uNiQuE_VarName = x", p1
  eval "x=#{y_uNiQuE_VarName}", p1  # naive version
  result = p1.call
  eval "x=old_x_uNiQuE_VarName", p1
  result
}

# Less naive version that should work with any object
p3 = Proc.new {|y_uNiQuE_VarName| 
  eval "old_x_uNiQuE_VarName = x", p1
  eval "x=ObjectSpace._id2ref(#{y_uNiQuE_VarName.id})", p1  # less naive
  result = p1.call
  eval "x=old_x_uNiQuE_VarName", p1
  result
}

puts p2.call(5)  #=> 10
puts p3.call(15) #)=> 20

puts "Dang!" if x != nil
</CODE>

-- 
(\[ Kent Dahl ]/)_    _~_    __[ http://www.stud.ntnu.no/~kentda/ ]___/~
 ))\_student_/((  \__d L b__/  NTNU - graduate engineering - 4. year  )
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