From: avi@...4.com (Avi Bryant)
Date: 2004-02-01T12:35:06+09:00
Subject: Re: Save the flip/flop op!

ptkwt@aracnet.com (Phil Tomson) wrote:

> Seems to work and be nestable, as in the usage here:

> ff = FlipFlop.new
> ff2= FlipFlop.new
> f = File.open('filename')
> f.each_line {|l|
>  ff.from(l=~/BEGIN/, l=~/END/) do
>    #inside the BEGIN .. END block
>     ff2.from(l=~/def/,l=~/end/) do
>       #inside def .. end block
>       puts l
>     end
>  end
> }

> But it's more cumbersome having to predeclare all the FlipFlops.

Now that raises an interesting point.  Say you had this input:

---
BEGIN
def baz 
#oops, syntax error
END

BEGIN
#we shouldn't see this line
END
---

Your code above would break with it (at least, wouldn't produce what I
think most people would expect it to). So would the builtin flipflop
syntax.  However, it's easy to solve with a slight modification to the
FlipFlop class:

class FlipFlop
  def initialize
     @in_range = false
  end

  def from(start_cond, end_cond)
      if start_cond
         @in_range = true
         @nested = FlipFlop.new
      end
      yield @nested if @in_range
      @in_range = false if end_cond
   end
end


ff = FlipFlop.new
f = File.open('test')
f.each_line {|l|
  ff.from(l=~/BEGIN/, l=~/END/) do
     |nested|
     nested.from(l=~/def/,l=~/end/) do
       #inside def .. end block
       puts l
     end
  end
}