From: Paul Brannan <paul@...>
Date: 2002-03-01T00:34:49+09:00
Subject: Re: RCR?: parallel to until: as_soon_as

On Thu, Feb 28, 2002 at 06:37:58AM +0900, Tobias Reif wrote:
> I still think a very simple construct for firing a block as soon as / 
> each time a condition turns true would be useful. No code inside the 
> class is required; just do observe visible objects and their state.

Useful, but slow.

If you don't mind slow, then the following solution will work (I've
trimmed this down to make it succinct; the full version of this code has
a little more error checking):

# A hack to temporarily turn off $VERBOSE
def verbose_off
  verbose = $VERBOSE
  $VERBOSE = false
  yield
  $VERBOSE = verbose
end

# A hack to allow multiple trace funcs
verbose_off do
  $trace_funcs = []
  module Kernel
    set_trace_func_orig = method(:set_trace_func)
    @@fix_trace_func = proc {
      if $trace_funcs.size == 0 then
        set_trace_func_orig.call(nil)
      elsif $trace_funcs.size == 0 then
        set_trace_func_orig.call($trace_funcs[0])
      else
        set_trace_func_orig.call(proc { |*args|
          $trace_funcs.each { |trace_func| trace_func.call(*args) }
        })
      end
    }
    def set_trace_func(func)
      $trace_funcs.push(func)
      @@fix_trace_func.call()
    end
  end
end

# A hack to allow access to the binding of the caller
$call_stack = []
set_trace_func(proc {|*args|
  case args[0]
    when "call"
      $call_stack.unshift($current_call)
    when "return"
      $call_stack.shift()
  end
  $current_call = args
})
def caller_binding(level = 0)
  $call_stack[level.succ][4]
end

# A hack to allow a callback to be called the first time an expression
# evaluates to true
$as_soon_as_expressions = []
set_trace_func(proc {|*args|
  $as_soon_as_expressions.each_index do |idx|
    expr, expr_binding, cb = $as_soon_as_expressions[idx]
    if eval(expr, expr_binding) then
      $as_soon_as_expressions.delete_at(idx)
      cb.call()
    end
  end
})
def as_soon_as(expr, &block)
  $as_soon_as_expressions.push [expr, caller_binding(), block]
end

# Some test code
$x = 0
as_soon_as "$x == 42" do
  puts "woohoo!"
end

puts "1"
$x = 1
puts "2"
$x = 2
puts "3"
$x = 3
puts "42"
$x = 42
puts "done"

I hope you can see and appreciate just how sick this code really is.

Paul