From: Brian Marick <marick@...>
Date: 2001-08-23T17:18:53+09:00
Subject: [ruby-talk:20164] Re: breaking out of nested loops

While probably the right way to do this is to use a block...

def yield_matches(first_range, next_range)
   for i in first_range
     for j in next_range
       yield i if i == j
     end
   end
end

def find_special_number(first_range, next_range)
   puts "== Look for a special value in #{first_range} and #{next_range}"

   yield_matches(first_range, next_range) { | val |
     puts "Value is #{val.inspect}."
     if val + val == val * val
       puts "Isn't that special?"
       return
     end
   }
   puts "Nothing special here."
end

if __FILE__ == $0
   find_special_number(0..3, 1..5)
   find_special_number(3..8, 4..7)
end

% ruby continuation-passing.rb
== Look for a special value in 0..3 and 1..5
Value is 1.
Value is 2.
Isn't that special?
== Look for a special value in 3..8 and 4..7
Value is 4.
Value is 5.
Value is 6.
Value is 7.
Nothing special here.

===================
As I say, while probably the right way to do this is to use a block, the 
continuation version can be made to work like this:

def find_special_number(first_range, next_range)
   puts "== Look for a special value in #{first_range} and #{next_range}"

   val, restart = find_matches(first_range, next_range)
   return if val == nil
   puts "Value is #{val.inspect}."
   if val + val == val * val
     puts "Isn't that special?"
   else
     restart.call
   end
end

def find_matches(first_range, next_range)
   callcc do | out |
     for i in first_range
       for j in next_range
         return_resumably(i, out) if i == j
       end
     end
     nil
   end
end

def return_resumably(primary_retval, return_continuation)
   callcc do | resume_continuation |
     return_continuation.call(primary_retval, resume_continuation)
   end
end

if __FILE__ == $0
   find_special_number(0..3, 1..5)
   find_special_number(3..8, 4..7)
end

% ruby continuation-loop.rb
== Look for a special value in 0..3 and 1..5
Value is 1.
Value is 2.
Isn't that special?
== Look for a special value in 3..8 and 4..7
Value is 4.
Value is 5.
Value is 6.
Value is 7.

========
But that forces the programmer calling find_matches know obscure things. 
I'd still like to find a way to suspend an arbitrary computation, get a 
"resumer" that can be passed around and called anywhere to return the next 
value. Maybe time to try to find those Scheme documents from the mid-80's.

--
Brian Marick, marick@testing.com
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