tupling
horizontal loop combination

   A program transformation where several results are returned
   from a single traversal of a data structure.  E.g.

   	mean l = sum l / length l

   		==>

   	mean l = s/n
   		 where
   		 (s,n) = sumLen l

   	sumLen []     = (0,0)
   	sumLen (x:xs) = (s+x, n+1)
   			where
   			(s,n) = sumLen xs

   In procedural languages this technique is known as
   horizontal loop combination because it uses one loop to
   calculate several results.

   Another form of tupling transformation is used to avoid
   repeated evaluation where a function generates several
   identical calls to itself.  By analysing the pattern of
   recursion (see descent function) it is possible to arrange
   for these identical calls to share results.  E.g.

   	fib 0 = 1
   	fib 1 = 1
   	fib n = fib (n-1) + fib (n-2)

   		==>

   	fib n = v where (_,v) = fibt n
   	fibt 0 = (1,1)
   	fibt n = (u+v,u) where (u,v) = fibt (n-1)

   (1995-01-12)