Online Computer Terms Dictionary - H

horizontal loop combination ==>

tupling

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

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)