callcc.R

prod <- function (coll) {
	# callCC lets you jump out of ANY function early,
	# by specifying a breakpoint function (often called 'k').
	# if I used return instead of breakpoint here the
	# program would take a long time to terminate.

	# 1; breakpoint jumps up to this level if it is called, evalulating to zero
	callCC(
		function (breakpoint) {
			Reduce (
				function (acc, new) {
					# 2; using return would only break this step of the Reduce 
					# function; it wouldn't exit reduce altogether.

					if (new == 0) {
						breakpoint (0)
					} else {
						Sys.sleep(0.1)
						acc * new
					}
				},
				coll
			)
	})
}

prod(c(1, 2, 0, 1:100000))