Cook sugar, milk and pineapple to a soft ball stage. That is when it forms a soft ball when dropped in cold water. This must be cooked a fast rolling boil. If cooked slowly it becomes dark and sugary. Remove from fire and add marshmallow fluff, butter, vanilla and nuts. Beat until thick and turn into buttered pan and when set, cut into squares. This should be creamy in color and will keep indefinitely as the fruit keeps it moist.