I like this suggestion, I will give it a go! thanks, and also for the catMaybes function, I needed something like that some days ago. martin On 9/28/2010 5:20 PM, edgar klerks wrote:
I think it is simpler to use the Maybe type for this. If you only search for ternary relations, you can write a function, which only converts a list to a 3-tuple if it is of length three and otherwise it returns nothing. Then with catMaybes :: [Maybe a] -> [a], you can convert it back to a list with only 3-tuples.
import Data.Maybe
getLengthThree = catMaybes . fmap toTup where toTup (x:y:z:[]) = Just (x,y,z) toTup _ = Nothing
-- Martin Tomko Postdoctoral Research Assistant Geographic Information Systems Division Department of Geography University of Zurich - Irchel Winterthurerstr. 190 CH-8057 Zurich, Switzerland email: martin.tomko@geo.uzh.ch site: http://www.geo.uzh.ch/~mtomko mob: +41-788 629 558 tel: +41-44-6355256 fax: +41-44-6356848