Jeffrey Brown
head $ Data.Set.toList S. If I do that, am I correct that Haskell will not try to convert all of S to a list; instead it will only convert one element, and then return it, and leave the rest of the list unevaluated?
This is how toList from Data.Set.Base is defined in containers-0.5.0: {-------------------------------------------------------------------- Lists --------------------------------------------------------------------} -- | /O(n)/. Convert the set to a list of elements. Subject to list fusion. toList :: Set a -> [a] toList = toAscList -- | /O(n)/. Convert the set to an ascending list of elements. Subject to list fusion. toAscList :: Set a -> [a] toAscList = foldr (:) [] The buzzword you are looking for is list fusion: http://stackoverflow.com/questions/10945429/haskell-list-fusion-where-is-it-... Regards Thomas Bach.