Re: [Haskell-beginners] Laziness to efficiently get one element from a set

Fantastic! Thanks, Thomas! On Sat, Mar 7, 2015 at 3:49 PM, Thomas Bach wrote:

Jeffrey Brown writes:

...

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. _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners