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

[Jeffrey Brown]

Fantastic! Thanks, Thomas!

On Sat, Mar 7, 2015 at 3:49 PM, Thomas Bach <thbach at students.uni-mainz.de>
wrote:

> Jeffrey Brown <jeffbrown.the at gmail.com> 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-needed
>
> Regards
>
>         Thomas Bach.
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20150307/0199f749/attachment-0001.html>