What would be use-cases for (2)? As Joachim pointed out, for any reasonable data type inserting an equal element should have no difference.
alterF :: (Functor f, Ord a) => a -> (Bool -> f Bool) -> Set a -> f (Set a)
(with any reasonable name) which'd allow to examine a set and possibly modify it in one traversal. It should be smart enough not to modify the set at all if the output of a given function is the same as its input. And it'd also fit with lens. In particular, query+delete could be then expressed as
memberDelete :: (Ord a) => a -> Set a -> (Bool, Set a)
memberDelete k = alterF k (flip (,) False)
Petr
čt 5. 3. 2015 v 23:59 odesílatel David Feuer <
david.feuer@gmail.com> napsal:
There are a few rather conspicuously missing features:
1. A way to take the intersection of a list of sets. This shouldn't
really be a big deal, and it already exists for unions, but the
intersection version should probably sort the sets by size before
folding, or otherwise try to do something smart.
2. A way to insert an element if an == one is not already present
(insert replaces an existing one with a new one). Currently, as far as
I can tell, this can only be done using either
if e `member` s then s else insert e s
which potentially descends the tree twice for no reason
or
s `union` singleton e
which is documented as being O(|s|+1), although I wouldn't be shocked
if the documentation were too pessimistic in this case.
3. A way to delete an element and simultaneously find out whether it
was in the set.
David Feuer
_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries