Expand the Data.Set API

[David Feuer]

Regarding (2), I was thinking about situations where sharing is an issue.
Even if == is the right equality, you may want to settle on a single copy.
That would also suggest a version of member that returns a Maybe instead of
a Bool.
On Mar 6, 2015 3:42 AM, "Joachim Breitner" <mail at joachim-breitner.de> wrote:

> Hi,
>
>
> Am Donnerstag, den 05.03.2015, 17:59 -0500 schrieb David Feuer:
> > 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.
>
> +1
>
> > 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.
>
> I’m doubtful about that one.
>
> This is only needed if == is not „the right equality“, in which case a
> Set might be the wrong data type. Do we want to advocate that style?
>
> Also, it will make everyone wonder „do I need insert or theOtherInsert“,
> making the API a bit less concise and quick to grasp.
>
> Since "s `union` singleton e" does the right thing with almost the right
> performance, maybe that’s sufficient?
>
> > 3. A way to delete an element and simultaneously find out whether it
> > was in the set.
>
> +1, possibly with the type
>         a -> Set a -> Maybe (a, Set a)
> to get the remaining set as well.
>
> Greetings,
> Joachim
>
>
>
>
> --
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de • http://www.joachim-breitner.de/
>   Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
>   Debian Developer: nomeata at debian.org
>
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