Proposal for containers: Add 'lookup' function to Data.Set

[Edward Kmett]

As another color for the bikeshed: lookupWithKey would match the existing
vocabulary of insertWithKey, etc.

We could further document the rest of them to include the assertion that
the copy of the key they give back to the combinator is the one found in
the Map/Set and not the one given. Then this is just the continuation of an
existing pattern in the library.

-Edward

On Wed, Jun 29, 2016 at 4:18 AM, Joachim Breitner <mail at joachim-breitner.de>
wrote:

> Hi,
>
> good point. How about
>
>     lookupKey :: Ord a => a -> Set a -> Maybe a
>
> and
>
>     lookupKey :: Ord a => k -> Map k v -> Maybe (k, v)
>
>
> It might be a bit strange to talk about key in the context of Sets, but
> less so to someone who knows about sharing and pointers etc, and the
> naming convention works well for maps. There, looking up the key is
> likely the main purpose of the function; that we return the value along
> with it is a nice bonus.
>
> Greetings,
> Joachim
>
> Am Mittwoch, den 29.06.2016, 02:12 -0400 schrieb David Feuer:
> > Would we also want something similar for `Data.Map`, looking up a key
> > and returning the copy of the key found in the map along with the
> > associated value?
> >
> > lookupWithInternedKeyOrWhatever :: Ord k => k -> Map k v -> (k, v)
> >
> > If so, it might pay to think about a naming scheme that will make
> > sense for both Data.Map and Data.Set.
> >
> > On Tue, Jun 28, 2016 at 12:38 PM, David Turner
> > <dct25-561bs at mythic-beasts.com> wrote:
> > > +1
> > >
> > > When I previously searched for (and failed to find) this function
> > > in order
> > > to implement an intern table, I remember trying 'lookup',
> > > 'lookupEQ' and
> > > 'find' in approximately that order, so that's my order if
> > > preference for its
> > > name.
> > >
> > > 'lookupInterned' or similar could work for me too if you want to
> > > scare off
> > > beginners although I don't see a strong need myself. Certainly a
> > > mention of
> > > interning in the haddocks would help.
> > >
> > >
> > > WHAT
> > >
> > > It is proposed to add a ‘lookup' function on 'Set' in the
> > > "containers"
> > > package. Feedback during the next two weeks is welcome.
> > >
> > > The function
> > >
> > > > lookup :: Ord a => a -> Set a -> Maybe a
> > >
> > > is almost indentical to the 'member' function but, in addition,
> > > returns the
> > > value
> > > stored in the set.
> > >
> > > WHY
> > >
> > > The point of this proposal is to facilitate program-wide data
> > > sharing. The
> > > 'lookup'
> > > function gives access to a pointer to an object already stored in a
> > > Set and
> > > equal
> > > to a given argument. The 'lookup' function is a natural extension
> > > to the
> > > current
> > > 'lookupLT', 'lookupGT', 'lookupLE' and 'lookupGE' functions, with
> > > obvious
> > > semantics.
> > >
> > > Example use case: In a parser, the memory footprint can be reduced
> > > by
> > > collapsing
> > > all equal strings to a single instance of each string. To achieve
> > > this, one
> > > needs
> > > a way to get a previously seen string (internally, a pointer) equal
> > > to a
> > > newly
> > > parsed string. Amazingly, this is very difficult with the current
> > > "containers" library interface.
> > > One current option is to use a Map instead, e.g., 'Map String
> > > String'
> > > which stores twice as many pointers as necessary.
> > >
> > > HOW
> > >
> > > The git pull request at
> > > https://github.com/haskell/containers/pull/291
> > > contains the straight-forward implementation of the 'lookup’
> > > function on
> > > 'Set', with test cases,
> > > as a patch against the current containers master branch.
> > >
> > >
> > > Salutations,
> > > Nicolas.
> > >
> > > _______________________________________________
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> > >
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> --
>
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de • https://www.joachim-breitner.de/
>   XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
>   Debian Developer: nomeata at debian.org
>
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