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

[Joachim Breitner]

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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> > 
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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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