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

[David Feuer]

Let me be firm (co-maintainer's prerogative): the function will not be
named `lookup`. The current leader in my eyes is Andreas Abel's
`lookupEntry`, but I could be convinced to use something else if others
prefer
On Jun 29, 2016 5:12 PM, "Alec" <alec at deviant-logic.net> wrote:

> > Are there many cases where we want `lookup` where we don't actually
> want `intern`?
>
> Anecdotally, the majority of the times I've wanted `intern` functionality,
> I've been implementing something full-text-search-like.  This meant having
> a "load the lexicon" phase, where a set of blessed words get `intern`ed,
> and then a query phase where I want is `lookup` because my lexicon is doing
> double duty as a stoplist.  I have no sense of whether this would be a
> common case.
>
> Also, +1 for `intern`, +1 for `lookup`.  I'm fine with both names.
>
> On Wed, Jun 29, 2016 at 4:57 PM David Thomas <davidleothomas at gmail.com>
> wrote:
>
>> Well, "intern" includes a "and if it is not there, add it".  Calling
>> this "intern" without that behavior would be highly misleading.  That
>> said, maybe we just want an "intern" function?  It would save us a dip
>> into the set, for new strings and leave (marginally) less room to make
>> a mistake.  Are there many cases where we want `lookup` where we don't
>> actually want `intern`?
>>
>> On Tue, Jun 28, 2016 at 2:22 AM, Chris Wong <lambda.fairy at gmail.com>
>> wrote:
>> > Python uses "intern", so perhaps that can serve as the name.
>> >
>> > (See https://docs.python.org/2/library/functions.html#intern)
>> >
>> > On Tue, Jun 28, 2016 at 9:47 AM, David Feuer <david.feuer at gmail.com>
>> wrote:
>> >> +1 on the function. -1/2 on the name.
>> >>
>> >> On Jun 27, 2016 5:45 PM, "Nicolas Godbout" <nicolas.godbout at gmail.com>
>> >> wrote:
>> >>>
>> >>>
>> >>> 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.
>> >>>
>> >>> _______________________________________________
>> >>> Libraries mailing list
>> >>> Libraries at haskell.org
>> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>> >>
>> >>
>> >> _______________________________________________
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>> >>
>> >
>> >
>> >
>> > --
>> > Chris Wong (https://lambda.xyz)
>> >
>> > "I had not the vaguest idea what this meant and when I could not
>> > remember the words, my tutor threw the book at my head, which did not
>> > stimulate my intellect in any way." -- Bertrand Russell
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>
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