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

[Alec]

> 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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