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

[Oliver Charles]

I don't really see the problem with lookup. To me, that function always
takes a container, an index into that container, and returns the value
under than index or fails. It just so happens that sets are indexed by the
very values the contain, but I don't find the signature confusing. Sure, I
might not use it very often, but it's still clear enough to me what it's
doing.

Ollie

On Tue, 28 Jun 2016, 10:23 a.m. 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.
> >>
> >> _______________________________________________
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> >> 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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