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

[David Feuer]

+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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