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

I suspect we may want both functions. On Wed, Jun 29, 2016 at 4:57 PM, David Thomas 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 wrote:

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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 wrote:

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+1 on the function. -1/2 on the name.

On Jun 27, 2016 5:45 PM, "Nicolas Godbout" wrote:

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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

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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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