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

28 Jun 2016


      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:
...
+1 on the function. -1/2 on the name.
On Jun 27, 2016 5:45 PM, "Nicolas Godbout" 
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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Chris Wong (https://lambda.xyz)

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