Add Int index functions to Data.Set

Data.Map has some functions that allow a Map's entries to be accessed through Int indexes:

lookupIndex :: Ord k => k -> Map k a -> Maybe Int
findIndex :: Ord k => k -> Map k a -> Int
elemAt :: Int -> Map k a -> (k, a)
updateAt :: (k -> a -> Maybe a) -> Int -> Map k a -> Map k a
deleteAt :: Int -> Map k a -> Map k a

With the exception of updateAt, these functions are trivially adaptable to Data.Set.

I attach a patch to this ticket that does that. My Data.Set implementations of the functions are modeled on the ones from Data.Map. Functions implemented:

lookupIndex :: Ord a => a -> Set a -> Maybe Int
findIndex :: Ord a => a -> Set a -> Int
elemAt :: Int -> Set a -> a
deleteAt :: Int -> Set a -> Set a

Trac metadata

Trac field	Value
Version	7.0.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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