Set and Map from lazy/infinite lists.

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MIT licensed by Carlos Freund


A Lazy Set and Map implemented in Haskell.

Allows efficient, lazy lookups on sorted lists. The list may be of ininite size.

The Source-List must + contain elements that implement Ord + must be ascending (or descending: see fromDescList) + either produce an infinite number of elements or terminate

Set Sample usage

import Data.Set.Lazy

set = fromAscList $ map (*3) [1..]

3 `member` set -> True
4 `member` set -> False

Map Sample usage

import Prelude hiding(lookup)
import Data.Map.Lazier

sqrtmap = fromList $ map (\i->(i, sqrt i)) [1..]
lookup 2 sqrtmap -> Just 1.4142135623730951


Elements from the Source-List will be requested in batches of increasing size. By default the batch-size is increases by two. This would lead to batches of 1,2,4,8,16. This can be changed by using growFromAscList factor list. For Example a factor of 1.3 casues the batches to be 1,2,2,3,3,4,5.

Increasing the growth-factor reduces lookup times but increases the batch-size. When it is set to 1.0 it performs like a list.

lookup: O(m) = log m where m is the index of the element in the source-list.


This breaks the set, because the underlying list stops producing elements.

set = fromList $ filter (<4) [1..]
5 `member` set


Revision history for lazyset -- YYYY-mm-dd

  • First version. Released on an unsuspecting world.
  • Import of files from
  • making this a cabal-module
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