Assorted concrete container types

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LTS Haskell 21.12:0.6.7
Stackage Nightly 2023-09-22:0.6.7
Latest on Hackage:0.6.7

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BSD-3-Clause licensed
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This package contains efficient general-purpose implementations of various basic immutable container types. The declared cost of each operation is either worst-case or amortized, but remains valid even if structures are shared.


Changelog for containers package

  • Planned for GHC 8.2.

  • Use COMPLETE pragmas to declare complete sets of pattern synonyms for Data.Sequence. At last!

  • Make Data.IntMap.Strict.traverseWithKey force the values before installing them in the result. Previously, this function could be used to produce an IntMap containing undefined values.

  • Fix strictness bugs in various rewrite rules for Data.Map.Strict and Data.IntMap.Strict. Previously, rules could unintentionally reduce strictness. The most important change in this regard is the elimination of rules rewriting * coerce to coerce. To map a coercion over a structure for free, be sure to use the lazy map or fmap. It is possible to write rules that do a somewhat better job of this, but it turns out to be a bit messy.

  • Optimize Data.IntMap.restrictKeys and Data.IntMap.withoutKeys. The semantic fix in left them rather slow in certain cases.

  • Speed up size for IntSet and IntMap (thanks, Mike Ledger!).

  • Define a custom liftA2 in Applicative instances for base 4.10, and use liftA2 rather than <*> whenever it may be beneficial.

  • Add liftA2-related RULES for Data.Sequence.

  • Export non-deprecated versions of showTree and showTreeWith from Data.IntMap.Internal.Debug.

  • Fix completely incorrect implementations of Data.IntMap.restrictKeys and Data.IntMap.withoutKeys. Make the tests for these actually run. (Thanks to Tom Smalley for reporting this.)

  • Fix a minor bug in the Show1 instance of Data.Tree. This produced valid output, but with fewer parentheses than Show. (Thanks, Ryan Scott.)

  • Add MonadZip instance for Data.Sequence.

  • Remove meaningless stability annotations (Thanks, Simon Jakobi.)

  • Backport bug fixes from

  • Add merge and mergeA for Data.IntMap.

  • Add instances for Data.Graph.SCC: Foldable, Traversable, Data, Generic, Generic1, Eq, Eq1, Show, Show1, Read, and Read1.

  • Add lifted instances (from Data.Functor.Classes) for Data.Sequence, Data.Map, Data.Set, Data.IntMap, and Data.Tree. (Thanks to Oleg Grenrus for doing a lot of this work.)

  • Properly deprecate functions in Data.IntMap long documented as deprecated.

  • Rename several internal modules for clarity. Thanks to esoeylemez for starting this process.

  • Make Data.Map.fromDistinctAscList and Data.Map.fromDistinctDescList more eager, improving performance.

  • Plug space leaks in Data.Map.Lazy.fromAscList and Data.Map.Lazy.fromDescList by manually inlining constant functions.

  • Add lookupMin and lookupMax to Data.Set and Data.Map as total alternatives to findMin and findMax.

  • Add !? to Data.Map as a total alternative to !.

  • Avoid using deleteFindMin and deleteFindMax internally, preferring total functions instead. New implementations of said functions lead to slight performance improvements overall.

  • Backport bug fixes from Aug 2016

General package changes

  • Remove all attempts to support nhc98 and any versions of GHC before 7.0.

  • Integrate benchmarks with Cabal. (Thanks, Gabriel Gonzalez!)

  • Make Cabal report required extensions properly, and stop using default extensions. Note that we do not report extensions conditionally enabled based on GHC version, as doing so would lead to a maintenance nightmare with no obvious benefits.

  • Use BangPatterns throughout to reduce noise. This extension is now required to compile containers.

  • Improve QuickCheck properties taking arbitrary functions by using Test.QuickCheck.Function.Fun instead of evil Show instances for functions.

  • Expose several internal modules through Cabal (as requested by Edward Kmett). These remain completely unsupported.

New exports and instances

  • Add alterF, restrictKeys, and withoutKeys to Data.Map and Data.IntMap.

  • Add take, drop, splitAt, takeWhileAntitone, dropWhileAntitone, and spanAntitone for Data.Map and Data.Set. Thanks to Cale Gibbard for suggesting these.

  • Add merge, mergeA, and associated merge tactics for Data.Map. Many thanks to Cale Gibbard, Ryan Trinkle, and Dan Doel for inspiring the merge idea and helping refine the interface.

  • Add traverseMaybeWithKey, fromDescList, fromDescListWith, fromDescListWithKey, and fromDistinctDescList to Data.Map.

  • Add fromDescList and fromDistinctDescList to Data.Set.

  • Add Empty, :<|, and :|> pattern synonyms for Data.Sequence.

  • Add adjust', (!?), lookup, chunksOf, cycleTaking, insertAt, deleteAt, intersperse, foldMapWithIndex, and traverseWithIndex for Data.Sequence.

  • Derive Generic and Generic1 for Data.Tree.Tree, Data.Sequence.ViewL, and Data.Sequence.ViewR.

  • Add foldTree for Data.Tree. (Thanks, Daniel Wagner!)

Semantic changes

  • Make Data.Sequence.splitAt strict in its arguments. Previously, it returned a lazy pair.

  • Fix completely erroneous definition of length for Data.Sequence.ViewR.

  • Make Data.Map.Strict.traverseWithKey force result values before installing them in the new map.

  • Make drawTree handle newlines better. (Thanks, recursion-ninja!)


  • All functions in Data.Map proper that have been documented as deprecated since version 0.5 or before now have DEPRECATED pragmas and will actually be removed after another cycle or two.

  • Tree printing functions in Data.Map intended for library debugging are now deprecated. They will continue to be available for the foreseeable future in an internal module.

Performance changes

  • Substantially speed up splitAt, zipWith, take, drop, fromList, partition, foldl', and foldr' for Data.Sequence. Special thanks to Lennart Spitzner for digging into the performance problems with previous versions of fromList and finding a way to make it really fast. Slightly optimize replicateA. Stop traverse from performing many unnecessary fmap operations.

  • Most operations in Data.Sequence advertised as taking logarithmic time (including >< and adjust) now use their full allotted time to avoid potentially building up chains of thunks in the tree. In general, the only remaining operations that avoid doing more than they really need are the particular bulk creation and transformation functions that really benefit from the extra laziness. There are some situations where this change may slow programs down, but I think having more predictable and usually better performance more than makes up for that.

  • Add rewrite rules to fuse fmap with reverse for Data.Sequence.

  • Switch from hedge algorithms to divide-and-conquer algorithms for union, intersection, difference, and merge in both Data.Map and Data.Set. These algorithms are simpler, are known to be asymptotically optimal, and are faster according to our benchmarks.

  • Speed up adjust for Data.Map. Allow map to inline, and define a custom (<$). This considerably improves mapping with a constant function.

  • Remove non-essential laziness in Data.Map.Lazy implementation.

  • Speed up deletion and alteration functions for Data.IntMap. Dec 2015

  • Planned to bundle with GHC 8.0.1.

  • Add IsString instance to Data.Sequence.

  • Define Semigroup instances for Data.Map, Data.Set, Data.IntMap, Data.IntSet and Data.Sequence. Dec 2014

  • Bundled with GHC 7.10.1.

  • Add role annotations for Data.Map and Data.Set.

  • Add IsList instances for Data.Map, Data.Set, Data.IntMap and Data.IntSet.

  • Several performance improvements for Data.Sequence.

  • Add Data.Sequence.fromFunction and Data.Sequence.fromArray. Jan 2014

  • Bundled with GHC 7.8.1.

  • The Data.Map.fromList and Data.Set.fromList now use linear-time algorithm if the input is sorted, without need to call fromDistinctAscList.

  • Implement indexing operations (lookupIndex, findIndex, elemAt, deletaAt) for Data.Set too.

  • Add Applicative and Alternative instances for Data.Sequence.

  • Add foldMapWithKey to Data.Map and Data.IntMap.

  • Implement poly-kinded Typeable.

  • Add Functor instance for Data.Graph.SCC.

  • Add Data.Map.splitRoot and Data.Set.splitRoot. May 2012

  • Bundled with GHC 7.6.1.

  • Major improvements since last release:

    • a clearer distinction between value-lazy and value-strict containers,
    • performance improvements across the board,
    • a big internal clean-up, and
    • new functions for e.g. merging, updating, and searching containers.
  • While the old Data.Map and Data.IntMap modules will continue to exist for the foreseeable future, we’ve abandoned the practice of having the strict and lazy versions of each function distinguished by an apostrophe. The distinction is instead made at the module level, by introducing four new modules:

    • Data.Map.Strict
    • Data.Map.Lazy
    • Data.IntMap.Strict
    • Data.IntMap.Lazy

    This split has three benefits:

    • It makes the choice between value-strict and value-lazy containers more declarative; you pick once at import time, instead of having to remember to use the strict or lazy versions of a function every time you modify the container.
    • It alleviates a common source of performance issues, by forcing the user to think about the strictness properties upfront. For example, using insertWith instead of insertWith’ is a common source of containers-related performance bugs.
    • There are fewer functions per module, making it easier to get an overview of each module.
  • Note that the types used in the strict and lazy APIs are the same, so you can still use the same container in a “mixed” manner, if needed.

  • The Data.IntSet representation changed to store small sets using bits in an Word. Larger sets are stored as a collection of such dense small sets, connected together by a prefix trie. Feb 2012

  • Bundled with GHC 7.4.1.

  • Data.Map now exports foldr, foldr’, foldlandfoldl’`.

  • Data.Set now exports foldr, foldr’, foldlandfoldl’`.

  • Data.IntMap now exports foldr, foldr’, foldl, foldl’, foldrWithKey, foldrWithKey’, foldlWithKeyandfoldlWithKey’`.

  • Data.IntSet now exports foldr, foldr’, foldlandfoldl’`.

  • Data.Map.foldWithKey is no longer deprecated, although it is expected to be deprecated again in the future.

  • There are now NFData instance for Data.Map.Map, Data.Set.Set, Data.IntMap.IntMap, Data.IntSet.IntSet and Data.Tree.Tree. Aug 2011

  • Bundled with GHC 7.2.1.

  • Data.Map now exports new functions foldrWithKey' and foldlWithKey', which are strict variants of foldrWithKey and foldlWithKey respectively.

  • Data.IntMap now exports new functions insertWith' and insertWithKey', which are strict variants of insertWith and insertWithKey respectively. Nov 2010

  • Bundled with GHC 7.0.1.

  • Strictness is now more consistent, with containers being strict in their elements even in singleton cases.

  • There is a new function insertLookupWithKey' in Data.Map.

  • The foldWithKey function in Data.Map has been deprecated in favour of foldrWithKey. Dec 2009

  • Bundled with GHC 6.12.1.

  • mapAccumRWithKey has been added to Data.IntMap.

  • A Traversable instance has been added to Data.IntMap.IntMap.

  • The types of Data.IntMap.intersectionWith and Data.IntMap.intersectionWithKey have been changed from intersectionWith :: (a -> b -> a) -> IntMap a -> IntMap b -> IntMap a intersectionWithKey :: (Key -> a -> b -> a) -> IntMap a -> IntMap b -> IntMap a to intersectionWith :: (a -> b -> c) -> IntMap a -> IntMap b -> IntMap c intersectionWithKey :: (Key -> a -> b -> c) -> IntMap a -> IntMap b -> IntMap c

  • The types of Data.IntMap.findMin and Data.IntMap.findMax have been changed from findMin :: IntMap a -> a findMax :: IntMap a -> a to findMin :: IntMap a -> (Int,a) findMax :: IntMap a -> (Int,a)

  • Data.Map now exports mapAccumRWithKey, foldrWithKey, foldlWithKey and toDescList.

  • Data.Sequence now exports replicate, replicateA, replicateM, iterateN, unfoldr, unfoldl, scanl, scanl1, scanr, scanr1, tails, inits, takeWhileL, takeWhileR, dropWhileL, dropWhileR, spanl, spanr, breakl, breakr, partition, filter, sort, sortBy, unstableSort, unstableSortBy, elemIndexL, elemIndicesL, elemIndexR, elemIndicesR, findIndexL, findIndicesL, findIndexR, findIndicesR, foldlWithIndex, foldrWithIndex, mapWithIndex, zip, zipWith, zip3, zipWith3, zip4 and zipWith4. Nov 2008

  • Bundled with GHC 6.10.1.

  • Various result type now use Maybe rather than allowing any Monad. Nov 2007

  • Bundled with GHC 6.8.1.

  • Initial split off from GHC base.