The hashtables package

[Tags: bsd3, library]

This package provides a couple of different implementations of mutable hash tables in the ST monad, as well as a typeclass abstracting their common operations, and a set of wrappers to use the hash tables in the IO monad.

QUICK START: documentation for the hash table operations is provided in the Data.HashTable.Class module, and the IO wrappers (which most users will probably prefer) are located in the Data.HashTable.IO module.

This package currently contains three hash table implementations:

1. Data.HashTable.ST.Basic contains a basic open-addressing hash table using linear probing as the collision strategy. On a pure speed basis it should currently be the fastest available Haskell hash table implementation for lookups, although it has a higher memory overhead than the other tables and can suffer from long delays when the table is resized because all of the elements in the table need to be rehashed.

2. Data.HashTable.ST.Cuckoo contains an implementation of "cuckoo hashing" as introduced by Pagh and Rodler in 2001 (see Cuckoo hashing has worst-case O(1) lookups and can reach a high "load factor", in which the table can perform acceptably well even when more than 90% full. Randomized testing shows this implementation of cuckoo hashing to be slightly faster on insert and slightly slower on lookup than Data.Hashtable.ST.Basic, while being more space efficient by about a half-word per key-value mapping. Cuckoo hashing, like the basic hash table implementation using linear probing, can suffer from long delays when the table is resized.

3. Data.HashTable.ST.Linear contains a linear hash table (see, which trades some insert and lookup performance for higher space efficiency and much shorter delays when expanding the table. In most cases, benchmarks show this table to be currently slightly faster than Data.HashTable from the Haskell base library.

It is recommended to create a concrete type alias in your code when using this package, i.e.:

 import qualified Data.HashTable.IO as H

 type HashTable k v = H.BasicHashTable k v

 foo :: IO (HashTable Int Int)
 foo = do
     ht <-
     H.insert ht 1 1
     return ht

Firstly, this makes it easy to switch to a different hash table implementation, and secondly, using a concrete type rather than leaving your functions abstract in the HashTable class should allow GHC to optimize away the typeclass dictionaries.

This package accepts a couple of different cabal flags:

This package has been tested with GHC 7.0.3, on:

Please send bug reports to


Dependenciesbase (==4.*), hashable (>=1.1 && <2), primitive, vector (>=0.7)
Copyright(c) 2011, Google, Inc.
AuthorGregory Collins
Home page
UploadedFri Apr 20 11:50:57 UTC 2012 by GregoryCollins
DistributionsDebian:, Fedora:, FreeBSD:, NixOS:, Stackage:
Downloads25736 total (1202 in last 30 days)
StatusDocs uploaded by user
Build status unknown [no reports yet]




unsafe-tricksturn on unsafe GHC tricksEnabled
bounds-checkingif on, use bounds-checking array accessesDisabled
debugif on, spew debugging output to stdoutDisabled
sse41if on, use SSE 4.1 extensions to search cache lines very efficiently. The portable flag forces this off.Disabled
portableif on, use only pure Haskell code and no GHC extensions.Disabled

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