Package Info


Mutable hash tables in the ST monad


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.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 approaching 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.

  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.

  1. "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:

  • 'unsafe-tricks', default /ON/. If this flag is enabled, we use some unsafe

GHC-specific tricks to save indirections (namely 'unsafeCoerce#' and 'reallyUnsafePtrEquality#'. These techniques rely on assumptions about the behaviour of the GHC runtime system and, although they've been tested and should be safe under normal conditions, are slightly dangerous. Caveat emptor. In particular, these techniques are incompatible with HPC code coverage reports.

  • 'sse42', default /OFF/. If this flag is enabled, we use some SSE 4.2

instructions (see <>, first available on Intel Core 2 processors) to speed up cache-line searches for cuckoo hashing.

  • 'bounds-checking', default /OFF/. If this flag is enabled, array accesses are


  • 'debug', default /OFF/. If turned on, we'll rudely spew debug output to


  • 'portable', default /OFF/. If this flag is enabled, we use only pure Haskell

code and try not to use unportable GHC extensions. Turning this flag on forces 'unsafe-tricks' and 'sse42' /OFF/.

Please send bug reports to <>.

License: BSD-3-Clause



Package Version Update ID Released Package Hub Version Platforms Subpackages info GA Release 2022-05-09 15 SP4
  • AArch64
  • ppc64le
  • x86-64
  • ghc-hashtables
  • ghc-hashtables-devel info GA Release 2018-08-01 15
  • AArch64
  • ghc-hashtables
  • ghc-hashtables-devel info GA Release 2018-07-31 15
  • ppc64le
  • ghc-hashtables
  • ghc-hashtables-devel info GA Release 2018-07-30 15
  • x86-64
  • ghc-hashtables
  • ghc-hashtables-devel