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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulehashtables-1.4.2Haskell2010

Data.HashTable.Class

This module contains a HashTable typeclass for the hash table implementations in this package. This allows you to provide functions which will work for any hash table implementation in this collection.

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.new
    H.insert ht 1 1
    return ht

or

import qualified Data.HashTable.ST.Cuckoo as C
import qualified Data.HashTable.Class as H

type HashTable s k v = C.HashTable s k v

foo :: ST s (HashTable s k v)
foo = do
    ht <- H.new
    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.

Note that the functions in this typeclass are in the ST monad; if you want hash tables in IO, use the convenience wrappers in Data.HashTable.IO.

  • 1 class
  • 3 values
  • Packagehashtables-1.4.2
  • Exports4
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceClass.hs
classclass HashTable (h :: Type -> Type -> Type -> Type) where
#

A typeclass for hash tables in the ST monad. The operations on these hash tables are typically both key- and value-strict.

Methods

  • new :: ST s (h s k v)

    Creates a new, default-sized hash table. O(1).

  • newSized :: Int -> ST s (h s k v)

    Creates a new hash table sized to hold n elements. O(n).

  • mutate :: (Eq k, Hashable k) => h s k v -> k -> (Maybe v -> (Maybe v, a)) -> ST s a

    Generalized update. Given a key k, and a user function f, calls:

    • `f Nothing` if the key did not exist in the hash table

    • `f (Just v)` otherwise

    If the user function returns (Nothing, _), then the value is deleted from the hash table. Otherwise the mapping for k is inserted or replaced with the provided value.

    Returns the second part of the tuple returned by f.

  • mutateST :: (Eq k, Hashable k) => h s k v -> k -> (Maybe v -> ST s (Maybe v, a)) -> ST s a

    As mutate, except that the action can perform additional side effects.

  • insert :: (Eq k, Hashable k) => h s k v -> k -> v -> ST s ()

    Inserts a key/value mapping into a hash table, replacing any existing mapping for that key.

    O(n) worst case, O(1) amortized.

  • delete :: (Eq k, Hashable k) => h s k v -> k -> ST s ()

    Deletes a key-value mapping from a hash table. O(n) worst case, O(1) amortized.

  • lookup :: (Eq k, Hashable k) => h s k v -> k -> ST s (Maybe v)

    Looks up a key-value mapping in a hash table. O(n) worst case, (O(1) for cuckoo hash), O(1) amortized.

  • foldM :: (a -> (k, v) -> ST s a) -> a -> h s k v -> ST s a

    A strict fold over the key-value records of a hash table in the ST monad. O(n).

  • mapM_ :: ((k, v) -> ST s b) -> h s k v -> ST s ()

    A side-effecting map over the key-value records of a hash table. O(n).

  • lookupIndex :: (Eq k, Hashable k) => h s k v -> k -> ST s (Maybe Word)

    Looks up the index of a key-value mapping in a hash table suitable for passing to nextByIndex.

  • nextByIndex :: h s k v -> Word -> ST s (Maybe (Word, k, v))

    Returns the next key-value mapping stored at the given index or at a greater index. The index, key, and value of the next record are returned.

  • computeOverhead :: h s k v -> ST s Double

    Computes the overhead (in words) per key-value mapping. Used for debugging, etc; time complexity depends on the underlying hash table implementation. O(n).

Instances3HashTable
valuetoList :: HashTable h => h s k v -> ST s [(k, v)]
#

Given a hash table, produce a list of key-value pairs. O(n).