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.