How does one implement hash tables in a functional language?

Question

Is there any way to implement hash tables efficiently in a purely functional language? It seems like any change to the hash table would require creating a copy of the origi

Answer 1

Hash tables can be implemented with something like the ST monad in Haskell, which basically wraps IO actions in a purely functional interface. It does so by forcing the IO actions to be performed sequentially, so it maintains referential transparency: you can't access the old "version" of the hash-table.

See: hackage.haskell.org/package/hashtables