Persistent Hash Tables

This is a dead simple implementation of persistent hash tables on top of a port of David Va Horn's persistent random-access lists, SRFI-101.

These are like hash tables in most respects, with similar, but not identical performance profiles, with the major exception that modification is functional, ptbl-set returns a new hash table with the new association in it. The previous table is left unmodified.

Example Usage:

(setq tbl ({} the-empty-ptbl :x 10))
(ptbl-get tbl :x) -> 10
(ptbl-get the-empty-ptbl :x) -> nil

The function {} allows you to quickly create a new ptbl or to update the associations in a ptbl, when the first argument is itself a ptbl. Eg:

({} :x 10 :y 11)

Creates a new ptbl with :x and :y associations.

({} a-table :x 10 :y 11)

Returns a new ptbl with the same associations as a-table except where :x and :y are concerned.