On Lisp -> Clojure: ProloG (pt. 1)
; As always, I will post when the code is “complete”, but my progress can be followed on Github. Also, this post is executable, just copy and paste into a Clojure REPL.
(comment “
Posts in this series: ch. 2, ch. 2 redux, ch. 3, ch. 4, ch. 5, ProloG pt. 1
First, let me thank Stuart Halloway for picking up the On Lisp -> Clojure port where I left off. I do not know if that was intentional, but in any case it makes for nice reading to start with my first 5 chapters and then transition right into Mr. Halloway’s posts (which are much higher quality IMO).
“)
; On of my favorite parts from On Lisp is his implementation of an embedded Prolog interpreter based on a simple database and inference system. My main goal for starting the On Lisp -> Clojure series was to eventually have this embedded Prolog system to use in my own applications. I will start out by just talking about a few preliminary structures and functions and then expand on them in future installments.
;
;;
;; The original Lisp make-hash-table function works on a
;; cons-cell structure, however Clojure provides a persistent
;; hash structure instead. I am not going to port the Common
;; Lisp function to Clojure, but instead will modify the Prolog
;; implementation to fit a more idiomatic Clojure approach.
;;
(defn make-db []
nil)
(def *default-db* (ref (make-db)))
(defn clear-db
"Takes a db and clears it"
([] (clear-db *default-db*))
([db] (dosync (ref-set db nil))))
(defn db-query
"Takes a db and a key and returns the mapped val"
([key] (db-query *default-db* key))
([db key] (get @db key)))
;;
;; db-push in Clojure is a bit trickier since it will be working
;; on a referenced persistent hash map. This version is larger
;; than the On Lisp version do to the fact that the database
;; being queried is an in-transaction value.
;;
(defn db-push
"Stores a value in a the db"
([key val] (db-push *default-db* key val))
([db key val]
(dosync
(commute db
(fn [the-db key val]
(assoc the-db key (cons val (get the-db key))))
key
val))))
;;
;; fact is almost a direct translation save for minor syntax diffs
;;
(defmacro fact
[pred & args]
`(do
(db-push '~pred '~args)
'~args))
(defn test-pt1 []
(fact painter reynolds joshua english)
(fact painter canale antonio venetian)
(db-query 'painter))
;
; There is definitely some room for improvement (suggestions/criticisms always welcome), but these initial functions work the same as the On Lisp originals, so that’s all for now. I’ll come back later for more.
;-m
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