; ArrowLISP Micro KANREN
; Copyright (C) 2006 Nils M Holm
;
; Based on "The Reasoned Schemer" by Daniel P. Friedman, et al.
; Inspired by "Sokuza" Mini-KANREN by Oleg Kiselyov.
;
; See the file LICENSE of the ArrowLISP distribution
; for conditions of use.

(define amk t)

(require '=nmath)

(define ,_ ',_)

(package amk)

(define (fail x) ())

(define (succeed x) (list x))

(define failed null)

(define (var x)
  (cons '? x))

(define (var-p x)
  (and (not (atom x))
       (eq (car x) '?)))

(define empty-s ())

(define :BOTTOM (var 'bottom))

(define (atomic x)
  (or (atom x) (eq (car x) 'closure)))

(define (ext-s x v s)
  (cons (cons x v) s))

(define (walk x s)
  (cond ((not (var-p x)) x)
    (t (let ((v (assq x s)))
         (cond (v (walk (cdr v) s))
           (t x))))))

(define (unify x y s)
  (let ((x (walk x s))
        (y (walk y s)))
    (cond
      ((eq x y) s)
      ((var-p x) (ext-s x y s))
      ((var-p y) (ext-s y x s))
      ((or (atomic x) (atomic y)) :F)
      (t (let ((s (unify (car x) (car y) s)))
           (and s (unify (cdr x) (cdr y) s)))))))

(define (== g1 g2)
  (lambda (s)
    (let ((s2 (unify g1 g2 s)))
      (cond (s2 (succeed s2))
        (t (fail s))))))

(define (disj* . g*)
  (lambda (s)
    (letrec
      ((try (lambda g*
        (cond ((null g*) (fail s))
          (t (append ((car g*) s)
                     (apply try (cdr g*))))))))
      (apply try g*))))

(define (conj* . g*)
  (lambda (s)
    (letrec
      ((try (lambda (g* s*)
        (cond ((null g*) s*)
          (t (try (cdr g*)
                  (apply append
                    (map (car g*) s*))))))))
      (try g* (succeed s)))))

(define (disja* . g*)
  (lambda (s)
    (letrec
      ((try (lambda g*
        (cond ((null g*) (fail s))
          (t (let ((out ((car g*) s)))
               (cond ((failed out)
                   (apply try (cdr g*)))
                 (t out))))))))
      (apply try g*))))

(define (neg g)
  (lambda (s)
    (let ((out (g s)))
      (cond ((failed out) (succeed s))
        (t (fail s))))))

(define (choice x lst)
  (cond ((null lst) fail)
    (t (disj* (== x (car lst))
              (choice x (cdr lst))))))

(define (occurs x y s)
  (let ((v (walk y s)))
    (cond
      ((var-p y) (eq x y))
      ((var-p v) (eq x v))
      ((atomic v) :F)
      (t (or (occurs x (car v) s)
             (occurs x (cdr v) s))))))

(define (circular x s)
  (let ((v (walk x s)))
    (cond ((eq x v) :F)
      (t (occurs x (walk x s) s)))))

(define (walk* x s)
  (letrec
    ((w* (lambda (x s)
      (let ((x (walk x s)))
        (cond
          ((var-p x) x)
          ((atomic x) x)
          (t (cons (w* (car x) s)
                   (w* (cdr x) s))))))))
    (cond ((circular x s) :BOTTOM)
      ((eq x (walk x s)) empty-s)
      (t (w* x s)))))

(define (preserve-bottom s)
  (cond ((occurs :BOTTOM s s) ())
    (t s)))

(define (reify-name n)
  (implode (append '#_ '#, n)))

(define (reify v)
  (letrec
    ((reify-s (lambda (v s)
      (let ((v (walk v s)))
        (cond ((var-p v)
            (ext-s v (reify-name (length s)) s))
          ((atomic v) s)
          (t (reify-s (cdr v)
                (reify-s (car v) s))))))))
    (reify-s v empty-s)))

(define (run* x g)
  (preserve-bottom
    (map (lambda (s)
           (walk* x (append s (reify (walk* x s)))))
         (g empty-s))))

(define (expose_ x)
  (letrec
    ((expose (lambda (x)
      (cond
        ((eq x ,_)
          (list 'var (list 'quote ',_)))
        ((atom x) x)
        ((var-p x) x)
        (t (cons (expose (car x))
                 (expose (cdr x))))))))
    (expose x)))

(define (arun* x g)
  (run* x (eval (expose_ g))))

(package)