;;; "sql-null.scm" -- SQL NULL object and the ternary logic  -*- Scheme -*-

;;; Ivan Shmakov, 2007  This code is in public domain.

(module sql-null
  (sql-null sql-null? sql-not sql-or sql-or sql-and sql-coalesce)

(import scheme (chicken base))

;; We could also (define-record sql-null) and alias sql-null to make-sql-null
;; but that implies creating many new objects, which we don't want.
(define-record sql-null-type)
(define sql-null-object (make-sql-null-type))
(define sql-null (constantly sql-null-object))
(define sql-null? sql-null-type?)

(define (sql-not o)
  (if (sql-null? o) o (not o)))

(define-syntax sql-or
  (syntax-rules ()
    ((sql-or a ...)
     (sql-or/null #f a ...))))

(define-syntax sql-or/null
  (syntax-rules ()
    ((sql-or/null null)
     null)
    ((sql-or/null null a b ...)
     (let ((ea a))
       (cond ((sql-null? ea) (sql-or/null ea    b ...))
             ((not ea)       (sql-or/null null b ...))
             (else           ea))))))

(define-syntax sql-and
  (syntax-rules ()
    ((sql-and a ...)
     (sql-and/null #t a ...))))

(define-syntax sql-and/null
  (syntax-rules ()
    ((sql-and/null null)
     null)
    ((sql-and/null null a b ...)
     (let ((ea a))
       (cond ((sql-null? ea) (sql-and/null ea    b ...))
             (ea             (sql-and/null null b ...))
             (else           ea))))))

(define-syntax sql-coalesce
  (syntax-rules ()
    ((sql-coalesce)
     (sql-null))
    ((sql-coalesce a b ...)
     (let ((ea a))
       (if (sql-null? ea)
           (sql-coalesce b ...)
           ea)))))

)