◊(Local Yarn Code "Diff")

  [int->bool                 (exact-integer? . -> . boolean?)]

  ;; Simple SQL makers
  [make-table-schema         ((string? (listof stringish?))
                              (#:primary-key-cols (listof stringish?))
                              . ->* . string?)]
  [make-insert/replace-query (stringish? (listof stringish?) . -> . string?)]
  [make-select-query         (stringish? (listof stringish?) #:where stringish? . -> . string?)]

  ;; Database operations
  [init-db!                  ((pathish?) #:rest (listof string?) . ->* . void?)]
  [query!                    ((string?) #:rest (listof any/c) . ->* . void?)]
  [select-rows!              (case->
                              (stringish? (listof stringish?) any/c . -> . (or/c empty? hash?))
                              (string? (listof stringish?) . -> . (or/c empty? hash?)))]))

          primary-key))

;; Create a query that inserts a row if it doesn’t exist (based on the first
;; column only), or updates it if it does. The returned query is parameterized,
;; and must be used with a list of values equal in length to the number of
;; fields given.
(define (make-insert/replace-query tablename fields)
  (format "INSERT OR REPLACE INTO `~a` (`rowid`, ~a) VALUES ((SELECT `rowid` FROM `~a` WHERE `~a`= ?1), ~a)"
          tablename
          (list->sql-fields fields)
          tablename
          (first fields)
          (list->sql-parameters fields)))

;; Simple row selection

  (check-equal? (make-table-schema "posts" '("title" "date"))
                "CREATE TABLE IF NOT EXISTS `posts` (`title`, `date`, PRIMARY KEY (`title`));")
  (check-equal? (make-table-schema 'posts '(title date) #:primary-key-cols '(author date))
                "CREATE TABLE IF NOT EXISTS `posts` (`title`, `date`, PRIMARY KEY (`author`, `date`));")

  (check-equal? (make-insert/replace-query 'posts '(author title))
                (string-append "INSERT OR REPLACE INTO `posts` (`rowid`, `author`, `title`) "
                               "VALUES ((SELECT `rowid` FROM `posts` WHERE `author`= ?1), ?1, ?2)"))

  (check-equal? (make-select-query 'posts '(author title) #:where 1)
                "SELECT `author`, `title` FROM `posts` WHERE 1"))

(define (good-connection?)
  (and (connection? (sqltools:dbc)) (connected? (sqltools:dbc))))

;; Initialize the database connection, creating the database if it does not yet exist
;; and running any provided queries (e.g., "CREATE TABLE IF NOT EXISTS...")
(define (init-db! filename . qs)
  (cond [(good-connection?) (disconnect (sqltools:dbc))])
  (sqltools:dbc (sqlite3-connect #:database filename #:mode 'create))
  (unless (empty? qs)
    (for ([q (in-list qs)])
         (query! q))))

;; Run a query with logging (if enabled) and return the result
(define (query! q . parameters)