chickadee » postgresql » row-fold

row-fold KONS KNIL RESULTprocedure
row-fold* KONS KNIL RESULTprocedure

This is the fundamental result set iterator. It calls (kons row seed) for every row, where row is the list of values in the current row and seed is the accumulated result from previous calls (initially knil), ie its pattern looks like (KONS ROWN ... (KONS ROW2 (KONS ROW1 KNIL))). It returns the final accumulated result.

The starred version works the same, except it calls (kons rowN-col1 rowN-col2 ... seed) instead of (kons rowN seed), so the procedure must know how many columns you have in the result set.

(use postgresql)

(let ((conn (connect '((dbname . test)))))
   (row-fold (lambda (row sum) (+ (car row) sum))
             (query conn "SELECT 1 UNION SELECT 2")))
 => 3

(let ((conn (connect '((dbname . test)))))
   (row-fold* (lambda (value str) (string-append str value))
              (query conn "SELECT 'hello, ' UNION SELECT 'world'")))
 => "hello, world"