Higher-order functions in C and Racket via the FFI

qsort.rkt

#lang racket

(require ffi/unsafe srfi/67)
(define-fun-syntax _unptr 
  (syntax-rules () [(_ t) (type: _pointer pre: (x => (ptr-ref x t)))]))

(define qsort 
  (get-ffi-obj 'qsort #f
               (_fun (l : (_list io _int len))
                     (len : _int = (length l))
                     (size : _int = (ctype-sizeof _int))
                     (compare : (_fun (_unptr _int) (_unptr _int) -> _int))
                     -> _void -> l)))

(qsort
 '(7 1 2 3 5 6 4 8 0 9)
 number-compare)