#lang scheme

(require (for-syntax srfi/1))

; from John: 

;; define/checks
;; won't work with more than eight arguments (at least it's a static failure...)
(define-syntax (define/checks stx)
  (syntax-case stx ()
      [(_ (fun-name (argname checker) ...) body ...)
       (let* ([num-args (length (syntax->list #`(argname ...)))]
              [posn-names (take (list #'"first" #'"second" #'"third" #'"fourth" #'"fifth" #'"sixth" #'"seventh" #'"eighth")
                                num-args)])
         (with-syntax ([(posn-name ...) (datum->syntax #f posn-names)])
           (with-syntax ([(check-call ...) #`((checker (quote fun-name) argname posn-name) ...)])
             #`(define (fun-name argname ...)
                 check-call
                 ...
                 body
                 ...))))]))


(define (check-image f s x)
  (unless (string? s) 
    (error f "string expected as ~a argument, given ~e " x s)))

(define-struct position (x y))

(define (check-coordinate f s x)
  (unless (position? s)
    (error f "position expected as ~a argument, given ~e" x s)))

;; plonk a shape down at some given set of coordinates
;; plonk-at : shape number number -> shapelist
(define/checks (plonk-at [shape check-image]
                         [x check-coordinate]
                         [y check-coordinate])
  (list (make-position (make-position x y) shape)))