;;; Generating code from a type's own definition.
;;;
;;; `serialize-struct' is handed nothing but a struct's name. It asks
;;; the compiler's type database what fields that struct has and what
;;; type each one is, and writes a printer to match. Add a field to the
;;; struct and the printer grows with it, with no other edit.
;;;
;;; The database is filled in as toplevel forms are processed, in order,
;;; so a struct has to be declared before the macro call that asks about
;;; it -- the same rule C has.

(include stdio.h)

(defmacro (serialize-struct type-name)
  ;; map-fields walks the declaration; type-match picks a printf
  ;; conversion per field. Both come from the compiler's type database,
  ;; so the macro never takes a type apart itself.
  (let ((printers
         (map-fields type-name
           (lambda (name type)
             `(fprintf out
                       ,(string-append
                         " " (symbol->string name) "="
                         (type-match type
                           (int             "%d")
                           (char            "%c")
                           (long            "%ld")
                           (unsigned        "%u")
                           (float           "%g")
                           (double          "%g")
                           ((* const char)  "%s")
                           ((* char)        "%s")
                           (else (error "serialize-struct: unsupported field type"
                                        type-name name type))))
                       (-> v ,name))))))
    (if (not printers)
        (error "serialize-struct: no such struct" type-name)
        `(pub fn ,(cat 'serialize- type-name)
              ((v (* const struct ,type-name)) (out (* FILE)))
              void
           (fprintf out ,(string-append (symbol->string type-name) " {"))
           ,@printers
           (fprintf out " }\n")))))

(struct point ((x int) (y int)))

(struct person
  ((name (* const char))
   (age int)
   (height float)))

;;; Two printers, written by the compiler from the declarations above.
(serialize-struct point)
(serialize-struct person)

(pub fn main () int
  (var origin (struct point) #(0 0))
  (var corner (struct point) #(640 -480))
  (var alex (struct person) #("Alex" 34 1.82))

  (serialize-point (& origin) stdout)
  (serialize-point (& corner) stdout)
  (serialize-person (& alex) stdout)
  (return 0))