(define-module (data monoid)
  #:use-module (oop goops)
  #:use-module (srfi srfi-1)
  #:export (null mappend <>))

(define-generic null)
(define-generic mappend)

(define (<> . args)
  (fold mappend (null (car args)) (reverse args)))

;;; Lists

(define-method (mappend (a <pair>) (b <pair>))
  (append a b))
(define-method (mappend (a <pair>) (b <null>)) a)
(define-method (mappend (a <null>) (b <pair>)) b)
(define-method (mappend (a <null>) (b <null>)) '())
(define-method (null (a <pair>)) '())
(define-method (null (a <null>)) '())

;;; Strings
(define-method (mappend (a <string>) (b <string>))
  (string-append a b))
(define-method (null (a <string>)) "")