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(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>)) "")
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