1 files changed, 25 insertions, 0 deletions
diff --git a/data/monoid.scm b/data/monoid.scm
new file mode 100644
index 0000000..45d30cd
--- /dev/null
+++ b/data/monoid.scm
@@ -0,0 +1,25 @@
+(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>)) "")