Template assignment 12

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+implementation module cashModel
+/*
+	Pieter Koopman, Radboud University, 2017
+	pieter@cs.ru.nl
+	Advanced programming
+
+	A simple state model for an automated cash register
+*/
+
+import StdEnv, GenEq
+
+class euro a :: a -> Euro
+instance euro Product
+where
+	euro Pizza = euro (4,99)
+	euro Beer  = euro (0,65)
+	euro _     = euro 1
+
+instance euro Int where euro e = {euro = e, cent = 0}
+instance euro (Int, Int) where euro (e,c) = {euro = e, cent = c}
+instance euro [e] | euro e where euro l = sum (map euro l)
+instance euro Euro where euro e = e
+
+instance + Euro
+where
+	+ x y = {euro = c / 100, cent = (abs c) rem 100}
+	where
+		c = (x.euro + y.euro) * 100 + sign x.euro * x.cent + sign y.euro * y.cent
+
+instance - Euro
+where
+	- x y = {euro = c / 100, cent = (abs c) rem 100}
+	where
+		c = (x.euro - y.euro) * 100 + sign x.euro * x.cent - sign y.euro * y.cent
+
+instance zero Euro where zero = {euro = 0, cent = 0}
+derive gEq Euro, Product
+instance == Product where (==) p q = p === q
+instance == Euro where (==) p q = p === q
+
+instance ~ Euro where ~ e = {e & euro = ~e.euro}
+
+model :: [Product] Action -> ([Product],[Euro])
+model list (Add p) = ([p:list],[euro p])
+model list (Rem p)
+| isMember p list = (removeMember p list,[~ (euro p)])
+| otherwise       = (list,[])
+model list Pay = ([],[euro list])