Hi, I am studying type classes using examples from the literature [1]. The attached code is a formalization of basic object oriented ideas. The particular approach seems to be built on the concepts of: thing, object, and identifier. I have no intension to implement anything or significantly change the code below. Rather, I am trying to understand the code as it stands. I include a number of test runs which seem OK, but I cannot get the *obj* function to work. obj :: t -> i -> o t i obj t i = Obj t i Any hints would be appreciated. Thanks, Pat [1] ftp://ftp.geoinfo.tuwien.ac.at/frank/frank97executableAxiomaticSpecification.pdf -- A property of a thing data Color = Blue | Green | Red | White deriving Show -- A thing class Cars c where car :: Color -> c getColor :: c -> Color putColor :: Color -> c -> c paint :: Color -> c -> c paint color car = putColor color car data Car = Car Color deriving Show instance Cars Car where car c = Car c putColor color (Car c) = Car color getColor (Car c) = c -- Identifiers for objects class (Integral i) => IDs i where startId :: i newId :: i -> i newId i = succ i sameId, notSameId :: i -> i -> Bool sameId i j = i == j notSameId i j = not (sameId i j) instance IDs Integer where startId = 1 -- Objects consist of Things, with Identifiers class (IDs i, Show i) => Objects o t i where obj :: t -> i -> o t i getId :: o t i -> i getThing :: o t i -> t doThing :: (t -> t) -> o t i -> o t i doThing f o = obj (f (getThing o)) (getId o) same :: o t i -> o t i -> Bool same i j = sameId (getId i) (getId j) isId :: i -> o t i -> Bool isId i o = sameId i (getId o) -- A general type of Obj data Object t i = Obj t i deriving Show -- A particular Car Obj, which an instance of Objects class (in Haskell terms, not OO terms) instance Objects Object Car Integer where obj t i = Obj t i getId (Obj t i) = i getThing (Obj t i) = t -- Create some actual Objects x = (Obj (Car Blue) (startId::Integer)) y = (Obj (Car Green) (newId startId::Integer)) -- Some tests on car thing, seem OK -- getColor (Car Blue) -- putColor Green (Car Blue) -- getColor (putColor Green (Car Blue)) -- Some tests on objects, seem OK -- same x y -- Obj (Car Blue) (newId startId::Integer) -- Obj (Car Blue) (startId::Integer) -- getThing (Obj (Car Blue) (startId::Integer)) -- getId (Obj (Car Blue) (startId::Integer)) -- isId 2 (Obj (Car Blue) (startId::Integer)) This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie