Hi, I am trying to understand the data type declaration below. What is the relation between class C3 and the data type Address below? Where is such a technique used? Thanks, Pat module A where data Person = Person String Integer deriving Show data Employee = Employee String Integer deriving Show class C1 c1 where age :: c1 -> Integer instance C1 Person where age(Person "P1" 1) = 1 instance C1 Employee where age(Employee "E1" 1) = 1 class C1 c2 => C2 c2 where name :: c2 -> String instance C2 Person where name(Person "P2" 1) = "P2" instance C2 Employee where name(Employee "E2" 1) = "E2" class C2 c3 => C3 c3 a where address :: c3 -> a instance C3 Person String where -- ** What are the semantics or purpose of this construct. -- ** Is the type declared in the context of a class. data C3 c3 a => Address c3 a = Address c3 a instance C3 Person (Address String String) where 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