Raul Sierra writes: | Hi all, | | What is the difference between regular classes and constructor classes | and how do you specify that a class is a constructor class? | | Thanks in advance, | Raul The term `constructor class' is meant to include classes like Functor and Monad, whose instances are type constructors but not types. instance Functor Maybe where ... -- OK, and Functor is a constructor class foo :: Maybe -- Error, because Maybe isn't a type instance Eq () where ... -- OK, and Eq is a type class bar :: () -- OK, because () is a type There's no particular syntax to distinguish constructor classes from type classes. It's inferred from the method signatures. class Functor f where fmap :: (a -> b) -> (f a -> f b) -- Here f's kind is inferred as *->* (unary type constructor) class Eq a where (==), (/=) :: a -> a -> Bool -- Here a's kind is inferred as * (nullary type constructor, or type) Here's a previous thread about kind inference, if you're interested. http://haskell.cs.yale.edu/pipermail/haskell/2001-February/000489.html Regards, Tom