Hi. To answer your original question anyway: In Haskell 2010, all type variables that appear in any type annotation are implicitly quantified. They're not "scoped". So e.g. here: instance (Arbitrary a, Eq a) => Arbitrary (Temporal a) where arbitrary = do d <- arbitrary (CList cs) <- arbitrary :: (Arbitrary a, Eq a) => Gen (CList a) -- <=== return (Temporal d cs) if you say just "Gen (CList a)", it's the same as if you'd write "Gen (CList b)", and it means "forall b. Gen (CList b)". But that's not true! The term "arbitrary" cannot have the type "forall b. Gen (CList b)", because the instance for "Gen (CList c)" requires "(Arbitray c, Eq c)". That's why GHC wants you to add the constraint. However, you can make type variables scoped in GHC, by enabling the "ScopedTypeVariables" extension. So if you add the pragma {-# LANGUAGE ScopedTypeVariables #-} as the first line of your module, then the type variables that occur in the instance declaration scope over the body of the instance, and your annotations should mean what you originally expected and work without the extra constraints. But of course, as has already been stated, the type annotations are unnecessary here, because GHC can infer everything anyway. Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com Registered in England & Wales, OC335890 250 Ice Wharf, 17 New Wharf Road, London N1 9RF, England