When using template haskell (via Derive) to generate this (exact) instance: instance Foldable ((->) Int) => Foldable Data.Derivable.InterpreterLib.Test.List where foldMap f (Cons x0 x1) = (const mempty Cons `mappend` foldMap f x0) `mappend` foldMap f x1 foldMap f (Nil) = const mempty Nil I realize the context is insatisfiable. My issue, is that I can't even reach that "challenge". I get this error instead: Malformed type AppT ArrowT (ConT GHC.Base.Int) When splicing generated code into the program I couldn't find an existing ticket or discussion for this issue relying on the phrase "malformed type". I couldn't even find the source that generates that string in haskell-src, template-haskell, or ghc-6.8.2. Can anybody help? Thanks for your time.