Haskell already has the capability to write type-level representations of numbers, so we can have types like zipVec :: Vector a n -> Vector b n -> Vector (

Haskell already has the capability to write type-level representations of numbers, so we can have types like zipVec :: Vector a n -> Vector b n -> Vector (a, b) n where Vector a b is the type of a vector of elements of type a that is of length n. The above function will therefore only zip together two vectors if they have the same length; otherwise it's a compile-time error. Right now, people usually roll their own representations of numbers, but this new extension means not only do we have a standard one, we can now write something like myBuffer :: Buffer Int 256 where 256 is a type-level number. With type-level symbols, we could do something like data StringWrapper (a :: Symbol) = S String getUserData :: IO (StringWrapper "unsanitized") writeToDB :: StringWrapper "sanitized" -> IO () sanitize :: StringWrapper "unsanitized" -> StringWrapper "sanitized" stringLength :: StringWrapper a -> Int so that writing code like do x <- getUserData putStrLn ("Length is " ++ show (stringLength x)) writeToDb x -- forgetting to sanitize produces a compile-time error because the types don't match up.