With the changes to ScopedTypeVariables in GHC you can't pick up the type from the return type of your function directly, so you'll need either a combinator to do the work or to pass the type in question in as an argument to a helper function.
normalize :: (Modular s a, Integral a) => a -> (M s a)
normalize = normalize' undefined where
normalize' :: (Modular s a, Integral a) => s -> a -> (M s a)
normalize' s a = M (a `mod` modulus s)
There is an implementation of the reflection code with minor modifications to work with the modern version of GHC's ScopedTypeVariables in hackage as 'reflection' and a minimalist implementation of modular arithmetic from the same paper (if not yet including the residue number system based optimizations) available in 'monoids' as Data.Ring.ModularArithmetic. Both have only been fleshed out as far as I've needed them for other purposes, but should be usable.
-Edward Kmett