../haskell/prepose.lhs:707:0: Parse error in pattern which is pointing at: normalize a :: M s a = M (mod a (modulus (undefined :: s)))

The code indeed used lexically scoped type variables -- which GHC at that time implemented differently. Incidentally, on the above line, M s a is the type annotation on the result of (normalize a) rather than on the argument 'a'. Therefore, putting a parentheses like (a :: M s a) is wrong. Since the implementation of local type variables in GHC changed over the years, it's best to get rid of them. Here is the same line in the Hugs version of the code

normalize :: (Modular s a, Integral a) => a -> M s a normalize a = r a __ where r :: (Modular s a, Integral a) => a -> s -> M s a r a s = M (mod a (modulus s))

Of course the signature for normalize can be omitted. Hugs did not support lexically scoped type variables then (and probably doesn't support now). Today I would have written this definition simpler:

normalize :: (Modular s a, Integral a) => a -> M s a normalize a = r where r = M (mod a (modulus (tcar2 r)))

tcar2:: f s x -> s tcar2 = undefined

(the function tcar2 is used in a few other places in the Hugs code, for a similar purpose).