[Haskell-cafe] type families with non-type indices

[Doug McIlroy]

Is there an analog or variant of type families that is indexed
by values rather than by types? An example of such a family
is integers mod p. One wants to define the operations once
for all p, so that a type for any particular p can be
introduced by a one-line declaration.

Of course this can be done with p being a run-time parameter:

data Modp = Modp Integer Int
instance Num (Modp) where
        (Modp x p) + (Modp y q) =
                if p==q then Modp ((x+y) 'mod' p) p
                        else error "unequal moduli in Modp operation"

But it would be better to catch the error at compile time and
not have to check at every operation.

Doug