In the paper Functional Pearl: Implicit Configurations, the authors use modular arithmetic as a running example to demonstrate the many ways of injecting the modulus into the arithmetic operations. For example, one approach is to explicitly pass around the modulus

> add1 :: Integral a => a -> a -> a -> a
> add1 m x y = (x+y) `mod` m
> 
> mult1 :: Integral a => a -> a -> a -> a
> mult1 m x y = (x*y) `mod` m

But this is error prone and cumbersome because someone using these two functions to do something like

> example :: Integral a => a -> a -> a -> a
> example m x y = (((x+y) `mod` x) * y) `mod` m

when they actually intended to mod by m both times. You could say add a newtype wrapper to m to fix this but this still doesn’t stop the user from using two different modulus operations in example.

The paper suggests many different ways to solving this issue and one of them is to use a reader monad and write add as add :: (Integral a, MonadReader a m) => a -> a -> a which would certainly thread the same modulus through all operations. But it forces us to write monadic code when it is unnecessary.

What I realized was that we can still the reader structure but without the monad instance if we roll our own type with the reader state.

> newtype M a = M (a -> a)
> 
> withModulus :: Integral a => a -> M a -> a
> withModulus m (M f) = f m
> 
> instance Integral a => Num (M a) where
>   (M f) + (M g) = M $ \s -> (f s + g s) `mod` s
>   (M f) - (M g) = M $ \s -> (f s - g s) `mod` s
>   (M f) * (M g) = M $ \s -> (f s * g s) `mod` s
>   negate (M f)  = M $ \s -> (- f s) `mod` s
>   abs _         = error "Modular numbers are not signed"
>   signum _      = error "Modular numbers are not signed"
>   fromInteger n = M $ \s -> fromIntegral n `mod` s

This is very convenient.

ghci> withModulus 7 $ (10+3)*(3-4) + 8
  2

This solution seems as convenient and safe as what the paper does for this particular example. Of course, the paper is solving a much more general problem but this struck me as a good solution for modular arithmetic.